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Journal Cover Journal of Geophysical Research : Atmospheres
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   Partially Free Journal Partially Free Journal
   ISSN (Online) 2169-8996
   Published by AGU Homepage  [17 journals]
  • Observed and Simulated Spring and Summer Dryness in the United States: the
           Impact of the Pacific Sea Surface Temperature and Beyond
    • Authors: Siyu Zhao; Yi Deng, Robert X. Black
      Abstract: This study characterizes major modes of variability in the spring and summer U.S. dryness, as measured by the seasonal total number of dry (no-precipitation) days, and assesses the impact of the Pacific sea surface temperature. The most severe spring and summer dry conditions typically occur in the western United States. Maximum Covariance Analysis (MCA) reveals that the Pacific Decadal Oscillation (PDO) is the primary driver of interannual variability in the U.S. dryness. El Niño Southern Oscillation (ENSO) also contributes in part (especially during spring). Beyond the PDO and ENSO impact, interannual variations in spring dryness exhibit a meridional dipole structure while variations during summer are related to a northwest-southeast (NW-SE) oriented dipole. The spring meridional dipole is associated with circulation anomalies resembling the West Pacific (WP) teleconnection pattern while the summer NW-SE dipole is a downstream manifestation of a quasi-stationary wave train. A parallel analysis of CMIP5 models’ historical simulations demonstrates that such models generally capture the relation between U.S. dryness and the PDO, albeit with varying degrees of accuracy. The models also show reasonable skill in simulating the residual meridional dipole in spring dryness variability, but have difficulty representing the NW-SE oriented dipole occurring during summer. The model shortcomings isolated here largely arise from a misrepresentation of the corresponding large-scale circulation and moisture transport anomalies. These model biases suggest that great challenges exist in our ongoing pursuit of reliable projections of the U.S. hydroclimate variability.
      PubDate: 2017-11-24T07:45:31.623607-05:
      DOI: 10.1002/2017JD027279
  • Quantifying the uncertainties in an ensemble of decadal climate
    • Authors: Ehud Strobach; Golan Bel
      Abstract: Meaningful climate predictions must be accompanied by their corresponding range of uncertainty. Quantifying the uncertainties is non-trivial, and different methods have been suggested and used in the past. Here, we propose a method that does not rely on any assumptions regarding the distribution of the ensemble member predictions. The method is tested using the CMIP5 1981-2010 decadal predictions and is shown to perform better than two other methods considered here. The improved estimate of the uncertainties is of great importance for both practical use and for better assessing the significance of the effects seen in theoretical studies.
      PubDate: 2017-11-23T22:50:34.816936-05:
      DOI: 10.1002/2017JD027249
  • Simulation of Precipitation Extremes Using a Stochastic Convective
           Parameterization in the NCAR CAM5 under Different Resolutions
    • Authors: Yong Wang; Guang J. Zhang, Yu-Jun He
      Abstract: With the incorporation of the Plant-Craig (PC) stochastic deep convection scheme into the Zhang-McFarlane (ZM) deterministic parameterization in the Community Atmospheric Model version 5 (CAM5), its impact on extreme precipitation at different resolutions (2o, 1o and 0.5o) is investigated. CAM5 with the stochastic deep convection scheme (EXP) simulates the precipitation extremes indices better than the standard version (CTL). At 2o and 1o resolutions, EXP increases high percentile (> 99th) daily precipitation over the United States, Europe and China, resulting in a better agreement with observations. However, at 0.5o resolution, due to enhanced grid-scale precipitation with increasing resolution, EXP overestimates extreme precipitation over southeastern U.S. and eastern Europe. The reduced biases in EXP at each resolution benefit from a broader probability distribution function (PDF) of convective precipitation intensity simulated. Among EXP simulations at different resolutions, if the spatial averaging area over which input quantities used in convective closure are spatially averaged in the stochastic convection scheme is comparable, the modeled convective precipitation intensity decreases with increasing resolution, when gridded to the same resolution, while the total precipitation is not sensitive to model resolution, exhibiting some degree of scale awareness. Sensitivity tests show that for the same resolution increasing the size of spatial averaging area decreases convective precipitation, but increases the grid-scale precipitation.
      PubDate: 2017-11-22T01:35:41.632198-05:
      DOI: 10.1002/2017JD026901
  • Seasonal and diurnal variations of Tropical Tropopause Layer (TTL) over
           the Indian Peninsula
    • Authors: M. Muhsin; S. V. Sunilkumar, M. Venkat Ratnam, B. V. Krishna Murthy, K. Parameswaran
      Abstract: Seasonal and diurnal variations in the thermal structure of the troposphere and lower stratosphere (TLS) with special reference to the tropical tropopause layer (TTL) are studied using data from 3 hour interval radiosonde launches carried out simultaneously from Trivandrum (8.5°N, 76.9°E) and Gadanki (13.5°N,79°E) during December 2010 to March 2014. TTL is defined as the region extending from the level of minimum stability (LMinS) to the level of maximum stability (LMaxS). Above the cold point tropopause (CPT), temperature showed warm anomaly (4-5K) during summer monsoon (June-September) and cold anomaly (up to −4 K) during winter (December-February). The temperature in the troposphere showed a clear diurnal variation (±0.5K) with a cold anomaly during early morning and warm anomaly during the day in all the seasons. At Gadanki, the diurnal temperature anomaly in the lower stratosphere (LS) showed its phase propagating downwards with time; whereas at Trivandrum such variations are not clearly evident. At diurnal timescales, the TTL showed significant variations at LMinS (0.5-1.5km/5-7K) and smaller variations at CPT (0.2-0.5km/2-3K). In general, the amplitude of the diurnal component is greater than the semi-diurnal component for all the TTL-parameters; and these amplitudes are relatively larger at Trivandrum than at Gadanki. The observed diurnal variations could be the manifestation of tidal oscillations and/or due to the influence of local convection. Correlation analysis between different TTL parameters indicated a slow transition from a governing adiabatic process in the TTL-base to a diabatic process at the TTL-top.
      PubDate: 2017-11-21T04:25:37.026155-05:
      DOI: 10.1002/2017JD027056
  • Iron mineralogy and speciation in clay-sized fractions of Chinese desert
    • Authors: Wanyi Lu; Wancang Zhao, William Balsam, Huayu Lu, Pan Liu, Zunli Lu, Junfeng Ji
      Abstract: Iron released from Asian desert dust may be an important source of bioavailable iron for the North Pacific Ocean and thereby may stimulate primary productivity. However, the Fe species of the fine dusts from this source region are poorly characterized. Here we investigate iron species and mineralogy in the clay-sized fractions (< 2 μm, CSFS), the size fraction most prone to long-distance transport as dust. Samples were analyzed by sequential chemical extraction, X-ray diffraction and diffuse reflectance spectrometry. Our results show that Fe dissolved from easily reducible iron phases (ferrihydrite and lepidocrocite) and reducible iron oxides (dominated by goethite) are 0.81 wt.% and 2.39 wt.% respectively, and Fe dissolved from phyllosilicates extracted by boiling HCl (dominated by chlorite) is 3.15 wt.%. Dusts originating from deserts in north-western China, particularly the Taklimakan desert, are relatively enriched in easily reducible Fe phases, probably due to abundant Fe contained in fresh weathering products resulting from the rapid erosion associated with active uplift of mountains to the west. Data about Fe speciation and mineralogy in Asian dust sources will be useful for improving the quantification of soluble Fe supplied to the oceans, especially in dust models.
      PubDate: 2017-11-21T03:41:03.140745-05:
      DOI: 10.1002/2017JD027733
  • Water soluble organic nitrogen (WSON) in ambient fine particles over a
           megacity in south China: Spatiotemporal variations and source
    • Authors: Xu Yu; Qingqing Yu, Ming Zhu, Mingjin Tang, Sheng Li, Weiqiang Yang, Yanli Zhang, Wei Deng, Guanghui Li, Yuegang Yu, Zhonghui Huang, Wei Song, Xiang Ding, Qihou Hu, Jun Li, Xinhui Bi, Xinming Wang
      Abstract: Organic nitrogen aerosols are complex mixtures and important compositions in ambient fine particulate matters (PM2.5), yet their sources and spatiotemporal patterns are not well understood particularly in regions influenced by intensive human activities. In this study, filter-based ambient PM2.5 samples at four stations (one urban, two rural plus one urban roadside) and PM samples from combustion sources (vehicle exhaust, ship emission and biomass burning) were collected in the coastal megacity Guangzhou, south China for determining water soluble organic nitrogen (WSON) along with other organic and inorganic species. The annual average WSON concentrations, as well as the ratios of WSON to water soluble total nitrogen (WSON/WSTN), were all significantly higher at rural sites than urban sites. Average WSON concentrations at the four sites during the wet season were quite near each other, ranging from 0.41 to 0.49 ug/m3; however, they became 2 times higher at the rural sites than at the urban sites during the dry season. Five major sources for WSON were identified through positive matrix factorization (PMF) analysis. Vehicle emission (29.3%), biomass burning (22.8%) and secondary formation (20.2%) were three dominant sources of WSON at the urban station while vehicle emission (45.4%) and dust (28.6%) were two dominant sources at the urban roadside station. At the two rural sites biomass burning (51.1% and 34.1%, respectively) and secondary formation (17.8% and 30.5%, respectively) were dominant sources of WSON. Ship emission contributed 8-12% of WSON at the four sites. Natural vegetation seemed to have very minor contribution to WSON.
      PubDate: 2017-11-21T03:40:45.253173-05:
      DOI: 10.1002/2017JD027327
  • Evaluating modeled impact metrics for human health, agriculture growth,
           and near-term climate
    • Authors: K. M. Seltzer; D. T. Shindell, G. Faluvegi, L. T. Murray
      Abstract: Simulated metrics that assess impacts on human health, agriculture growth, and near-term climate were evaluated using ground based and satellite observations. The NASA GISS ModelE2 and GEOS-Chem models were used to simulate the near present chemistry of the atmosphere. A suite of simulations that varied by model, meteorology, horizontal resolution, emissions inventory, and emissions year were performed, enabling an analysis of metric sensitivities to various model components. All simulations utilized consistent anthropogenic global emissions inventories (ECLIPSE V5a or CEDS) and an evaluation of simulated results were carried out for 2004–2006 and 2009–2011 over the United States and 2014–2015 over China. Results for O3 and PM2.5 based metrics featured minor differences due to the model resolutions considered here (2.0∘×2.5∘ and 0.5∘×0.666∘), and model, meteorology, and emissions inventory each played larger roles in variances. Surface metrics related to O3 were consistently high biased, though to varying degrees, demonstrating the need to evaluate particular modeling frameworks before O3 impacts are quantified. Surface metrics related to PM2.5 were regionally diverse, indicating a multi-model mean with robust results are valuable tools in predicting PM2.5 related impacts. Oftentimes, the configuration that captured the change of a metric best over time differed from the configuration that captured the magnitude of the same metric best, demonstrating the challenge in skillfully simulating impacts. These results highlight the strengths and weaknesses of these models in simulating impact metrics related to air quality and near-term climate. With such information, the reliability of historical and future simulations can be better understood.
      PubDate: 2017-11-20T22:35:18.81724-05:0
      DOI: 10.1002/2017JD026780
  • FDTD Modeling of LEMP Propagation in the Earth-Ionosphere Waveguide with
           Emphasis on Realistic Representation of Lightning Source
    • Authors: Thang H. Tran; Yoshihiro Baba, Vijaya B. Somu, Vladimir A. Rakov
      Abstract: The finite-difference time-domain (FDTD) method in the 2D cylindrical coordinate system was used to compute the nearly-full-frequency-bandwidth vertical electric field and azimuthal magnetic field waveforms produced on the ground surface by lightning return strokes. The lightning source was represented by the modified transmission-line model with linear current decay with height (MTLL model), which was implemented in the FDTD computations as an appropriate vertical phased-current-source array. The conductivity of atmosphere was assumed to increase exponentially with height, with different conductivity profiles being used for daytime and nighttime conditions. The fields were computed at distances ranging from 50 to 500 km. Skywaves (reflections from the ionosphere) were identified in computed waveforms and used for estimation of apparent ionospheric reflection heights. It was found that our model reproduces reasonably well the daytime electric field waveforms measured at different distances and simulated (using a more sophisticated propagation model) by Qin et al. [2017]. Sensitivity of model predictions to changes in the parameters of atmospheric conductivity profile, as well as influences of the lightning source characteristics (current waveshape parameters, return-stroke speed, and channel length) and ground conductivity were examined.
      PubDate: 2017-11-18T15:55:21.796917-05:
      DOI: 10.1002/2017JD027305
  • Gravity Wave Dynamics in a Mesospheric Inversion Layer: 1. Reflection,
           Trapping, and Instability Dynamics
    • Authors: D. C. Fritts; B. Laughman, L. Wang, T. S. Lund, R. L. Collins
      Abstract: An anelastic numerical model is employed to explore the dynamics of gravity waves (GWs) encountering a mesosphere inversion layer (MIL) having a moderate static stability enhancement and a layer of weaker static stability above. Instabilities occur within the MIL when the GW amplitude approaches that required for GW breaking due to compression of the vertical wavelength accompanying the increasing static stability. Thus MILs can cause large-amplitude GWs to yield instabilities and turbulence below the altitude where they would otherwise arise. Smaller amplitude GWs encountering a MIL do not lead to instability and turbulence, but do exhibit partial reflection and transmission, and the transmission is a smaller fraction of the incident GW when instabilities and turbulence arise within the MIL. Additionally, greater GW transmission occurs for weaker MILs and for GWs having larger vertical wavelengths relative to the MIL depth and for lower GW intrinsic frequencies. These results imply similar dynamics for inversions due to other sources, including the tropopause inversion layer, the high stability capping the polar summer mesopause, and lower-frequency GWs or tides having sufficient amplitudes to yield significant variations in stability at large and small vertical scales. MILs also imply much stronger reflections and less coherent GW propagation in environments having significant fine structure in the stability and velocity fields than in environments that are smoothly varying.
      PubDate: 2017-11-18T15:35:21.596129-05:
      DOI: 10.1002/2017JD027440
  • Decadal transition of moisture sources and transport in northwestern China
           during summer from 1982 to 2010
    • Authors: Lijuan Hua; Linhao Zhong, Zhuguo Ma
      Abstract: The northwestern corner of China (NWCC) experienced a decadal transition in summer precipitation during 1982–2010, with a significant upward trend in 1982–2000 (P1) but a downward one in 2001–2010 (P2). A spatially unbounded dynamic recycling model is developed to estimate the moisture sources and moisture transport variations based on ERA-Interim data. The results suggest that more than 88% of NWCC precipitation has external moisture origins in the southwest and northwest terrestrial areas. The increasing precipitation trend during P1 can be explained by the increasing moisture contribution from the southwest and decreasing contribution from the northwest. However, the opposite trends cause the decreasing precipitation trend during P2. In general, the decadal precipitation transition is mainly determined by the variation of short-distance moisture transport from Central Asia, although opposite moisture transport variations exist in the Ural Mountains and Northeast Europe. The variation of the precipitation trend is closely associated with a well-organized wave train propagation from the North Atlantic to Central Asia. During P1, the wave-train structure consists of a titled positive-phase North Atlantic Oscillation (NAO), an anticyclonic circulation over Europe and a cyclonic anomaly over Central Asia, which promotes the southwest moisture flux to NWCC. But the opposite circulation pattern dominates P2. The energy dispersion due to the breakdown of the NAO determines the phase and strength of the downstream wave anomalies over Eurasia. This suggests that the summer NAO might influence the decadal variation of NWCC precipitation through the decadal modulation of the Eurasia wave train.
      PubDate: 2017-11-18T15:05:25.958073-05:
      DOI: 10.1002/2017JD027728
  • Gravity Wave Dynamics in a Mesospheric Inversion Layer: 2. Instabilities,
           Turbulence, Fluxes, and Mixing
    • Authors: D. C. Fritts; L. Wang, B. Laughman, T. S. Lund, R. L. Collins
      Abstract: A companion paper by Fritts et al. [2017a] employed an anelastic numerical model to explore the dynamics of gravity waves (GWs) encountering a mesospheric inversion layer (MIL) having a moderate static stability enhancement and a layer of weaker static stability above. That study revealed that MIL responses, including GW transmission, reflection, and instabilities, are sensitive functions of GW parameters. This paper expands on two of the Fritts et al. [2017a] simulations to examine GW instability dynamics and turbulence in the MIL, forcing of the mean wind and stability environments by GW, instability, and turbulence fluxes, and associated heat and momentum transports. These direct numerical simulations resolve turbulence inertial-range scales and yield the following results:GW breaking and turbulence in the MIL occur below where they would otherwise due to enhancements of GW amplitudes and shears in the MIL,2D GW and instability heat and momentum fluxes are ~20-30 times larger than 3D instability and turbulence fluxes,mean fields are driven largely by 2D GW and instability dynamics rather than 3D instabilities and turbulence,2D and 3D heat fluxes in regions of strong turbulence yield small departures from initial T(z) and N2(z) profiles, hence do not yield nearly adiabatic “mixed” layers, andour MIL results are consistent with the relation between the turbulent vertical velocity variance and energy dissipation rate proposed by Weinstock [1981] for the limited intervals evaluated.
      PubDate: 2017-11-18T14:25:33.100498-05:
      DOI: 10.1002/2017JD027442
  • The potential of existing cellular networks for detecting the precursors
           of fog
    • Authors: N. David; H. O. Gao
      Abstract: In the last decade, published research has indicated the potential of commercial microwave links that comprise the data transmission infrastructure of cellular communication networks as an environmental monitoring technology. Different weather phenomena cause interference in the wireless communication links which can therefore essentially act as a low cost sensor network, already deployed worldwide, for atmospheric monitoring.In this study we focus on the attenuation effect caused in commercial microwave networks due to gradients in the atmospheric refractive index with altitude as a result of the combination of temperature inversions and falls in the atmospheric humidity trapped beneath them. These conditions, when combined with high relative humidity near ground level are precursors to the creation of fog. The current work utilizes this novel approach to demonstrate the potential for detecting these preconditions of fog, a phenomenon associated with severe visibility limitations which can lead to dangerous accidents, injuries and loss of lives.
      PubDate: 2017-11-18T09:10:24.592293-05:
      DOI: 10.1002/2017JD027360
  • Prominent mid-latitude circulation signature in High Asia's surface
           climate during monsoon
    • Authors: Thomas Mölg; Fabien Maussion, Emily Collier, John C. H. Chiang, Dieter Scherer
      Abstract: High Asia has experienced strong environmental changes in recent decades, as evident in records of glaciers, lakes, tree rings, and vegetation. The multi-scale understanding of the climatic drivers, however, is still incomplete. In particular, few systematic assessments have evaluated to what degree, if at all, the mid-latitude westerly circulation modifies local surface climates in the reach of the Indian Summer Monsoon. This paper shows that a southward shift of the upper-tropospheric westerlies contributes significantly to climate variability in the core monsoon season (July-September) by two prominent dipole patterns at the surface: cooling in the west of High Asia contrasts with warming in the east, while moist anomalies in the east and northwest occur with drying along the southwestern margins. Circulation anomalies help to understand the dipoles and coincide with shifts in both the westerly wave train and the South Asian High, which imprint on air mass advection and local energy budgets. The relation of the variabilities to a well-established index of mid-latitude climate dynamics allows future research on climate proxies to include a fresh hypothesis for the interpretation of environmental changes.
      PubDate: 2017-11-18T08:35:25.652524-05:
      DOI: 10.1002/2017JD027414
  • Satellite Ocean Aerosol Retrieval (SOAR) algorithm extension to S-NPP
           VIIRS as part of the ‘Deep Blue’ aerosol project
    • Authors: A. M. Sayer; N. C. Hsu, J. Lee, C. Bettenhausen, W. V. Kim, A. Smirnov
      Abstract: The Suomi National Polar-Orbiting Partnership (S-NPP) satellite, launched in late 2011, carries the Visible Infrared Imaging Radiometer Suite (VIIRS) and several other instruments. VIIRS has similar characteristics to prior satellite sensors used for aerosol optical depth (AOD) retrieval, allowing the continuation of space-based aerosol data records. The Deep Blue algorithm has previously been applied to retrieve AOD from Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and Moderate Resolution Imaging Spectroradiometer (MODIS) measurements over land. The SeaWiFS Deep Blue data s et also included a SeaWiFS Ocean Aerosol Retrieval (SOAR) algorithm to cover water surfaces. As part of NASA's VIIRS data processing, Deep Blue is being applied to VIIRS data over land, and SOAR has been adapted from SeaWiFS to VIIRS for use over water surfaces. This study describes SOAR as applied in version 1 of NASA's S-NPP VIIRS Deep Blue data product suite. Several advances have been made since the SeaWiFS application, as well as changes to make use of the broader spectral range of VIIRS. A preliminary validation against Maritime Aerosol Network (MAN) measurements suggests a typical uncertainty on retrieved 550nm AOD of order ±(0.03+10%), comparable to existing SeaWiFS/MODIS aerosol data products. Retrieved Ångström exponent and fine mode AOD fraction are also well-correlated with MAN data, with small biases and uncertainty similar to or better than SeaWiFS/MODIS products.
      PubDate: 2017-11-17T04:41:40.973435-05:
      DOI: 10.1002/2017JD027412
  • An improved triple collocation analysis algorithm for decomposing
           auto-correlated and white soil moisture retrieval errors
    • Authors: Jianzhi Dong; Wade Crow
      Abstract: If not properly account for, auto-correlated retrieval errors can lead to inaccurate results in soil moisture data analysis and reanalysis. Here, we propose a more generalized form of the triple collocation analysis algorithm (GTC) capable of decomposing the total error variance of remotely-sensed surface soil moisture retrievals into their auto-correlated and the serially-white components. Synthetic tests demonstrate the robustness and accuracy of GTC - even in the presence of significant temporal data gaps. However, the accuracy of GTC error autoregressive parameter estimates is relatively more sensitive to temporal data availability. In addition, land surface model soil moisture predictions collected from phase 2 of the North American Land Data Assimilation System (NLDAS-2) and remotely-sensed surface soil moisture retrievals obtained from the European Space Agency Climate Change Initiative (ESA CCI) are applied for a real data demonstration. Despite expectations to the contrary, significant error auto-correlation is found in the remotely-sensed-based ESA CCI soil moisture datasets. In particular, ESA CCI-Act (i.e., the subset of ESA CCI SM retrievals based on active scattomotter data) demonstrates the largest autoregressive parameters over low biomass areas. Conversely, ESA CCI-Pas retrievals (based on passive radiometer data) have larger error autoregressive parameters over high biomass areas. As such, results clarify circumstances in which errors in remotely-sensed surface soil moisture retrievals cannot be considered serially white.
      PubDate: 2017-11-17T02:05:22.318344-05:
      DOI: 10.1002/2017JD027387
  • Regional responses to black carbon aerosols: The importance of air-sea
    • Authors: A. Gnanadesikan; A. A. Scott, M. A. Pradal, W. J. M. Seviour, D. W. Waugh
      Abstract: The impact of modern black carbon aerosols on climate via their changes in radiative balance is studied using a coupled model where sea surface temperatures (SSTs) are allowed to vary and an atmosphere-only version of the same model where SSTs are held fixed. Allowing the ocean to respond is shown to have a profound impact on the pattern of temperature change. Particularly large impacts are found in the North Pacific (which cools by up to 1K in the coupled model) and in North Central Asia (which warms in the coupled simulation and cools in the fixed SST simulation). Neither set of experiments shows large changes in surface temperatures in the Southeast Asian region where the atmospheric burden of black carbon is highest. These results are related to the stabilization of the atmosphere and changes in oceanic heat transport. Over the North Pacific, atmospheric stabilization results in an increase in stratiform clouds. The resulting shading reduces evaporation, freshening the surface layer of the ocean and reducing the inflow of warm subtropical waters. Over the land, a delicate balance between greater atmospheric absorption, shading of the surface and changes in latent cooling of the surface helps to determine whether warming or cooling is seen. Our results emphasize the importance of coupling in determining the response of the climate system to black carbon and suggest that black carbon may play an important role in modulating climate change over the North Pacific.
      PubDate: 2017-11-17T01:56:38.86637-05:0
      DOI: 10.1002/2017JD027589
  • Case studies of low visibility forecasting in falling snow with WRF model
    • Authors: Haibo Huang; Guangxing Zhang
      Abstract: Accurate low visibility forecasts in falling snow are critical to the safety and efficiency of air traffic. The Weather and Research Forecast (WRF) model successfully captured two unusual snowstorms occurred in Urumqi. On this basis, the quality of 15 parameterizations for predicting visibility in snow are evaluated, using both observations and forecasts of the meteorological variables from WRF model. The parameterizations are mainly based on the relations between the extinction efficient (β) or visibility (Vis) and the snowfall rate (S). Comprehensive evaluations show that most of these parameterizations (13 of 15) are skillful as well as having the ability to predict low visibilities to some extent. Among them, the parameterization Vis = 0.62R−0.59 performs the best, followed by the approach of Stoelinga and Warner. It is also found that the visibility forecasts based on the observations always have considerably higher quality than the visibility forecasts from WRF model. The results suggest that more than one parameterization is promising if the WRF model is able to provide accurate predictions of the relevant meteorological variables. Furthermore, the forecast accuracy of low visibilities strongly depends on the accurate predictions of snowfall rate of greater than or equal to 1.0 mm h-1.
      PubDate: 2017-11-16T14:40:21.988532-05:
      DOI: 10.1002/2017JD026459
  • Diagnosing cloud biases in the GFDL AM3 model with atmospheric
    • Authors: Stuart Evans; Roger Marchand, Thomas Ackerman, Leo Donner, Jean-Christophe Golaz, Charles Seman
      Abstract: We define a set of 21 atmospheric states, or recurring weather patterns, for a region surrounding the ARM Program's Southern Great Plains site using an iterative clustering technique. The states are defined using dynamic and thermodynamic variables from reanalysis, tested for statistical significance with cloud radar data from the SGP site, and are determined every 6 hours for 14 years, creating a time-series of atmospheric state. The states represent the various stages of the progression of synoptic systems through the region (e.g. warm fronts, warm sectors, cold fronts, cold northerly advection, and high-pressure anticyclones) with a subset of states representing summertime conditions with varying degrees of convective activity.We use the states to classify output from the NOAA/GFDL AM3 model to test the model's simulation of the frequency of occurrence of the states and of the cloud occurrence during each state. The model roughly simulates the frequency of occurrence of the states, but exhibits systematic cloud occurrence biases. Comparison of observed and model-simulated ISCCP histograms of cloud top pressure and optical thickness show the model lacks high thin cloud under all conditions, but biases in thick cloud are state-dependent. Frontal conditions in the model do not produce enough thick cloud, while fairweather conditions produce too much. We find that increasing the horizontal resolution of the model improves the representation of thick clouds under all conditions, but has little effect on high thin clouds. However, increasing resolution also changes the distribution of states, causing an increase in total cloud occurrence bias.
      PubDate: 2017-11-16T13:30:24.858755-05:
      DOI: 10.1002/2017JD027163
  • Using Long-term Satellite Observations to Identify Sensitive Regimes and
           Active Regions of Aerosol Indirect Effects for Liquid Clouds over Global
    • Authors: Xuepeng Zhao; Yangang Liu, Fangquan Yu, Andrew K. Heidinger
      Abstract: Long-term (1981-2011) satellite climate data records (CDRs) of clouds and aerosols are used to investigate the aerosol-cloud interaction of marine water cloud from a climatology perspective. Our focus is on identifying the regimes and regions where the aerosol indirect effect (AIE) are evident in long-term averages over the global oceans through analyzing the correlation features between aerosol loading and the key cloud variables including cloud droplet effective radius (CDER), cloud optical depth (COD), cloud water path (CWP), cloud top height (CTH), and cloud top temperature (CTT). An aerosol optical thickness (AOT) range of 0.13 < AOT < 0.3 is identified as the sensitive regime of the conventional first AIE where CDER is more susceptible to AOT than the other cloud variables. The first AIE that manifests as the change of long-term averaged CDER appears only in limited oceanic regions. The signature of aerosol invigoration of water clouds as revealed by the increase of cloud cover fraction (CCF) and CTH with increasing AOT at the middle/high latitudes of both hemispheres is identified for a pristine atmosphere (AOT < 0.08). Aerosol invigoration signature is also revealed by the concurrent increase of CDER, COD, and CWP with increasing AOT for a polluted marine atmosphere (AOT> 0.3) in the tropical convergence zones. The regions where the second AIE is likely to manifest in the CCF change are limited to several oceanic areas with high CCF of the warm water clouds near the western coasts of continents. The second AIE signature as represented by the reduction of the precipitation efficiency with increasing AOT is more likely to be observed in the AOT regime of 0.08 < AOT < 0.4. The corresponding AIE active regions manifested themselves as the decline of the precipitation efficiency are mainly limited to the oceanic areas downwind of continental aerosols. The sensitive regime of the conventional AIE identified in this observational study is likely associated with the transitional regime from the aerosol-limited regime to the updraft-limited regime identified for aerosol-cloud interaction in cloud model simulations.
      PubDate: 2017-11-16T01:47:21.256615-05:
      DOI: 10.1002/2017JD027187
  • Enhanced recent local moisture recycling on the northwestern Tibetan
           Plateau deduced from ice core deuterium excess records
    • Authors: Wenling An; Shugui Hou, Qiong Zhang, Wangbin Zhang, Shuangye Wu, Hao Xu, Hongxi Pang, Yetang Wang, Yaping Liu
      Abstract: Local moisture recycling plays an essential role in maintaining an active hydrological cycle of the Tibetan Plateau (TP). Previous studies were largely limited to the seasonal time scale due to short and sparse observations, especially for the northwestern TP. In this study, we used a two-component mixing model to estimate local moisture recycling over the past decades from the deuterium excess records of two ice cores (i.e., Chongce and Zangser Kangri) from the northwestern TP. The results show that on average almost half of the precipitation on the northwestern TP is provided by local moisture recycling. In addition, the local moisture recycling ratio has increased evidently on the northwestern TP, suggesting an enhanced hydrological cycle. This recent increase could be due to the climatic and environmental changes on the TP in the past decades. Rapid increases in temperature and precipitation have enhanced evaporation. Changes of land surface of plateau have significantly increased evapotranspiration. All of these have intensified local moisture recycling. However, the mixing model used in this study only includes a limited number of climate factors. Some of the extreme values of moisture recycling ratio could be caused by large scale atmospheric circulation and other climatic and weather events. Moreover, the potential mechanisms for the increase in local recycling need to be further examined, since the numeric simulations from climate models did not reproduce the increased contribution of local moisture recycling in precipitation.
      PubDate: 2017-11-16T01:00:30.515038-05:
      DOI: 10.1002/2017JD027235
  • The warming of large lakes on the Tibetan Plateau-evidence from a lake
           model simulation of Nam Co, China, during 1979-2012
    • Authors: Lei Huang; Junbo Wang, Liping Zhu, Jianting Ju, Gerhard Daut
      Abstract: Lakes are considered as indicators of climate change on the Tibetan Plateau (TP). In the present study, we use the General Lake Model (GLM) to simulate water temperature changes in Nam Co, the second largest lake on the central TP, for the period 1979-2012. The calibration and validation results demonstrate that this model is well suited for thermal simulation of Nam Co. The simulation results indicate that Nam Co has responded to the recent warming climate. The average summer surface water temperature fluctuated yearly, but its trend is positive at a rate of 0.52±0.25°C/decade. At the same time, the onset of summer stratification advanced by 4.20±2.02 d/decade, and the duration increased at a rate of 6.00±3.54 d/decade. To explore the roles of air temperature and longwave radiation in lake warming, three sensitivity experiments are conducted by removing long-term trends from time series of air temperature and longwave radiation in the forcing data. These experiments prove that both increased air temperature and downward longwave radiation are two driving factors responsible for the warming of Nam Co.
      PubDate: 2017-11-16T01:00:21.442457-05:
      DOI: 10.1002/2017JD027379
  • Confirmation of elevated methane emissions in Utah's Uintah Basin with
           ground-based observations and a high-resolution transport model
    • Authors: C. S. Foster; E. T. Crosman, L. Holland, D. V. Mallia, B. Fasoli, R. Bares, J. Horel, J. C. Lin
      Abstract: Large CH4 leak rates have been observed in the Uintah Basin of eastern Utah, an area with over 10,000 active and producing natural gas and oil wells. In this paper, we model CH4 concentrations at four sites in the Uintah Basin and compare the simulated results to in situ observations at these sites during two spring time periods in 2015 and 2016. These sites include a baseline location (Fruitland); two sites near oil wells (Roosevelt and Castlepeak); and a site near natural gas wells (Horsepool). To interpret these measurements and relate observed CH4 variations to emissions, we carried out atmospheric simulations using the Stochastic Time-Inverted Lagrangian Transport (STILT) model driven by meteorological fields simulated by the Weather Research and Forecasting (WRF) and High Resolution Rapid Refresh (HRRR) models. These simulations were combined with two different emission inventories: 1) Aircraft-derived basin-wide emissions allocated spatially using oil and gas well locations, from the National Oceanic and Atmospheric Administration (NOAA); 2) A bottom-up inventory for the entire U.S., from the Environmental Protection Agency (EPA). At both Horsepool and Castlepeak, the diurnal cycle of modeled CH4 concentrations was captured using NOAA emission estimates, but was underestimated using the EPA inventory. These findings corroborate emission estimates from the NOAA inventory, based on daytime mass balance estimates, and provide additional support for a suggested leak rate from the Uintah Basin that is higher than most other regions with natural gas and oil development.
      PubDate: 2017-11-15T17:25:25.767019-05:
      DOI: 10.1002/2017JD027480
  • A positive feedback process between tropical cyclone intensity and the
           moisture conveyor belt assessed with Lagrangian diagnostics
    • Authors: Keita Fujiwara; Ryuichi Kawamura, Hidetaka Hirata, Tetsuya Kawano, Masaya Kato, Taro Shinoda
      Abstract: Using a cloud-resolving regional model and Lagrangian diagnostics, we assess a positive feedback process between tropical cyclone (TC) intensity and the moisture conveyor belt (MCB), which connects a TC and the Indian Ocean (IO), the South China Sea (SCS), and the Philippine Sea (PS) vapors, from a macroscopic view. We performed sensitivity experiments that modified the observed sea surface temperature (SST) field over the IO and the SCS to regulate the MCB behavior, and we examined the remote response of a prototypical TC. The results show that the connection between MCB formation and TC development is very robust, which was also observed in another TC's case. The MCB plays a vital role in transporting lots of moist air parcels toward the TC from the IO, SCS, and PS regions. The transported parcels, which further gained the underlying ocean vapor along the MCB, are easily trapped in the inner core by radial inflow in the atmospheric boundary layer and, subsequently, release latent heat around the eyewall, resulting in the TC's intensifying. This acts to further penetrate the moist parcels of remote ocean origin into the inner core through the enhanced and expanded inflow. An additional experiment suggested that the MCB is not formed unless the westward propagation of equatorial waves induced by TC heating overlaps with the background monsoon westerlies. These findings support the reliability and validity of TC-MCB feedback.
      PubDate: 2017-11-15T16:30:27.805975-05:
      DOI: 10.1002/2017JD027557
  • Vertical structure of aerosols and mineral dust over the Bay of Bengal
           from multi-satellite observations
    • Authors: N. B. Lakshmi; Vijayakumar S. Nair, S. Suresh Babu
      Abstract: The vertical distribution of aerosol and dust extinction coefficient over the Bay of Bengal is examined using the satellite observations (Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP) and Moderate Resolution Imaging Spectroradiometer (MODIS)) for the period from 2006 to 2017. Distinct seasonal pattern is observed in the vertical structure of both aerosol and dust over the Bay of Bengal with an enhancement of 24% in the aerosol extinction above 1 km from winter (December, January and February) to pre-monsoon (March, April, and May). Significant contribution of dust is observed over the northern Bay of Bengal during pre-monsoon season where 22% of the total aerosol extinction is contributed by dust aerosols transported from the nearby continental regions. During winter, dust transport is found to be less significant with fractional contribution of ~10% - 13% to the total aerosol optical depth over the Bay of Bengal. MODIS derived dust fraction (fine-mode based) shows an overestimation up to 2 fold compared to CALIOP dust fraction (depolarization based) whereas the GOCART simulated dust fraction underestimates the satellite derived dust fractions over the Bay of Bengal. Though the long term variation in dust aerosol showed a decreasing trend over the Bay of Bengal, the confidence level is insufficient establish the robustness of the observed trend. However, significant dust induced heating is observed above the boundary layer during pre-monsoon season. This dust induced elevated heating can affect the convection over the Bay of Bengal which will have implication on the monsoon dynamics over the Indian region.
      PubDate: 2017-11-15T16:10:36.082045-05:
      DOI: 10.1002/2017JD027643
  • Synoptic conditions and moisture sources actuating extreme precipitation
           in Nepal
    • Authors: Patrik Bohlinger; Asgeir Sorteberg, Harald Sodemann
      Abstract: Despite the vast literature on heavy precipitation events in South Asia, synoptic conditions and moisture sources related to extreme precipitation in Nepal have not been addressed systematically. We investigate two types of synoptic conditions —low pressure systems and mid-level troughs —and moisture sources related to extreme precipitation events. To account for the high spatial variability in rainfall, we cluster station based daily precipitation measurements resulting in three well separated geographic regions: West, Central, and East Nepal. For each region, composite analysis of extreme events shows that atmospheric circulation is directed against the Himalayas during an extreme event. The direction of the flow is regulated by mid-tropospheric troughs and low pressure systems traveling toward the respective region. Extreme precipitation events feature anomalous high abundance of total column moisture. Quantitative Lagrangian moisture source diagnostic reveals that the largest direct contribution stems from land (ca. 75%), where in particular over the Indo-Gangetic Plain moisture uptake was increased. Precipitation events occurring in this region before the extreme event likely provided additional moisture.
      PubDate: 2017-11-14T01:50:41.699176-05:
      DOI: 10.1002/2017JD027543
  • Spatial representativeness of surface-measured variations of downward
           solar radiation
    • Authors: M. Schwarz; D. Folini, M. Z. Hakuba, M. Wild
      Abstract: When using time series of ground based surface solar radiation (SSR) measurements in combination with gridded data the spatial and temporal representativeness of the point observations must be considered. We use SSR data from surface observations and high resolution (0.05∘) satellite derived data to infer the spatiotemporal representativeness of observations for monthly and longer time scales in Europe. The correlation analysis shows that the squared correlation coefficients (R2) between SSR times series decrease linearly with increasing distance between the surface observations. For deseasonalized monthly mean time series R2 ranges from 0.85 for distances up to 25km between the stations, to 0.25 at distances of 500km. A decorrelation length (i.e. the e-folding distance of R2) in the order of 400km (with spread of 100-600km) was found. R2 from correlations between point observations and colocated grid box area means determined from satellite data were found to be 0.80 for a 1∘ grid. To quantify the error which arises when using a point observation as a surrogate for the area mean SSR of a larger surrounding we calculated a spatial sampling error (SSE) for a 1∘ grid of 8(3)W/m2 for monthly (annual) time series. The SSE based on a 1∘ grid, therefore, is of the same magnitude as the measurement uncertainty. The analysis generally reveals that monthly mean (or longer temporally aggregated) point observations of SSR capture the larger scale variability well. This finding shows that comparing time series of SSR measurements with gridded data is feasible for that timescales.
      PubDate: 2017-11-14T01:50:22.845484-05:
      DOI: 10.1002/2017JD027261
  • The impact of nonequilibrium and equilibrium fractionation on two
           different deuterium excess definitions
    • Authors: Marina Dütsch; Stephan Pfahl, Harald Sodemann
      Abstract: The deuterium excess (d) is a useful measure for nonequilibrium effects of isotopic fractionation, and can therefore provide information about the meteorological conditions in evaporation regions, or during ice cloud formation. In addition to nonequilibrium fractionation, two other effects can change d during phase transitions. The first is the dependence of the equilibrium fractionation factors on temperature, the second is the nonlinearity of the δ scale on which d is defined. The second effect can be avoided by using an alternative definition that is based on the logarithmic scale. However, in this case d is not conserved when air parcels mix, which can lead to changes without phase transitions. Here we provide a systematic analysis of the benefits and limitations of both deuterium excess definitions by separately quantifying the impact of the nonequilibrium effect, the temperature effect, the δ scale effect, and the mixing effect in a simple Rayleigh model simulating the isotopic composition of air parcels during moist adiabatic ascent. The δ scale effect is important in depleted air parcels, for which it can change the sign of the traditional deuterium excess in the remaining vapor from negative to positive. The alternative definition mainly reflects the nonequilibrium and temperature effect, while the mixing effect is about two orders of magnitude smaller. Thus, the alternative deuterium excess definition appears to be a more accurate measure for nonequilibrium effects in situations where moisture is depleted and the δ scale effect is large, for instance at high latitudes or altitudes.
      PubDate: 2017-11-13T08:20:21.547117-05:
      DOI: 10.1002/2017JD027085
  • Analysis of the diurnal variation of the global electric circuit obtained
           from different numerical models
    • Authors: Jaroslav Jánský; Greg M. Lucas, Christina Kalb, Victor Bayona, Michael J. Peterson, Wiebke Deierling, Natasha Flyer, Victor P. Pasko
      Abstract: This work analyzes different current source and conductivity parameterizations and their influence on the diurnal variation of the global electric circuit (GEC). The diurnal variations of the current source parameterizations obtained using electric field and conductivity measurements from plane overflights combined with global TRMM satellite data give generally good agreement with measured diurnal variation of the electric field at Vostok, Antarctica, where reference experimental measurements are performed. An approach employing 85-GHz passive microwave observations to infer currents within the GEC is compared and shows the best agreement in amplitude and phase with experimental measurements. To study the conductivity influence, GEC models solving the continuity equation in 3-D are used to calculate atmospheric resistance using yearly averaged conductivity obtained from the global circulation model CESM. Then, using current source parameterization combining mean currents and global counts of electrified clouds, if the exponential conductivity is substituted by the conductivity from CESM, the peak to peak diurnal variation of the ionospheric potential of the GEC decreases from 24% to 20%. The main reason for the change is the presence of clouds while effects of 222Rn ionization, aerosols and topography are less pronounced. The simulated peak to peak diurnal variation of the electric field at Vostok is increased from 15% to 18% from the diurnal variation of the global current in the GEC if conductivity from CESM is used.
      PubDate: 2017-11-13T04:47:33.641883-05:
      DOI: 10.1002/2017JD026515
  • An Analysis of Total Lightning Flash Rates Over Florida
    • Authors: Thomas O. Mazzetti; Henry E. Fuelberg
      Abstract: Although Florida is known as the "Sunshine State", it also contains the greatest lightning flash densities in the United States. Flash density has received considerable attention in the literature, but lightning flash rate has received much less attention. We use data from the Earth Networks Total Lightning Network (ENTLN) to produce a 5 yr (2010-2014) set of statistics regarding total flash rates over Florida and adjacent regions. Instead of tracking individual storms, we superimpose a 0.2o × 0.2o grid over the study region and count both cloud-to-ground (CG) and in-cloud (IC) flashes over 5-min intervals. Results show that the distribution of total flash rates is highly skewed toward small values, whereas the greatest rate is ~185 flashes min-1. Greatest average annual flash rates (~3 flashes min-1) are located near Orlando. The southernmost peninsula, North Florida, and the Florida Panhandle exhibit smaller average annual flash rates (~1.5 flashes min-1) . Large flash rates>~ 100 flashes min-1 can occur during any season, at any time during the 24 h period, and at any location within the domain. However, they are most likely during the afternoon and early evening in East Central Florida during the spring and summer months.
      PubDate: 2017-11-13T00:10:27.066894-05:
      DOI: 10.1002/2017JD027579
  • Developing an a priori Database for Passive Microwave Snow Water
           Retrievals over Ocean
    • Authors: Mengtao Yin; Guosheng Liu
      Abstract: A physically optimized a priori database is developed for Global Precipitation Measurement Microwave Imager (GMI) snow water retrievals over ocean. The initial snow water content profiles are derived from CloudSat Cloud Profiling Radar (CPR) measurements. A radiative transfer model in which the single scattering properties of nonspherical snowflakes are based on the discrete dipole approximate (DDA) results is employed to simulate brightness temperatures and their gradients. Snow water content profiles are then optimized through a one–dimensional variational (1D–Var) method. The standard deviations of the difference between observed and simulated brightness temperatures are in a similar magnitude to the observation errors defined for observation error covariance matrix after the 1D–Var optimization, indicating that this variational method is successful. This optimized database is applied in a Bayesian retrieval snow water algorithm. The retrieval results indicated that the 1D–Var approach has a positive impact on the GMI retrieved snow water content profiles by improving the physical consistency between snow water content profiles and observed brightness temperatures. Global distribution of snow water contents retrieved from the a priori database is compared with CloudSat CPR estimates. Results showed that the two estimates have a similar pattern of global distribution, and the difference of their global means is small. In addition, we investigate the impact of using physical parameters to subset the database on snow water retrievals. It is shown that using total precipitable water to subset the database with 1D–Var optimization is beneficial for snow water retrievals.
      PubDate: 2017-11-12T23:40:25.258463-05:
      DOI: 10.1002/2017JD027636
  • Seasonality of Formic Acid (HCOOH) in London during the ClearfLo Campaign
    • Authors: Thomas J. Bannan; A. Murray Booth, Michael Le Breton, Asan Bacak, Jennifer B. A. Muller, Kimberley E. Leather, M. Anwar H. Khan, James D. Lee, Rachel E. Dunmore, James R. Hopkins, Zoë L. Fleming, Leonid Sheps, Craig A. Taatjes, Dudley E. Shallcross, Carl J. Percival
      Abstract: Following measurements in the winter of 2012, formic acid (HCOOH) and nitric acid (HNO3) were measured using a chemical ionisation mass spectrometer (CIMS) during the Summer Clean Air for London (ClearfLo) campaign in London, 2012. Consequently, the seasonal dependence of formic acid sources could be better understood. A mean formic acid concentration of 1.3 ppb and a maximum of 12.7 ppb was measured which is significantly greater than that measured during the winter campaign (0.63 ppb and 6.7 ppb respectively). Daily calibrations of formic acid during the summer campaign gave sensitivities of 1.2 ion counts s−1 pptv−1 and a limit of detection of 34 ppt. During the summer campaign, there was no correlation between formic acid and anthropogenic emissions such as NOx and CO or peaks associated with the rush hour as was identified in the winter. Rather, peaks in formic acid were observed that correlated with solar irradiance. Analysis using a photochemical trajectory model (PTM) has been conducted to determine the source of this formic acid. The contribution of formic acid formation through ozonolysis of alkenes is important but the secondary production from biogenic VOCs could be the most dominant source of formic acid at this measurement site during the summer.
      PubDate: 2017-11-09T11:05:38.707668-05:
      DOI: 10.1002/2017JD027064
  • A Case Study in Low Aerosol Number Concentrations over the Eastern North
           Atlantic: Implications for Pristine Conditions in the Remote Marine
           Boundary Layer
    • Authors: Sam Pennypacker; Robert Wood
      Abstract: We present a case study (September 20 – October 13, 2015) of synergistic, multi-instrument observations of aerosols, clouds and the marine boundary layer (MBL) at the Eastern North Atlantic (ENA) ARM site centered on a period of exceptionally low (20 – 50 cm-3) surface accumulation mode (0.1 – 1 μm) aerosol particle number concentrations. We divide the case study into three regimes (high, clean and ultra-clean) based on daily median number concentrations, and compare finer resolution (hourly or less) observations between these regimes. The analysis focuses on the possibility of using these ultra-clean events to study pristine conditions in the remote MBL, as well as examining evidence for a recently proposed conceptual model for the large-scale depletion of CCN-sized particles in post-frontal air masses. Relative to the high and clean regimes, the ultra-clean regime tends to exhibit significantly fewer particles between 0.1 and 0.4 μm in diameter and a relatively increased prevalence of larger accumulation mode particles. In addition, supermicron particles tend to dominate total scattering in the ultra-clean regime, and there is little evidence for absorbing aerosol. These observations are more in line with a heavily scavenged but natural marine aerosol population and minimal contribution from continental sources such as anthropogenic pollution, biomass burning or dust. The air masses with the consistently lowest accumulation mode aerosol number concentrations are largely dominated by heavily drizzling clouds with high liquid water path (LWP) cores, deep decoupled boundary layers, open cellular organization and notable surface forcing of sub-cloud turbulence, even at night.
      PubDate: 2017-11-09T10:35:40.579668-05:
      DOI: 10.1002/2017JD027493
  • Simulated historical (1901–2010) changes in the permafrost extent and
           active layer thickness in the Northern Hemisphere
    • Authors: Donglin Guo; Huijun Wang
      Abstract: A growing body of simulation research has considered the dynamics of permafrost, which has an important role in the climate system of a warming world. Previous studies have concentrated on the future degradation of permafrost based on global climate models (GCMs) or data from GCMs. An accurate estimation of historical changes in permafrost is required to understand the relations between changes in permafrost and the Earth's climate and to validate the results from GCMs. Using the Community Land Model 4.5 driven by the CRUNCEP atmospheric dataset and observations of changes in soil temperature and active layer thickness and present day areal extent of permafrost, this study investigated the changes in permafrost in the Northern Hemisphere from 1901 to 2010. The results showed that the model can reproduce the inter-annual variations in the observed soil temperature and active layer thickness. The simulated area of present day permafrost fits well with observations, with a bias of 2.02 × 106 km2. The area of permafrost decreased by 0.06 (0.62) × 106 km2 decade−1 from 1901 to 2009 (1979 to 2009). A clear decrease in the area of permafrost was found in response to increases in air temperatures during the period from about the 1930s to the 1940s, indicating that permafrost is sensitive to even a temporary increase in temperature. From a regional perspective, high-elevation permafrost decreases at a faster rate than high-latitude permafrost; permafrost in China shows the fastest rate of decrease, followed by Alaska, Russia, and Canada. Discrepancies in the rate of decrease in the extent of permafrost among different regions were linked to the sensitivity of permafrost in the regions to increases in air temperatures rather than to the amplitude of the increase in air temperatures. An increase in the active layer thickness of 0.009 (0.071) m decade−1 was shown during the period 1901–2009 (1979–2009). These results are useful in understanding the response of permafrost to a historical warming climate and for validating the results from GCMs.
      PubDate: 2017-11-08T12:20:20.142306-05:
      DOI: 10.1002/2017JD027691
  • OMI satellite and ground-based Pandora observations and their application
           to surface NO2 estimations at terrestrial and marine sites
    • Authors: Debra E. Kollonige; Anne M. Thompson, Miroslav Josipovic, Maria Tzortziou, Johan P. Beukes, Roelof Burger, Douglas K. Martins, Pieter G. Zyl, Ville Vakkari, Lauri Laakso
      Abstract: The Pandora spectrometer that uses direct-sun measurements to derive total column amounts of gases provides an approach for (1) validation of satellite instruments and (2) monitoring of total column (TC) ozone (O3) and nitrogen dioxide (NO2). We use for the first time Pandora and OMI observations to estimate surface NO2 over marine and terrestrial sites downwind of urban pollution and compared with in situ measurements during campaigns in contrasting regions: (1) the South African Highveld (at Welgegund, 26°34'10"S, 26°56'21"E, 1480 m asl, ~120 km south-west of the Johannesburg-Pretoria megacity); (2) shipboard US mid-Atlantic coast during the 2014 Deposition of Atmospheric Nitrogen to Coastal Ecosystems (DANCE) cruise. In both cases there were no local NOx sources, but intermittent regional pollution influences. For TC NO2, OMI and Pandora difference is ~ 20% with Pandora higher most times. Surface NO2 values estimated from OMI and Pandora columns are compared to in situ NO2 for both locations. For Welgegund, the planetary boundary layer (PBL) height, used in converting column to surface NO2 value, has been estimated by three methods: co-located Atmospheric InfraRed Sounder (AIRS) observations; a model simulation; radiosonde data from Irene, 150 km northeast of the site. AIRS PBL heights agree within 10% of radiosonde-derived values. Absolute differences between Pandora- and OMI-estimated surface NO2 and the in situ data are better at the terrestrial site (~0.5ppbv and ~1 ppbv or greater, respectively) than under clean marine air conditions, with differences usually>3 ppbv. Cloud cover and PBL variability can influence these estimations.
      PubDate: 2017-11-08T11:25:26.528542-05:
      DOI: 10.1002/2017JD026518
  • Concentration and viability of airborne bacteria over the Kuroshio
           Extension region in the northwestern Pacific Ocean: data from three
    • Authors: Wei Hu; Kotaro Murata, Shinichiro Fukuyama, Yoshimi Kawai, Eitarou Oka, Mitsuo Uematsu, Daizhou Zhang
      Abstract: Airborne bacteria have been shown to act as condensation and ice nuclei in mixed-phase clouds and are consequently hypothesized to have significant effects on atmospheric processes and even the global climate. However, few data are available regarding their concentration and variation in the air over the open ocean. Aerosol samples were collected during three cruises in the early summers of 2013, 2014 and 2016 over the Kuroshio Extension region of the northwest Pacific Ocean. The concentrations of viable and non-viable bacterial cells in the marine surface air were quantified using epifluorescence enumeration with the LIVE/DEAD BacLight stain. The concentrations of total bacteria varied between 1.0×104 and 2.5×105 cells m−3 and averaged 5.2×104, 1.0×105, and 7.5×104 cells m−3 in the three respective cruises. The viabilities, i.e., the ratios of the concentration of viable bacterial cells to that of total bacterial cells, ranged from 80% to 100% (average 93%), and the respective means were 93%, 89%, and 96% in the cruises. The total bacterial concentration had a close correlation with the wind speed near the sea surface, and the bacterial viability correlated negatively with the air temperature, sea surface temperature and concentration of coarse particles (size>1 μm). The deposition and sea spray fluxes of bacteria were roughly estimated as hundreds of cells m−2 s−1 on average. The limited data on bacterial concentration and viability from the three cruises indicate the rapid air-sea exchange of bacteria over the Kuroshio Extension region of the northwest Pacific Ocean.
      PubDate: 2017-11-08T11:00:28.713943-05:
      DOI: 10.1002/2017JD027287
  • Sensitivity of historical simulation of the permafrost to different
           atmospheric forcing datasets from 1979 to 2009
    • Authors: Donglin Guo; Huijun Wang, Aihui Wang
      Abstract: Numerical simulation is of great importance to the investigation of changes in frozen ground on large spatial and long temporal scales. Previous studies have focused on the impacts of improvements in the model for the simulation of frozen ground. Here, the sensitivities of permafrost simulation to different atmospheric forcing datasets are examined using the Community Land Model, version 4.5 (CLM4.5), in combination with three sets of newly developed and reanalysis-based atmospheric forcing datasets (CFSR, ERA-I, and MERRA). All three simulations were run from 1979 to 2009 at a resolution of 0.5° × 0.5°, and validated with what is considered to be the best available permafrost observations (soil temperature, active layer thickness and permafrost extent). Results show that the use of reanalysis-based atmospheric forcing dataset reproduce the variations in soil temperature and active layer thickness, but produce evident biases in their climatologies. Overall, the simulations based on the CFSR and ERA-I datasets give more reasonable results than the simulation based on the MERRA dataset, particularly for the present-day permafrost extent and the change in active layer thickness. The three simulations produce ranges for the present-day climatology (permafrost area: 11.31–13.57 × 106 km2; active layer thickness: 1.10–1.26 m) and for recent changes (permafrost area: −5.8% to −9.0%; active layer thickness: 9.9%–20.2%). The differences in air temperature increase, snow depth, and permafrost thermal conditions in these simulations contribute to the differences in simulated results.
      PubDate: 2017-11-08T10:45:20.21416-05:0
      DOI: 10.1002/2017JD027477
  • A Systematic Evaluation of Noah-MP in Simulating Land-Atmosphere Energy,
           Water and Carbon Exchanges over the Continental United States
    • Authors: Ning Ma; Guo-Yue Niu, Youlong Xia, Xitian Cai, Yinsheng Zhang, Yaoming Ma, Yuanhao Fang
      Abstract: Accurate simulation of energy, water, and carbon fluxes exchanging between the land surface and the atmosphere is beneficial for improving terrestrial ecohydrological and climate predictions. We systematically assessed the Noah land surface model (LSM) with mutiparameterization options (Noah-MP) in simulating these fluxes and associated variations in terrestrial water storage (TWS) as well as snow cover fraction (SCF) against various reference products over 18 USGS two-digital hydrological unit code regions of the continental United States (CONUS). In general, Noah-MP captures better the observed seasonal and inter-region variability of net radiation, SCF, and runoff than other variables. With a dynamic vegetation model, it overestimates gross primary productivity by 40% and evapotranspiration (ET) by 22% over the whole CONUS domain; however, with a prescribed climatology of leaf area index, it greatly improves ET simulation with relative bias dropping to 4%. It accurately simulates regional TWS dynamics in most regions except those with large lakes or severely affected by irrigation and/or impoundments. Incorporating the lake water storage variations into the modeled TWS variations largely reduces the TWS simulation bias more obviously over the Great Lakes with model efficiency increasing from 0.18 to 0.76. Noah-MP simulates runoff well in most regions except an obvious overestimation (underestimation) in the Rio Grande and Lower Colorado (New England). Compared with NLDAS-2 LSMs, Noah-MP shows a better ability to simulate runoff and a comparable skill in simulating Rn, but a worse skill in simulating ET over most regions. This study suggests that future model developments should focus on improving the representations of vegetation dynamics, lake water storage dynamics, and human activities including irrigation and impoundments.
      PubDate: 2017-11-08T10:30:37.853108-05:
      DOI: 10.1002/2017JD027597
  • In-flight observation of gamma-ray glows by ILDAS
    • Authors: P. Kochkin; A. P. J. van Deursen, M. Marisaldi, A. Ursi, A. I. de Boer, M. Bardet, C. Allasia, J.-F. Boissin, F. Flourens, N. Østgaard
      Abstract: An Airbus A340 aircraft flew over Northern Australia with the In-Flight Lightning Damage Assessment System (ILDAS) installed on-board. A long-duration gamma-ray emission was detected. The most intense emission was observed at 12 km altitude and lasted for 20 s. Its intensity was 20 times the background counts and it was abruptly terminated by a distant lightning flash. In this work we reconstruct the aircraft path and event timeline. The glow terminating flash triggered a discharge from the aircraft wing that was recorded by a video camera operating on-board. Another count rate increase was observed 6 min later and lasted for 30 s. The lightning activity as reported by ground networks in this region was analyzed. The measured spectra characteristics of the emission were estimated.
      PubDate: 2017-11-08T02:56:27.718761-05:
      DOI: 10.1002/2017JD027405
  • A dynamical explanation of the topographically bound easterly low-level
           jet surrounding Antarctica
    • Authors: Scott R. Fulton; Wayne H. Schubert, Zhengqing Chen, Paul E. Ciesielski
      Abstract: This study investigates the topographically bound easterly low-level jet surrounding Antarctica. This jet is modeled as a balanced flow that satisfies the potential vorticity invertibility principle, based on local linear balance in spherical coordinates and expressed in isentropic coordinates. In this way, this easterly low-level jet is shown to be the balanced flow associated with the topography of the Antarctic plateau, moderated by a shallow potential vorticity anomaly atop the plateau produced by radiative cooling. The dynamical connection of the jet with katabatic winds can be understood through the meridional circulation equation. Model results based on the simple theoretical arguments developed here are found to be consistent with high resolution reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF) for the 2008–2010 period.
      PubDate: 2017-11-07T09:06:34.630531-05:
      DOI: 10.1002/2017JD027192
  • The climate response to stratospheric aerosol geoengineering can be
           tailored using multiple injection locations
    • Authors: Douglas G. MacMartin; Ben Kravitz, Simone Tilmes, Jadwiga H. Richter, Michael J. Mills, Jean-Francois Lamarque, Joseph J. Tribbia, Francis Vitt
      Abstract: By injecting different amounts of SO2 at multiple different latitudes, the spatial pattern of aerosol optical depth (AOD) can be partially controlled. This leads to the ability to influence the climate response to geoengineering with stratospheric aerosols, providing the potential for design. We use simulations from the fully-coupled whole-atmosphere chemistry-climate model CESM1(WACCM) to demonstrate that by appropriately combining injection at just four different locations, 30° S, 15° S, 15° N, and 30° N, then three spatial degrees of freedom of AOD can be achieved: an approximately spatially-uniform AOD distribution, the relative difference in AOD between Northern and Southern hemispheres, and the relative AOD in high versus low latitudes. For forcing levels that yield 1–2° C cooling, the AOD and surface temperature response are sufficiently linear in this model so that the response to different combinations of injection at different latitudes can be estimated from single-latitude injection simulations; nonlinearities associated with both aerosol growth and changes to stratospheric circulation will be increasingly important at higher forcing levels. Optimized injection at multiple locations is predicted to improve compensation of CO2-forced climate change relative to a case using only equatorial aerosol injection (which overcools the tropics relative to high latitudes). The additional degrees of freedom can be used, for example, to balance the interhemispheric temperature gradient and the equator to pole temperature gradient in addition to the global mean temperature.Further research is needed to better quantify the impacts of these strategies on changes to long-term temperature, precipitation, and other climate parameters.
      PubDate: 2017-11-06T09:50:05.173243-05:
      DOI: 10.1002/2017JD026868
  • Radiative and chemical response to interactive stratospheric sulfate
           aerosols in fully coupled CESM1(WACCM)
    • Authors: Michael J. Mills; Jadwiga H. Richter, Simone Tilmes, Ben Kravitz, Douglas G. MacMartin, Anne A. Glanville, Joseph J. Tribbia, Jean-François Lamarque, Francis Vitt, Anja Schmidt, Andrew Gettelman, Cecile Hannay, Julio T. Bacmeister, Douglas E. Kinnison
      Abstract: We present new insights into the evolution and interactions of stratospheric aerosol using an updated version of the Whole Atmosphere Community Climate Model (WACCM). Improved horizontal resolution, dynamics, and chemistry now produce an internally generated quasi-biennial oscillation, and significant improvements to stratospheric temperatures and ozone compared to observations. We present a validation of WACCM column ozone and climate calculations against observations. The prognostic treatment of stratospheric sulfate aerosols accurately represents the evolution of stratospheric aerosol optical depth and perturbations to solar and longwave radiation following the June 1991 eruption of Mt. Pinatubo. We confirm the inclusion of interactive OH chemistry as an important factor in the formation and initial distribution of aerosol following large inputs of sulfur dioxide (SO2) to the stratosphere. We calculate that depletion of OH levels within the dense SO2 cloud in the first weeks following the Pinatubo eruption significantly prolonged the average initial e-folding decay time for SO2 oxidation to 47 days. Previous observational and model studies showing a 30-day decay time have not accounted for the large (30-55%) losses of SO2 on ash and ice within 7-9 days post-eruption, and have not correctly accounted for OH depletion. We examine the variability of aerosol evolution in free-running climate simulations due to meteorology, with comparison to simulations nudged with specified dynamics. We assess calculated impacts of volcanic aerosols on ozone loss with comparisons to observations. The completeness of the chemistry, dynamics, and aerosol microphysics in WACCM qualify it for studies of stratospheric sulfate aerosol geoengineering.
      PubDate: 2017-11-06T09:50:04.335181-05:
      DOI: 10.1002/2017JD027006
  • First simulations of designing stratospheric sulfate aerosol
           geoengineering to meet multiple simultaneous climate objectives
    • Authors: Ben Kravitz; Douglas G. MacMartin, Michael J. Mills, Jadwiga H. Richter, Simone Tilmes, Jean-Francois Lamarque, Joseph J. Tribbia, Francis Vitt
      Abstract: We describe the first simulations of stratospheric sulfate aerosol geoengineering using multiple injection locations to meet multiple simultaneous surface temperature objectives. Simulations were performed using CESM1(WACCM), a coupled atmosphere-ocean general circulation model with fully interactive stratospheric chemistry, dynamics (including an internally generated quasi-biennial oscillation), and a sophisticated treatment of sulfate aerosol formation, microphysical growth, and deposition. The objectives are defined as maintaining three temperature features at their 2020 levels against a background of the RCP8.5 scenario over the period 2020-2099. These objectives are met using a feedback mechanism in which the rate of sulfur dioxide injection at each of the four locations is adjusted independently every year of simulation. Even in the presence of uncertainties, nonlinearities, and variability, the objectives are met, predominantly by SO2 injection at 30° N and 30° S. By the last year of simulation, the feedback algorithm calls for a total injection rate of 51 Tg SO2 per year. The injections are not in the tropics, which results in a greater degree of linearity of the surface climate response with injection amount than has been found in many previous studies using injection at the equator. Because the objectives are defined in terms of annual mean temperature, the required geongineering results in “overcooling” during summer and “undercooling” during winter. The hydrological cycle is also suppressed as compared to the reference values corresponding to the year 2020. The demonstration we describe in this study is an important step toward understanding what geoengineering can do and what it cannot do.
      PubDate: 2017-11-06T09:50:03.501028-05:
      DOI: 10.1002/2017JD026874
  • Stratospheric Dynamical Response and Ozone Feedbacks in the Presence of
           SO2 Injections
    • Authors: Jadwiga H. Richter; Simone Tilmes, Michael J. Mills, Joseph J. Tribbia, Ben Kravitz, Douglas G. MacMartin, Francis Vitt, Jean-Francois Lamarque
      Abstract: Injections of sulfur dioxide into the stratosphere are among several proposed methods of solar radiation management. Such injections could cool the Earth's climate. However, they would significantly alter the dynamics of the stratosphere. We explore here the stratospheric dynamical response to sulfur dioxide injections ∼ 5 km above the tropopause at multiple latitudes (equator, 15° S, 15° N, 30° S and 30° N) using a fully coupled Earth system model, Community Earth System Model, version 1, with the Whole Atmosphere Community Climate Model as its atmospheric component (CESM1(WACCM)). We find that in all simulations, the tropical lower stratosphere warms primarily between 30° S and 30° N, regardless of injection latitude. The quasi-biennial oscillation (QBO) of the tropical zonal wind is altered by the various sulfur dioxide injections. In a simulation with a 12 Tg yr−1 equatorial injection, and with fully interactive chemistry, the QBO period lengthens to ∼ 3.5 years, but never completely disappears. However, in a simulation with specified (or non-interactive) chemical fields, including O3 and prescribed aerosols taken from the interactive simulation, the oscillation is virtually lost. In addition we find that geoengineering does not always lengthen the QBO. We further demonstrate that the QBO period changes from 24 to 12 - 17 months in simulations with sulfur dioxide injections placed poleward of the equator. Our study points to the importance of understanding and verifying of the complex interactions between aerosols, atmospheric dynamics, and atmospheric chemistry as well as understanding the effects of sulfur dioxide injections placed away from the Equator on the QBO.
      PubDate: 2017-11-06T09:50:02.774564-05:
      DOI: 10.1002/2017JD026912
  • Sensitivity of aerosol distribution and climate response to stratospheric
           SO2 injection locations
    • Authors: Simone Tilmes; Jadwiga H. Richter, Michael J. Mills, Ben Kravitz, Douglas G. MacMartin, Francis Vitt, Joseph J. Tribbia, Jean-Francois Lamarque
      Abstract: Injection of SO2 into the stratosphere has been proposed as a method to in part counteract anthropogenic climate change. So far, most studies investigated injections at the equator or in a region in the tropics. Here, we use CESM1(WACCM) to explore the impact of continuous single grid point SO2 injections at 7 different latitudes and 2 altitudes in the stratosphere on aerosol distribution and climate. For each of the 14 locations, 3 different constant SO2 emission rates were tested to identify linearity in aerosol burden, aerosol optical depth, and climate effects. We found that injections at 15° N and 15° S and at 25 km altitude have equal or greater effect on radiation and surface temperature than injections at the equator. Non-equatorial injections transport SO2 and sulfate aerosols more efficiently into middle and high latitudes and result in particles of smaller effective radius and larger aerosol burden in middle and high latitudes. Injections at 15° S produce the largest increase in global average aerosol optical depth, and increase the change in radiative forcing per Tg SO2/yr by about 15% compared to equatorial injections. High altitude injections at 15° N produce the largest reduction in global average temperature of 0.2 degrees per Tg S/yr for the last 7 years of a ten year experiment. Injections at higher altitude are generally more efficient at reducing surface temperature, with the exception of large equatorial injections of at least 12 Tg SO2/yr. These findings have important implications for designing a strategy to counteract global climate change.
      PubDate: 2017-11-06T09:50:01.830357-05:
      DOI: 10.1002/2017JD026888
  • Creating Aerosol Types from CHemistry (CATCH): a new algorithm to extend
           the link between remote sensing and models
    • Authors: K. W. Dawson; N. Meskhidze, S. P. Burton, M. S. Johnson, M. S. Kacenelenbogen, C. A. Hostetler, Y. Hu
      Abstract: Current remote sensing methods can identify aerosol types within an atmospheric column, presenting an opportunity to incrementally bridge the gap between remote sensing and models. Here, a new algorithm was designed for Creating Aerosol Types from CHemistry (CATCH). CATCH derived aerosol types – dusty mix, maritime, urban, smoke and fresh smoke – are based on High Spectral Resolution Lidar (HSRL-1) retrievals during the Ship-Aircraft Bio-Optical Research (SABOR) campaign, July/August 2014. CATCH is designed to derive aerosol types from model output of chemical composition. CATCH derived aerosol types are determined by multivariate clustering of model-calculated variables that have been trained using retrievals of aerosol types from HSRL-1. CATCH-derived aerosol types (with the exception of smoke) compare well with HSRL-1 retrievals during SABOR with an average difference in AOD < 0.03. Data analysis shows that episodic free tropospheric transport of smoke is under-predicted by GEOS-Chem. Spatial distributions of CATCH-derived aerosol types for the North American model domain during July/August 2014 show that aerosol type-specific AOD values occurred over representative locations: urban over areas with large population, maritime over oceans, smoke and fresh smoke over typical biomass burning regions. This study demonstrates that model-generated information on aerosol chemical composition can be translated into aerosol types analogous to those retrieved from remote sensing methods. In the future, spaceborne HSRL-1 and CATCH can be used to gain insight into chemical composition of aerosol types, reducing uncertainties in estimates of aerosol radiative forcing.
      PubDate: 2017-11-06T09:27:14.747927-05:
      DOI: 10.1002/2017JD026913
  • Examining the impact of smoke on frontal clouds and precipitation during
           the 2002 Yakutsk wildfires using the WRF-Chem-SMOKE model and satellite
    • Authors: Zheng Lu; Irina N. Sokolik
      Abstract: In 2002, an enormous amount of smoke has been emitted from Yakutsk wildfires. In this study, we examine the impact of smoke on cloud properties and precipitation associated with frontal systems using the WRF-Chem-SMOKE model and satellite data. The smoke emissions are computed using the fire radiative power technique. Smoke particles are represented as an internal mixture of OM, BC and other inorganic matter, and their microphysical and radiative effects are explicitly modeled. After examining the fire activities, we identified two fire periods (FP1 and FP2). During FP1, in the cloud deck with the high cloud droplet number concentration (CDNC), but the relatively small amount of ice nuclei (IN), the rain and snow water contents (RWC and SWC) were strongly reduced, because of suppressed collision-coalescence and riming processes. The cloud cells acquired the longer lifetime and traveled further downwind. During FP2, in the cloud deck with relatively high CDNC and IN, RWC was reduced; however, the large amounts of IN triggered the glaciation indirect effect, and leaded to increased SWC. Due to the competing effects of CDNC and IN, changes in the cloud lifetime were small. Consequently, smoke-induced changes in the total cloudiness cause a dipole feature. After the smoke was nearly consumed during FP1, the large-scale dynamics of the frontal system was altered by smoke. The onset of the precipitation was delayed by one day. In FP2, the onset of the precipitation was not delayed, but occurred at different locations, and the area-averaged precipitation was slightly reduced (~0.5 mm/day).
      PubDate: 2017-11-06T08:51:12.141998-05:
      DOI: 10.1002/2017JD027001
  • Global land surface temperature from the Along-Track Scanning Radiometers
    • Authors: D. J. Ghent; G. K. Corlett, F.-M. Göttsche, J. J. Remedios
      Abstract: The Leicester ATSR and SLSTR Processor for LAnd Surface Temperature (LASPLAST) provides global land surface temperature (LST) products from thermal infra-red radiance data. In this paper, the state-of-the-art version of LASPLAST, as deployed in the GlobTemperature project, is described and applied to data from the Advanced Along-Track Scanning Radiometer (AATSR). The LASPLAST retrieval formulation for LST is a nadir-only, two channel, split-window algorithm, based on biome classification, fractional vegetation and across-track water vapor dependences. It incorporates globally robust retrieval coefficients derived using highly sampled atmosphere profiles. LASPLAST benefits from appropriate spatial resolution auxiliary information and a new probabilistic based cloud flagging algorithm. For the first time for a satellite-derived LST product, pixel-level uncertainties characterized in terms of random, locally correlated, and systematic components, are provided. The new GlobTemperature GT_ATS_2P Version 1.0 product has been validated for one year of AATSR data (2009) against in situ measurements acquired from “gold standard reference” stations: Gobabeb, Namibia and Evora, Portugal; seven SURFRAD stations and the ARM station at Southern Great Plains. These data show average absolute biases for the GT_ATS_2P Version 1.0 product of 1.00 K in the daytime and 1.08 K in the night-time. The improvements in data provenance including better accuracy, fully traceable retrieval coefficients, quantified uncertainty and more detailed information in the new harmonized format of the GT_ATS_2P product will allow for more significant exploitation of the historical LST data record from the ATSRs and a valuable near real-time service from the Sea and Land Surface Temperature Radiometers (SLSTRs).
      PubDate: 2017-11-03T20:30:31.966717-05:
      DOI: 10.1002/2017JD027161
  • Retrievals of the Far Infrared surface emissivity over the Greenland
           Plateau using the Tropospheric Airborne Fourier Transform Spectrometer
    • Authors: Christophe Bellisario; Helen E. Brindley, Jonathan E. Murray, Alan Last, Juliet Pickering, R. Chawn Harlow, Stuart Fox, Cathryn Fox, Stuart M. Newman, Maureen Smith, Doug Anderson, Xianglei Huang, Xiuhong Chen
      Abstract: The Tropospheric Airborne Fourier Transform Spectrometer (TAFTS) measured near surface upwelling and downwelling radiances within the far infrared (FIR) over Greenland during two flights in March 2015. Here we exploit observations from one of these flights to provide in-situ estimates of FIR surface emissivity, encompassing the range 80-535 cm-1. The flight campaign and instrumental set-up is described as well as the retrieval method, including the quality control performed on the observations. The combination of measurement and atmospheric profile uncertainties means that the retrieved surface emissivity has the smallest estimated error over the range 360-535 cm-1, (18.7-27.8 μm), lying between 0.89 and 1 with an associated error which is of the order ± 0.06. Between 80 and 360 cm-1, the increasing opacity of the atmosphere, coupled with the uncertainty in the atmospheric state, means that the associated errors are larger and the emissivity values cannot be said to be distinct from 1. These FIR surface emissivity values are, to the best of our knowledge, the first ever from aircraft-based measurements. We have compared them to a recently developed theoretical database designed to predict the infrared surface emissivity of frozen surfaces. When considering the FIR alone, we are able to match the retrievals within uncertainties. However, when we include contemporaneous retrievals from the mid infrared (MIR), no single theoretical representation is able to capture the FIR and MIR behaviour simultaneously. Our results point towards the need for model improvement and further testing, ideally including in-situ characterisation of the underlying surface conditions.
      PubDate: 2017-11-03T20:15:35.565948-05:
      DOI: 10.1002/2017JD027328
  • Tall tower vertical profiles and diurnal trends of ammonia in the Colorado
           Front Range
    • Authors: A. G. Tevlin; Y. Li, J. L. Collett, E. E. McDuffie, E. V. Fischer, J. G. Murphy
      Abstract: Ammonia (NH3) mixing ratios were measured between the surface and 280 m above ground level from a moveable carriage at the Boulder Atmospheric Observatory (BAO) tower in summer 2014 as part of the Front Range Air Pollution and Photochemistry Éxperiment (FRAPPÉ). The campaign median mixing ratio was 3.3 ppb, ranging from below detection limits to 192 ppb. Median vertical profiles show an overall increase in NH3 mixing ratios towards the surface of 6.7 ppb (89 %) during the day, and 3.9 ppb (141%) at night. In contrast to the overall increasing trend, some individual profiles show decreasing NH3 in the lowest 10 m. This suggests that the local surface is capable of acting as either a source or a sink, depending on the relative amounts of NH3 at the surface, and in advected air parcels. We further use this dataset to investigate the variation in diurnal patterns of NH3 as a function of height above the surface. At higher altitudes (100 ± 5 and 280 ± 5 m), NH3 mixing ratios reach a gradual maximum during the day between 9:00 and 16:00 local time, likely driven by changes in source region. At lower altitudes (10 ± 5 m), the daytime maximum begins earlier at about 7:00 local time, followed by a sharper increase at 9:00 local time. At this height we also observe a peak in NH3 mixing ratios during the night, likely driven by the trapping of emitted NH3 within the shallower nocturnal boundary layer.
      PubDate: 2017-11-03T20:05:35.816968-05:
      DOI: 10.1002/2017JD026534
  • The Role of Jet Adjustment Processes in Sub-Tropical Dust Storms
    • Authors: Ashok Kumar Pokharel; Michael L. Kaplan, Stephanie Fiedler
      Abstract: Meso-α/β/γ scale atmospheric processes of jet dynamics responsible for generating Harmattan, Saudi Arabian, and Bodélé Depression dust storms are analyzed with observations and high-resolution modeling. The analysis of the role of jet adjustment processes in each dust storm shows similarities as follows: (1) the presence of a well-organized baroclinic synoptic scale system, (2) cross mountain flows which produced a leeside inversion layer prior to the large scale dust storm, (3) the presence of thermal wind imbalance in the exit region of the mid-tropospheric jet streak in the lee of the respective mountains shortly after the time of the inversion formation, (4) dust storm formation accompanied by large magnitude ageostrophic isallobaric low-level winds as part of the meso-β scale adjustment process, (5) substantial low-level turbulence kinetic energy (TKE), and (6) emission and uplift of mineral dust in the lee of nearby mountains. The thermally-forced meso-γ scale adjustment processes, which occurred in the canyons/small valleys, may have been the cause of numerous observed dust streaks leading to the entry of the dust into the atmosphere due to the presence of significant vertical motion and TKE generation. This study points to the importance of meso-β to meso-γ scale adjustment processes at low atmospheric levels due to an imbalance within the exit region of an upper level jet streak for the formation of severe dust storms. The low level TKE, which is one of the pre-requisite to deflate the dust from the surface, can’t be detected with the low resolution data sets; so our results show that a high spatial resolution is required for better representing TKE as a proxy for dust emission.
      PubDate: 2017-11-03T19:40:32.282024-05:
      DOI: 10.1002/2017JD026672
  • Estimates of the size distribution of meteoric smoke particles from
           rocket-borne impact probes
    • Authors: Tarjei Antonsen; Ove Havnes, Ingrid Mann
      Abstract: Ice particles populating noctilucent clouds (NLC) and being responsible for polar mesospheric summer echoes (PMSE) exist around the mesopause in the altitude range from 80 to 90 km during polar summer. The particles are observed when temperatures around the mesopause reach a minimum and it is presumed that they consist of water ice with inclusions of smaller mesospheric smoke particles (MSPs). This work provides estimates of the mean size distribution of MSPs through analysis of collision fragments of the ice particles populating the mesospheric dust layers. We have analyzed data from two triplets of mechanically identical rocket probes, MUDD, which are Faraday bucket detectors with impact grids that partly fragments incoming ice particles. The MUDD probes were launched from Andøya Space Center (69∘17′N 16∘ree1′E) on two payloads during the MAXIDUSTY campaign on the 30th of June and 8th of July 2016, respectively. Our analysis show that it is unlikely that ice particles produce significant current to the detector, and that MSPs dominate the recorded current. The size distributions obtained from these currents, which reflect the MSP sizes, are described by inverse power laws with exponents of k∼[3.3 ± 0.7, 3.7 ± 0.5] and k∼[3.6 ± 0.8, 4.4 ± 0.3] for the respective flights. We derived two k-values for each flight depending on whether the charging probability is proportional to area or volume of fragments. We also confirm that MSPs are probably abundant inside mesospheric ice particles larger than a few nanometers, and the volume filling factor can be a few percent for reasonable assumptions of particle properties.
      PubDate: 2017-11-03T12:25:24.303866-05:
      DOI: 10.1002/2017JD027220
  • The Response of Simulated Arctic Mixed-Phase Stratocumulus to Sea Ice
           Cover Variability in the Absence of Large-Scale Advection
    • Authors: Zhujun Li; Kuan-Man Xu, Anning Cheng
      Abstract: This study examines the responses of Arctic Mixed-Phase Stratocumulus (AMPS) boundary layer to sea ice cover variability near the sea ice margins using large eddy simulations. The simulations are conducted for two different atmospheric conditions, based on observations from the Surface Heat Budget of the Arctic Ocean Experiment (SHEBA) (100% sea ice-covered) and the Mixed-Phase Arctic Cloud Experiment (M-PACE) (open ocean). The effect of sea ice cover variability is investigated for both atmospheric conditions by conducting a series of simulations prescribed with varying amounts of sea ice cover and no large-scale advection. As sea ice cover amount decreases, the SHEBA boundary layer deepens and becomes decoupled. The relative strength of turbulence driven by surface heating to that driven by cloud top radiative cooling increases. Cloud ice and snow grow more efficiently than cloud liquid with moisture transported from the lower boundary layer. On the other hand, as sea ice cover amount increases, the M-PACE boundary layer becomes shallower and more coupled with the surface as turbulence mainly driven by cloud top radiative cooling. Moisture supply from the surface is reduced while cloud droplets are generated from turbulence at cloud top with little ice formation. In both atmospheric conditions, the boundary layer turbulence structure is modified according to change in the relative strength of boundary-layer turbulent sources as sea ice amount changes, resulting in the growth/decay of the cloud layer. Simulations with smaller sea ice cover amounts are associated with more cloud ice but not necessarily more cloud liquid.
      PubDate: 2017-11-03T10:20:42.482459-05:
      DOI: 10.1002/2017JD027086
  • Indian monsoon low pressure systems feed up-and-over moisture transport to
           the southwestern Tibetan Plateau
    • Authors: Wenhao Dong; Yanluan Lin, Jonathon S. Wright, Yuanyu Xie, Fanghua Xu, Wenqing Xu, Yan Wang
      Abstract: As an integral part of the South Asian summer monsoon system, monsoon low pressure systems (LPSs) bring large amounts of precipitation to agrarian north−central India during their passage across the subcontinent. In this study, we investigate the role of LPSs in supplying moisture from north−central India to the southwestern Tibetan Plateau (SWTP) and quantify the contribution of these systems to summer rainfall over the SWTP. The results show that more than 60% of total summer rainfall over the SWTP is related to LPS occurrence. LPSs are associated with a 15% rise in average daily rainfall and a 10% rise in rainy days over the SWTP. This relationship is maintained primarily through up-and-over transport, in which convectively-lifted moisture over the Indian subcontinent is swept over the SWTP by southwesterly winds in the middle troposphere. LPSs play two roles in supplying up-and-over moisture transport. First, these systems elevate large amounts of water vapor and condensed water to the mid-troposphere. Second, the circulations associated with LPSs interact with the background westerlies to induce southwesterly flow in the mid-troposphere, transporting elevated moisture and condensate over the Himalayan mountains. Our findings indicate that LPSs are influential in extending the northern boundary of the South Asian monsoon system across the Himalayas into the interior of the SWTP. The strength of this connection depends on both LPS characteristics and the configuration of the mid-tropospheric circulation, particularly the prevailing westerlies upstream of the SWTP.
      PubDate: 2017-11-03T10:20:33.074175-05:
      DOI: 10.1002/2017JD027296
  • DEBRA—A Dynamic Enhancement with Background Reduction Algorithm:
           Overview and Application to Satellite-Based Dust Storm Detection
    • Authors: Steven D. Miller; Richard L. Bankert, Jeremy E. Solbrig, John M. Forsythe, Yoo-Jeong Noh
      Abstract: This paper describes a Dynamic Enhancement Background Reduction Algorithm (DEBRA) applicable to multi-spectral satellite imaging radiometers. DEBRA uses ancillary information about the clear-sky background to reduce false detections of atmospheric parameters in complex scenes. Applied here to the detection of lofted dust, DEBRA enlists a surface emissivity database coupled with a climatological database of surface temperature to approximate the clear-sky equivalent signal for selected infrared-based multispectral dust detection tests. This background allows for suppression of false alarms caused by land surface features while retaining some ability to detect dust above those problematic surfaces. The algorithm is applicable to both day and nighttime observations and enables weighted combinations of dust detection tests. The results are provided quantitatively, as a detection confidence factor [0,1], but are also readily visualized as enhanced imagery. Utilizing the DEBRA confidence factor as a scaling factor in false color Red/Green/Blue (R/G/B) imagery enables depiction of the targeted parameter in the context of the local meteorology and topography. In this way, the method holds utility to both automated clients and human analysts alike. Examples of DEBRA performance from notable dust storms and comparisons against other detection methods and independent observations are presented.
      PubDate: 2017-11-03T06:34:19.03115-05:0
      DOI: 10.1002/2017JD027365
  • Dust uplift potential in the Central Sahel: an analysis based on 10 years
           of meteorological measurements at high temporal resolution
    • Authors: G. Bergametti; B. Marticorena, J. L. Rajot, B. Chatenet, A. Féron, C. Gaimoz, G. Siour, M. Coulibaly, I. Koné, A. Maman, A. Zakou
      Abstract: A 10-year data set of wind speed and precipitation recorded in two Sahelian stations located in Niger and Mali is used to investigate the duration and the diurnal and seasonal cycles of high wind speeds and Dust Uplift Potential (DUP). The results indicate that high wind speeds, those greater than the threshold wind velocity required to initiate wind erosion (TWV) over a bare soil occurred in the mid and late morning during the dry and wet seasons but also at nighttime during the wet season. However, the morning wind speeds are only slightly greater than TWV leading to low DUP. On the opposite, the high wind velocities associated to the nocturnal mesoscale convective systems crossing the Sahel during the wet season are responsible for the highest potential wind erosion events. This leads to a strong seasonality of DUP with more than 70% occurring in less than 90 days, from mid-April to mid-July. The duration of the high wind speed events is very short since more than 80% last for less than 3 hours, suggesting that the frequency of the observations performed in SYNOP meteorological stations is not sufficient to correctly quantify the contribution of such events to DUP. Finally, by combining precipitation and DUP we estimated that precipitation should have a relatively limited role in terms of inhibition of wind erosion in this region with precipitation only affecting 25% of total DUP.
      PubDate: 2017-11-03T04:35:22.020325-05:
      DOI: 10.1002/2017JD027471
  • The impact of bias correction and model selection on passing temperature
    • Authors: L. K. Gohar; J. A. Lowe, D. Bernie
      Abstract: Knowledge of when specific global or local temperature levels are reached is important for decision makers in that it provides a time frame over which adaptation strategies for temperature related climate impacts need to be put in place. The time frame varies depending on the adaptation strategy but can range from a few years to the order of decades. Climate models, however, show a high degree of uncertainty in the timing of passing specific warming levels, limiting their use in adaptation policy development. This study examines the impact of two approaches, which may reduce the uncertainty in modeled timing of reaching specific warming levels. Firstly, the use of different performance metrics to preferentially weight model ensembles and secondly, the application of four bias correction approaches. Using the CMIP5 simulations of the RCPs, our results show that selecting models based on performance or bias correcting model data both reduce the spread in timing of specific warming levels reached in the first half of the century by up to 50% in some regions. This implies the potential of these approaches to support adaptation planning.
      PubDate: 2017-11-03T04:00:47.467512-05:
      DOI: 10.1002/2017JD026797
  • Features of upward positive leaders initiated from towers in natural
           cloud-to-ground lightning based on simultaneous high-speed videos,
           measured currents and electric fields
    • Authors: Silverio Visacro; Miguel Guimaraes, Maria Helena Murta Vale
      Abstract: Original simultaneous records of currents, close electric field and high-speed videos of natural negative CG lightning striking the tower of Morro do Cachimbo Station are used to reveal typical features of upward positive leaders before the attachment, including their initiation and mode of propagation. According to the results, upward positive leaders initiate some hundreds of microseconds prior to the return stroke, while a continuous uprising current of about 4 A and superimposed pulses of a few tens amperes flow along the tower. Upon leader initiation, the electric field measured 50 m away from the tower at ground level is about 60 kV/m. The corresponding average field roughly estimated 0.5 m above the tower top is higher than 0.55 MV/m. As in laboratory experiments, the common propagation mode of upward positive leaders is developing continuously, without steps, from their initiation. Unlike downward negative leaders, upward positive leaders typically do not branch off, though they can bifurcate under the effect of a downward negative leader's secondary branch approaching their lateral surface. The upward positive leader's estimated average two-dimensional propagation speed, in the range of 0.06 × 106 to 0.16 × 106 m/s, has the same order of magnitude as that of downward negative leaders. Apparently, the speed tends to increase just before attachment.
      PubDate: 2017-11-03T04:00:28.145909-05:
      DOI: 10.1002/2017JD027016
  • What controls springtime fine dust variability in the western United
           States' Investigating the 2002-2015 increase in fine dust in the U.S.
    • Authors: Pattanun Achakulwisut; Lu Shen, Loretta J. Mickley
      Abstract: We use empirical orthogonal function (EOF) analysis to investigate the role of meteorology in controlling the interannual variability of fine dust concentrations in the western United States during 2002-2015 March-May. We then develop a prediction model to explore the causes of an observed increase in fine dust concentrations during March in the Southwest. For each spring month, 54-61% of the total variance in fine dust anomalies can be explained by the first two leading EOF modes, which consist of a coherent pattern of co-variability across the West and a dipole Northwest-Southwest pattern. We identify the key meteorological controlling factors to be regional precipitation, temperature, and soil moisture, which are in turn mostly driven by large-scale changes in sea surface temperature and/or atmospheric circulation patterns, including the El Niño-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO). In addition, fluctuations in the trans-Pacific transport of Asian dust likely contribute to fine dust variability in March and April. We find that March fine dust concentrations have increased from 2002 to 2015 in the Southwest (0.06 ± 0.04 μg m-3 a-1, p < 0.05). Multiple linear regression analysis suggests that these increases are associated with: (1) regionally drier and warmer conditions driven by constructive interference between ENSO and PDO; (2) soil moisture reductions in areas spanning the North American deserts; and (3) enhanced trans-Pacific transport. Our results provide an observational basis for improving dust emission schemes and for assessing future dust activity under climate change.
      PubDate: 2017-11-03T02:00:35.396845-05:
      DOI: 10.1002/2017JD027208
  • Do lateral flows matter for the hyper-resolution land surface
    • Authors: Peng Ji; Xing Yuan, Xin-Zhong Liang
      Abstract: Hyper-resolution land surface modeling provides an unprecedented opportunity to simulate locally relevant water and energy cycle, but lateral surface and/or subsurface flows that are essential at fine scale are often neglected by most one-dimensional land surface models (LSMs). To analyze effects of lateral flows across scales, a Conjunctive Surface-Subsurface Process (CSSP) model which considers soil moisture-surface flow interaction and quasi-three-dimensional subsurface flow, is implemented over a mountainous HyperHydro testbed in southwestern USA at different resolutions. Validation over more than 70 International Soil Moisture Network stations shows that there are significant improvements in soil moisture simulations from 30km to 4km as finer soil property and precipitation data are used, with correlation increased by 5%-16% and error decreased by 5%. Lateral surface flow has a significant influence on surface soil moisture and ground evaporation even at coarse resolution. Effect of lateral subsurface flow on soil moisture is nontrivial at 1km or finer resolution especially over wet areas. At 100m resolution, topography-induced lateral subsurface flow causes drier peaks and wetter valleys, decreases latent heat by 8% at peaks while increases it by 12% at valleys. Furthermore, influences of lateral subsurface flow on ground evaporation and vegetation transpiration are more significant during dry season due to a stronger coupling between soil moisture and evapotranspiration. Therefore, it is worthy to incorporate lateral flow processes in hyper-resolution LSMs to better represent water and energy heterogeneity even with limited hyper-resolution meteorological and surface data.
      PubDate: 2017-11-03T02:00:29.016872-05:
      DOI: 10.1002/2017JD027366
  • The advantages of hybrid-4DEnVar in the context of the forecast
           sensitivity to initial conditions
    • Authors: Hyo-Jong Song; Seoleun Shin, Ji-Hyun Ha, Sujeong Lim
      Abstract: Hybrid four-dimensional ensemble-variational data assimilation (hybrid-4DEnVar) is a prospective successor to three-dimensional variational data assimilation (3DVar) in operational weather prediction centers currently developing a new weather prediction model and those that do not operate adjoint models. In experiments using real observations, hybrid-4DEnVar improved Northern Hemisphere (NH; 20°N-90°N) 500-hPa geopotential height forecasts up to 5 days in a NH summer month compared to 3DVar, with statistical significance. This result is verified against ERA-Interim through a Monte-Carlo test. By a regression analysis, the sensitivity of 5-d forecast is associated with the quality of the initial condition. The increased analysis skill for mid-tropospheric midlatitude temperature and sub-tropical moisture has the most apparent effect on forecast skill in the NH including a typhoon prediction case. Through attributing the analysis improvements by hybrid-4DEnVar separately to the ensemble background error covariance (BEC), its four-dimensional (4D) extension, and climatological BEC, it is revealed that the ensemble BEC contributes to the sub-tropical moisture analysis whereas the 4D extension does to the mid-tropospheric midlatitude temperature. This result implies that hourly wind-mass correlation in 6-h analysis window is required to extract the potential of hybrid-4DEnVar for the midlatitude temperature analysis to the maximum. However, the temporal ensemble correlation, in hourly time-scale, between moisture and another variable is invalid so that it could not work for improving the hybrid-4DEnVar analysis.
      PubDate: 2017-11-02T20:35:44.464563-05:
      DOI: 10.1002/2017JD027598
  • Discrepancies in the climatology and trends of cloud cover in global and
           regional climate models for the Mediterranean region
    • Authors: Aaron Enriquez-Alonso; Josep Calbó, Arturo Sanchez-Lorenzo, Elcin Tan
      Abstract: The present study aims at comparing total cloud cover (TCC) as simulated by regional climate models (RCM) from CORDEX project with the same variable as simulated by the driving global climate models (GCM), which are part of the CMIP5 ensemble. The comparison is performed for the Mediterranean region, and for the 1971-2005 period, when results from the “historical” scenario can also be compared with two datasets of ground based cloud observations. We work with 14 modeling results (resolution, 0.11° × 0.11°), which are a combination of five GCMs and five RCMs. In general, RCMs improve only very slightly the climatic estimation of TCC when compared with observations. Indeed, not all RCMs behave the same, and some indicators (monthly evolution of the relative bias) show an enhancement, while other indices (overall mean bias and annual range difference) improve only very slightly respect to GCMs. Changes in the estimate of TCC in summer might be the most relevant value added by RCMs, as these should describe in a more proper way several mesoscale processes, which play a more relevant role in summer. Noticeably, RCMs are unable to capture the observed decadal trend in TCC. Thus, TCC simulated by RCMs is almost stable, in contradiction with observations and GCMs, which both show statistically significant decreasing trends in the Mediterranean area. This result is somewhat unsatisfactory, as if RCMs cannot reproduce past trends in TCC, their skill in projecting TCC into the future may be questioned.
      PubDate: 2017-10-31T07:35:23.347545-05:
      DOI: 10.1002/2017JD027147
  • Aerosol Impacts on the Structure, Intensity and Precipitation of the
           Landfalling Typhoon Saomai (2006)
    • Authors: Yi Qu; Baojun Chen, Jie Ming, Barry H. Lynn, Ming-Jen Yang
      Abstract: Typhoon Saomai (2006) was simulated using the Weather Research and Forecasting Model (WRF) with explicit spectral (bin) microphysics (SBM) to investigate the effects of aerosol from mainland China on the intensity, structure and precipitation of the landfalling storm. MAR (maritime), MIX (semi-continental) and CON (continental) experiments were conducted with different initial aerosol concentrations. Varying aerosol concentrations had little influence on the storm track but resulted in pronounced deviations in intensity and structures. The experiment with a high initial aerosol concentration showed invigorated convection at the periphery of the tropical cyclone (TC), which interfered with the reformation of the eyewall, leading to TC weakening. The minimum pressures in the CON and MIX experiments were increased by more than 30 hPa and 14.6 hPa, and their maximum wind speeds were 20 m s-1 and 13 m s-1 weaker than that in the MAR experiment, respectively. The rain rates in the MIX and CON experiments were 16.6% and 56.2% greater than that in the MAR run, with the differences mainly occurring in the outer rainbands. These results indicate that the aerosol concentration substantially affects the spatial distributions of cloud hydrometeors and rainfall. The increase of rainfall was triggered by an increase in the melting of graupel and cloud droplets collected by raindrops. Similarly, the graupel melting process also enhanced in the outer rainbands with increasing aerosol. Furthermore, a positive microphysics feedback associated with the rainwater in the outer rainbands played an important role in increasing the rain rate in more aerosol scenarios.
      PubDate: 2017-10-30T08:55:36.104923-05:
      DOI: 10.1002/2017JD027151
  • An experimental investigation of kinetic fractionation of open-water
           evaporation over a large lake
    • Authors: Wei Xiao; Xuhui Lee, Yongbo Hu, Shoudong Liu, Wei Wang, Xuefa Wen, Martin Werner, Chengyu Xie
      Abstract: The oxygen isotopes of water (H218O and H216O) are tracers widely used for the investigation of earth science problems. The tracer applications are based on the premise that the 18O/16O ratio of open-water evaporation (δ18OE) can be calculated from environmental conditions. A long-standing issue concerns the role of kinetic fractionation, or diffusion transport, in the evaporation process. Here we deployed an optical instrument at a large lake (area 2400 km2) to make in-situ measurement of δ18O and δD of atmospheric vapor, then determined δ18O and δD of open-water evaporation using the gradient-diffusion method. Our results show a much weaker kinetic effect than suggested by the kinetic factor εk adopted in some previous studies of lake hydrology (14.2 ‰). By incorporating into the H218O isotopic mass balance of the lake a lower εk value (about 6.2 ‰) used for ocean evaporation in global climate models, we obtain an annual lake evaporation rate that agrees with an independent eddy-covariance observation, but the rate is 72% higher than if the commonly used lake εk value of 14.2 ‰ is applied. The applicability of this results to small lakes is uncertain and in need of field-based assessment.
      PubDate: 2017-10-30T08:51:20.148364-05:
      DOI: 10.1002/2017JD026774
  • Observations of acyl peroxy nitrates during the Front Range Air Pollution
           and Photochemistry Éxperiment (FRAPPÉ)
    • Authors: Jake Zaragoza; Sara Callahan, Erin E. McDuffie, Jeffrey Kirkland, Patrick Brophy, Lindsi Durrett, Delphine K. Farmer, Yong Zhou, Barkley Sive, Frank Flocke, Gabriele Pfister, Christoph Knote, Alex Tevlin, Jennifer Murphy, Emily V. Fischer
      Abstract: We report on measurements of acyl peroxy nitrates (APNs) obtained from two ground sites and the NSF/NCAR C-130 aircraft during the 2014 Front Range Air Pollution and Photochemistry Éxperiment (FRAPPÉ). The relative abundance of the APNs observed at the Boulder Atmospheric Observatory (BAO) indicates that anthropogenic emissions of volatile organic compounds (VOCs) are the dominant drivers of photochemistry during days with the most elevated PAN. Reduced major axis regression between PPN and PAN observed at BAO and from the C-130 produced a slope of 0.21 (R2 = 0.92). Periods of lower PPN/PAN ratios (~0.10) were associated with cleaner background air characterized by lower ammonia and formic acid abundances. The abundance of MPAN relative to PAN only exceeded 0.05 at BAO when PAN mixing ratios were < 300 pptv, implying low influence of isoprene oxidation during periods with substantial local PAN production. We show an example of a day (19 July) where high O3 was not accompanied by enhanced local PAN production. The contribution of biogenic VOCs to local O3 production on the other days in July with elevated O3 (22, 23, 28 and 29 July 2014) was small; evidence is provided in the high abundance of PPN to PAN (slopes between 0.18 – 0.26). The PAN chemistry observed from surface and aircraft platforms during FRAPPÉ implies that anthropogenic VOCs played a dominant role in PAN production during periods with the most O3, and that the relative importance of biogenic hydrocarbon chemistry decreased with increasing O3 production during FRAPPÉ.
      PubDate: 2017-10-27T15:21:04.113156-05:
      DOI: 10.1002/2017JD027337
  • A comparison study of summer season raindrop size distribution between
           Palau and Taiwan, two Islands in Western Pacific
    • Authors: Balaji Kumar Seela; Jayalakshmi Janapati, Pay-Liam Lin, K. Krishna Reddy, Ryuichi Shirooka, Pao K. Wang
      Abstract: Raindrop size distribution (RSD) characteristics in summer season rainfall of two observational sites [Taiwan (24o 58’ N, 121o 10’ E) and Palau (7o 20’ N, 134o 28’ E)] in western Pacific are studied by using five years of impact type disdrometer data. In addition to disdrometer data, TRMM, MODIS, and ERA-Interim data sets are used to illustrate the dynamical and microphysical characteristics associated with summer season rainfall of Taiwan and Palau. Taiwan and Palau's raindrop spectra showed a significant difference, with a higher concentration of mid and large drops in Taiwan than Palau rainfall. RSD stratified on the basis of rain rate showed a higher mass-weighted mean diameter (Dm) and a lower normalized intercept parameter (log10Nw) in Taiwan than Palau rainfall. Precipitation classification into stratiform and convective regimes showed higher Dm values in Taiwan than Palau. Furthermore, for both the locations, the convective precipitation has a higher Dm value than stratiform precipitation. The radar reflectivity - rain rate relations (Z= A*Rb) of Taiwan and Palau showed a clear variation in the coefficient and a less variation in exponent values. Terrain-influenced clouds extended to higher altitudes over Taiwan resulted with higher Dm and lower log10Nw values as compared to Palau.
      PubDate: 2017-10-27T15:20:54.211696-05:
      DOI: 10.1002/2017JD026816
  • Possible role of Hadley circulation strengthening in interdecadal
    • Authors: Botao Zhou; Zunya Wang, Ying Shi
      Abstract: This article revealed that strengthening of winter Hadley circulation in the context of climate change may partially contribute to interdecadal increasing of snowfall intensity over northeastern China in recent decades. This hypothesis is well supported by the process-based linkage between Hadley circulation and atmospheric circulations over the Asian-Pacific region on the interdecadal timescale. The strengthening of winter Hadley circulation corresponds to a weakening of the Siberian high, an eastward shifting of the Aleutian low, a reduction of the East Asian trough, and anomalous southwesterly prevailing over northeastern China. These atmospheric situations weaken the East Asian winter monsoon (EAWM) and lead to an increase of air temperature over northeastern China. Increased local evaporation due to the increase of air temperature, concurrent with more water vapor transported from the Pacific Ocean, can significantly enhance atmospheric water vapor content in the target region. Meanwhile, the ascending of airflows is also strengthened over northeastern China. All of these provide favorable interdecadal backgrounds for the occurrence of intense snowfalls, and thus snowfall intensity is intensified over northeastern China after the 1980s. Further analysis suggests that the circum-Pacific-like teleconnection pattern may play an important role in connecting Hadley circulation strengthening signal and atmospheric circulation anomalies favoring interdecadal intensification of snowfalls over northeastern China.
      PubDate: 2017-10-27T15:15:36.781458-05:
      DOI: 10.1002/2017JD027574
  • Evaporation kinetics of polyol droplets: determination of evaporation
           coefficients and diffusion constants
    • Authors: Yong-yang Su; Aleksandra Marsh, Allen E. Haddrell, Zhi-ming Li, Jonathan P. Reid
      Abstract: In order to quantify the kinetics of mass transfer between the gas and condensed phases in aerosol, physicochemical properties of the gas and condensed phases as well as kinetic parameters (mass/thermal accommodation coefficient) are crucial for estimating mass fluxes over a wide size range from the free-molecule to continuum regimes. In this study, we report measurements of the evaporation kinetics of droplets of 1-butanol, ethylene glycol (EG), diethylene glycol (DEG) and glycerol under well-controlled conditions (gas flow rates, temperature) using the previously-developed cylindrical electrode Electrodynamic Balance (EDB) technique. Measurements are compared with a model that captures the heat and mass transfer occurring at the evaporating droplet surface. The aim of these measurements is to clarify the discrepancy in the reported values of mass accommodation coefficient (αM, equals to evaporation coefficient based on microscopic reversibility) for 1-butanol, EG and DEG, and improve the accuracy of the value of the diffusion coefficient for glycerol in gaseous nitrogen. The uncertainties in the thermophysical and experimental parameters are carefully assessed, the literature values of the vapour pressures of these components are evaluated and the plausible ranges of the evaporation coefficients for 1-butanol, EG and DEG as well as uncertainty in diffusion coefficient for glycerol are reported. Results show that αM should be greater than 0.4, 0.2 and 0.4 for EG, DEG and 1-butanol, respectively. The refined values are helpful for accurate prediction of the evaporation / condensation rates.
      PubDate: 2017-10-26T11:45:22.203827-05:
      DOI: 10.1002/2017JD027111
  • Understanding Atmospheric Anomalies Associated with Seasonal
           Pluvial-Drought Processes Using Southwest China as an Example
    • Authors: Zhenchen Liu; Guihua Lu, Hai He, Zhiyong Wu, Jian He
      Abstract: Seasonal pluvial-drought transition processes are unique natural phenomena. To explore possible mechanisms, we considered Southwest China (SWC) as the study region and comprehensively investigated the temporal evolution or spatial patterns of large-scale and regional atmospheric variables with the simple method of Standardized Anomalies (SA). Some key procedures and results include: (1) Because regional atmospheric variables are more directly responsible for the transition processes, we investigate it in detail. The temporal evolution of net vertical integral water vapor flux (net VIWVF) across SWC, together with vertical SA-based patterns of regional horizontal divergence (D) and vertical motion (ω), coincide well with pluvial-drought transition processes. (2) With respect to large-scale circulation patterns, a well-organized Eurasian (EU) Pattern is one important feature during the pluvial-drought transitions over SWC. (3) Based on these large-scale and regional atmospheric anomalous features, relevant SA-based indices were built, to explore the possibility of simulating drought development using previous pluvial anomalies. As a whole, simulated drought development only with SA-indices of large-scale circulation patterns does not perform well. Further, it can be improved a lot when SA-based indices of regional D and net VIWVF are introduced. (4) In addition, the potential drought prediction using pluvial anomalies, together with the deep understanding of physical mechanisms responsible for pluvial-drought transitions, need to be further explored.
      PubDate: 2017-10-26T11:40:27.069233-05:
      DOI: 10.1002/2017JD026867
  • Impacts of summer extreme precipitation events on the hydrothermal
           dynamics of the active layer in the Tanggula permafrost region on the
           Qinghai-Tibet Plateau
    • Authors: Xiaofan Zhu; Tonghua Wu, Ren Li, Changwei Xie, Guojie Hu, Yanhui Qin, Weihua Wang, Junming Hao, Shuhua Yang, Jie Ni, Cheng Yang
      Abstract: The characteristics of long-term variation for extreme precipitation events were analyzed at the Tanggula site in the continuous permafrost regions of the Qinghai-Tibet Plateau (QTP). In addition, the impacts of extreme precipitation events in summer on soil thermal-moisture dynamics were also investigated. The results showed that local extreme precipitation indices fluctuated significantly, and that the trend magnitudes of local very wet days (R95p), annual total wet-day precipitation (PRCPTOT), number of heavy precipitation days (R10mm), maximum length of dry spell (CDD), and simple daily intensity index (SDII) were larger than those of the western QTP, other regions of China and even the global average. The freeze-thaw cycling in the local active layer occurred from October to the next September during 2006 to 2014. The influence of extreme precipitation event in summer on local soil hydrothermal conditions could reach soil depths up to 105 cm or so, and these were more pronounced than with light or moderate precipitation events. Soil temperature reacted more promptly to local extreme precipitation events than did soil moisture. The rate at which local soil temperature fell after an extreme precipitation event was greater than the rate of increasing temperature on non-precipitation days. Moreover, the amount of precipitation received during extreme precipitation events had a greater effect on local soil moisture and temperature than duration time for these events. Consecutive extreme precipitation events with a longer duration time did not necessarily to have a greater effect than a single precipitation event with a shorter duration. Finally, the thawing process of active layer and local water migration modes could also affect the response of soil hydrothermal conditions to an extreme precipitation event to a large extent.
      PubDate: 2017-10-26T11:35:31.104555-05:
      DOI: 10.1002/2017JD026736
  • Quantifying the impact of biomass burning emissions on major inorganic
           aerosols and their precursors in the US
    • Authors: Amir H. Souri; Yunsoo Choi, Wonbae Jeon, Adam K. Kochanski, Lijun Diao, Jan Mandel, Prakash V. Bhave, Shuai Pan
      Abstract: The primary sources for inorganic aerosols from biomass burning are rather negligible; but they are predominantly formed chemically following emission of their precursors (e.g., SO2, NH3, HOx, and NOx). The biomass burning contributions to some of the precursors can be considerable. Accordingly, we quantify the impact of the emissions on major inorganic aerosols in April-October 2012-2014 using a regional model simulation verified by extensive surface observations throughout the US. Simulated CO enhancements on an hourly basis are used to classify the US into weak-moderate (5
      PubDate: 2017-10-26T11:30:28.90221-05:0
      DOI: 10.1002/2017JD026788
  • Emission of methane and heavier alkanes from the La Brea Tar Pits seepage
           area, Los Angeles
    • Authors: G. Etiope; L. A. Doezema, C. Pacheco
      Abstract: Natural hydrocarbon (oil and gas) seeps are widespread in Los Angeles, California, due to gas migration, along faults, from numerous subsurface petroleum fields. These seeps may represent important natural contributors of methane (CH4) and heavier alkanes (C2-C4) to the atmosphere, in addition to anthropogenic fossil fuel and biogenic sources. We measured the CH4 flux by closed-chamber method from the La Brea Tar Pits park (0.1 km2), one of the largest seepage sites in Los Angeles. The gas seepage occurs throughout the park, not only from visible oil-asphalt seeps, but also diffusely from the soil, affecting grass physiology. About 500 kg CH4 day-1 are emitted from the park, especially along a belt of enhanced degassing that corresponds to the 6th Street Fault. Additional emissions are from bubble plumes in the lake within the park (order of 102-103 kg day-1) and at the intersection of Wilshire Blvd. and Curson Ave. (>130 kg day-1), along the same fault. The investigated area has the highest natural gas flux measured thus far for any onshore seepage zone in the USA. Gas migration, oil biodegradation and secondary methanogenesis altered the molecular composition of the original gas accumulated in the Salt Lake Oil Field (>300 m deep), leading to high C1/C2+ and i-butane/n-butane ratios. These molecular alterations can be important tracers of natural seepage and should be considered in the atmospheric modelling of the relative contribution of fossil fuel (anthropogenic fugitive emission and natural geologic sources) vs biogenic sources of methane, on local and global scales.
      PubDate: 2017-10-26T11:25:22.53234-05:0
      DOI: 10.1002/2017JD027675
  • Worsening of heat stress due to global warming in South Korea based on
           multi-RCM ensemble projections
    • Authors: Eun-Soon Im; Yeon-Woo Choi, Joong-Bae Ahn
      Abstract: This study assesses the future changes in summer (June-July-August; JJA) heat stress over South Korea under global warming. To better resolve the region-specific changes in terms of geographical patterns and severity of heat stress in the Korean peninsula, four regional climate models (RCMs) are used for dynamical downscaling of HadGEM2-AO global projections forced by two Representative Concentration Pathway (RCP4.5 and RCP8.5) scenarios. Dynamically downscaled simulations (horizontal resolution of 12.5 km and output interval of 3 hours) facilitate in-depth analysis of diurnal variation and extremes over South Korea, as well as focusing on the particular location, Daegu, that is characterized by high vulnerability to rising temperature. Both maximum temperature and heat stress indices such as wet-bulb globe temperature and apparent temperature, which include the effect of humidity, are examined in order to comprehensively interpret the behaviors of heat stress in response to anthropogenic climate change. Ensemble projections reveal robust patterns of temperature and resultant humidity increases that are roughly constrained by the approximate 7% / K increase in the moisture holding capacity. The changes in temperature and humidity are directly transmitted to the heat stress indices, showing a significant increase. The heat stress is exacerbated in a differentiated way, with more intensification in diurnal variation at nighttime and in regional variation at low-elevation basins. Both RCP4.5 and RCP8.5 scenarios project the statistical likelihood of a notable increase of extreme heat stress indices, much stronger and more extended heat waves, and the emergence of a long period of consecutive tropical nights.
      PubDate: 2017-10-25T06:21:43.538899-05:
      DOI: 10.1002/2017JD026731
  • Improving Lightning and Precipitation Prediction of Severe Convection
           using Lightning Data Assimilation with NCAR WRF-RTFDDA
    • Authors: Haoliang Wang; Yubao Liu, William Y. Y. Cheng, Tianliang Zhao, Mei Xu, Si Shen, Yuewei Liu, Kristin M. Calhoun
      Abstract: In this study, a lightning data assimilation (LDA) scheme was developed and implemented in the NCAR (National Center for Atmospheric Research) Weather Research and Forecasting – Real-Time Four-Dimensional Data assimilation (WRF-RTFDDA) system. In this LDA method, graupel mixing ratio (qg) is retrieved from observed total lightning. To retrieve qg on model grid-boxes, column-integrated graupel mass is first calculated using an observation-based linear formula between graupel mass and total lightning rate. Then the graupel mass is distributed vertically according to the empirical qg vertical profiles constructed from model simulations. Finally, a horizontal spread method is utilized to consider the existence of graupel in the adjacent regions of the lightning initiation locations. Based on the retrieved qg fields, latent heat is adjusted to account for the latent heat releases associated with the formation of the retrieved graupel and to promote convection at the observed lightning locations, which is conceptually similar to the method developed by Fierro et al. Three severe convection cases were studied to evaluate the LDA scheme for short-term (0 – 6 h) lightning and precipitation forecasts. The simulation results demonstrated that the LDA was effective in improving the short-term lightning and precipitation forecasts by improving the model simulation of the qg fields, updrafts, cold pool and front locations. The improvements were most notable in the first two hours, indicating a highly desired benefit of the LDA in lightning and convective precipitation nowcasting (0 – 2 h) applications.
      PubDate: 2017-10-24T12:50:21.414995-05:
      DOI: 10.1002/2017JD027340
  • Remote linkages to anomalous winter atmospheric ridging over the
           northeastern Pacific
    • Authors: Daniel L. Swain; Deepti Singh, Daniel E. Horton, Justinf S. Mankin, Tristan C. Ballard, Noah S. Diffenbaugh
      Abstract: Severe drought in California between 2013 and 2016 has been linked to the multi-year persistence of anomalously high atmospheric pressure over the Northeastern Pacific Ocean, which deflected the Pacific storm track northward and suppressed regional precipitation during California's winter “rainy season.” Multiple hypotheses have emerged regarding why this high pressure ridge near the west coast of North America was so resilient—including unusual sea surface temperature patterns in the Pacific Ocean, reductions in Arctic sea ice, random atmospheric variability, or some combination thereof. Here we explore relationships between previously documented atmospheric conditions over the North Pacific and several potential remote oceanic and cryospheric influences using both observational data and a large ensemble of climate model simulations. Our results suggest that persistent wintertime atmospheric ridging similar to that implicated in California's 2013-2016 drought can at least partially be linked to unusual Pacific sea surface temperatures, and that Pacific Ocean conditions may offer some degree of cool-season foresight in this region despite the presence of substantial internal variability.
      PubDate: 2017-10-24T12:45:40.913941-05:
      DOI: 10.1002/2017JD026575
  • Estimation of the isotopic composition and origins of winter precipitation
           over Japan using a regional isotope circulation model
    • Authors: Masahiro Tanoue; Kimpei Ichiyanagi, Kei Yoshimura, Jun Shimada, Yukiko Hirabayashi
      Abstract: The deuterium excess (d-excess) of precipitation aids in identifying vapor source regions because it reflects humidity conditions in those regions. For Japan, studies have assumed that the Sea of Japan was the dominant source of winter precipitation when the d-excess value in winter is>20‰ or higher than the average value in summer. Because this assumption is based on an interpretation that the high d-excess value is due to an interaction between the continental winter monsoon (WM) and warm Sea of Japan, it may not be appropriate for winter precipitation due to extratropical cyclones (ECs). Here, we clarify local patterns of water isotopic composition and the origins of precipitation in WM and EC types over Japan using a regional isotope circulation model. The results indicate that the Sea of Japan made the highest contribution to precipitation on the Sea of Japan side in the WM type, whereas the Pacific Ocean was the dominant source of precipitation over Japan in the EC type. Because d-excess values were higher in the WM type than the EC type, we can assume that the Sea of Japan is the dominant source of precipitation on the Sea of Japan side when the d-excess value is high. In comparison, we cannot identify the source regions from d-excess values alone for Honshu Island bordering the Pacific Ocean, because the difference in the d-excess value between the WM and EC types is unclear. WM variability can be estimated from observed d-excess values due to their clear positive correlation.
      PubDate: 2017-10-24T12:45:23.01031-05:0
      DOI: 10.1002/2017JD026751
  • A multi-model study on warm precipitation biases in global models compared
           to satellite observations
    • Authors: Xianwen Jing; Kentaroh Suzuki, Huan Guo, Daisuke Goto, Tomoo Ogura, Tsuyoshi Koshiro, Johannes Mülmenstädt
      Abstract: The cloud-to-precipitation transition process in warm clouds simulated by state-of-the-art global climate models (GCMs), including both traditional climate models and a high-resolution model, is evaluated against A-Train satellites observations. The models and satellite observations are compared in the form of the statistics obtained from combined analysis of multiple satellite observables that probe signatures of the cloud-to-precipitation transition process. One common problem identified among these models is the too frequent occurrence of warm precipitation. The precipitation is found to form when the cloud particle size and the liquid water path (LWP) are both much smaller than those in observations. The too efficient formation of precipitation is found to be compensated for by errors of cloud microphysical properties, such as underestimated cloud particle size and LWP, to an extent that varies among the models. However, this does not completely cancel the precipitation formation bias. Robust errors are also found in the evolution of cloud microphysical properties from non-precipitating to drizzling and then to raining clouds in some GCMs, implying unrealistic interaction between precipitation and cloud water. Nevertheless, auspicious information is found for future improvement of warm precipitation representations: the adoption of more realistic autoconversion scheme in the high-resolution model improves the triggering of precipitation, and the introduction of a sophisticated subgrid variability scheme in a traditional model improves the simulated precipitation frequency over subtropical eastern ocean. However, deterioration in other warm precipitation characteristics is also found accompanying these improvements, implying the multi-source nature of warm precipitation biases in GCMs.
      PubDate: 2017-10-23T17:45:23.875435-05:
      DOI: 10.1002/2017JD027310
  • Sources and potential photochemical roles of formaldehyde in an urban
           atmosphere in South China
    • Authors: Chuan Wang; Xiao-Feng Huang, Yu Han, Bo Zhu, Ling-Yan He
      Abstract: Formaldehyde (HCHO) is an important intermediate in tropospheric photochemistry. However, study of its evolution characteristics under heavy pollution conditions in China is limited, especially for high temporal resolutions, making it difficult to analyze its sources and environmental impacts. In this study, ambient levels of HCHO were monitored using a proton-transfer reaction mass spectrometer (PTR-MS) at an urban site in the Pearl River Delta of China. Continuous monitoring campaigns were conducted in the spring, summer, fall, and winter in 2016. The highest averaged HCHO concentrations were observed in autumn (5.1 ± 3.1 ppbv) and summer (5.0 ± 4.4 ppbv), followed by winter (4.2 ± 2.2 ppbv) and spring (3.4 ± 1.6 ppbv). The daily maximum of HCHO occurs in the early afternoon and shows good correlations with O3 and the secondary organic aerosol tracer during the day, revealing close relationships between ambient HCHO and secondary formations in Shenzhen, especially in summer and autumn. The daytime HCHO is estimated to be the major contributor to O3 formation and OH radical production, indicating that HCHO plays a key role in the urban atmospheric photochemical reactions. Anthropogenic secondary formation was calculated to be the dominant source of HCHO using a photochemical age-based parameterization method, with an average proportion of 39%. The contributions of biogenic sources in summer (41%) and autumn (39%) are much higher than those in spring (26%) and winter (28%), while the contributions of anthropogenic primary sources in spring (20%) and winter (18%) are twice those in summer (9%) and autumn (9%).
      PubDate: 2017-10-20T20:20:36.153231-05:
      DOI: 10.1002/2017JD027266
  • Relationships between Gravity Waves Observed at Earth's Surface and in the
           Stratosphere over the Central and Eastern United States
    • Authors: Catherine D. Groot-Hedlin; Michael A. H. Hedlin, Lars Hoffmann, M. Joan Alexander, Claudia C. Stephan
      Abstract: Observations of tropospheric gravity waves (GWs) made by the new and extensive USArray Transportable Array (TA) barometric network located east of the Rockies, in the central and eastern United States and of stratospheric (30-40 km above sealevel) GWs made by the Atmospheric InfraRed Sounder (AIRS) are compared over a 5-year time span from 2010 through 2014. GW detections in the period band from 2-6 hours made at the Earth's surface during the thunderstorm season from May through August each year exhibit the same broad spatial and temporal patterns as observed at stratospheric altitudes. At both levels, the occurrence frequency of GWs is higher at night than during the day and is highest to the west of the Great Lakes. Statistically significant correlations between the variance of the pressure at the TA, which is a proxy for GWs at ground level, with 8.1 μm brightness temperature measurements from AIRS and rain radar precipitation data, which are both proxies for convective activity, indicate that GWs observed at the TA are related to convective sources. There is little, if any, time lag between the two. Correlations between GWs in the stratosphere and at ground level are weaker, possibly due to the AIRS observational filter effect, but are still statistically significant at nighttime. We conclude that convective activity to the west of the Great Lakes is the dominant source of GWs both at ground level and within the stratosphere.
      PubDate: 2017-10-20T20:20:30.913451-05:
      DOI: 10.1002/2017JD027159
  • A Model and Satellite-Based Analysis of the Tropospheric Ozone
           Distribution in Clear versus Convectively Cloudy Conditions
    • Authors: Sarah A. Strode; Anne R. Douglass, Jerald R. Ziemke, Michael Manyin, J. Eric Nielsen, Luke D. Oman
      Abstract: Satellite observations of in-cloud ozone concentrations from the OMI and MLS instruments show substantial differences from background ozone concentrations. We develop a method for comparing a free-running chemistry-climate model (CCM) to in-cloud and background ozone observations using a simple criterion based on cloud fraction to separate cloudy and clear-sky days. We demonstrate that the CCM simulates key features of the in-cloud versus background ozone differences and of the geographic distribution of in-cloud ozone. Since the agreement is not dependent on matching the meteorological conditions of a specific day, this is a promising method for diagnosing how accurately CCMs represent the relationships between ozone and clouds, including the lower ozone concentrations shown by in-cloud satellite observations. Since clouds are associated with convection as well as changes in chemistry, we diagnose the tendency of tropical ozone at 400 hPa due to chemistry, convection and turbulence, and large-scale dynamics. While convection acts to reduce ozone concentrations at 400 hPa throughout much of the tropics, it has the opposite effect over highly polluted regions of South and East Asia.
      PubDate: 2017-10-20T20:15:33.644573-05:
      DOI: 10.1002/2017JD027015
  • Molecular Halogens above the Arctic Snowpack: Emissions, Diurnal
           Variations, and Recycling Mechanisms
    • Authors: Siyuan Wang; Kerri A. Pratt
      Abstract: Elevated levels of reactive bromine and chlorine species in the springtime Arctic boundary layer contribute to ozone depletion and mercury oxidation, as well as reactions with volatile organic compounds. Recent laboratory and field studies have revealed that snowpack photochemistry leads to Br2 and Cl2 production, the mechanisms of which remain poorly understood. In this work, we use a photochemical box model, with a simplified snow module, to examine the halogen chemistry occurring during the March 2012 BRomine, Ozone, and Mercury EXperiment (BROMEX) near Utqiaġvik (Barrow), Alaska. Elevated daytime Br2 levels (e.g., 6-30 ppt at around local noon) reported in previous studies and in this work may be explained by Br + BrNO2/BrONO2 reactions under conditions of depleted O3 (< ~10 ppb) and background NO2 (10-100 ppt). Even at low background NOx levels at Utqiaġvik, ClONO2 is predicted to be important in the production of Cl2 via heterogeneous reaction with Cl-. In the late afternoon, photolysis alone cannot explain the rapid decrease of Cl2 observed in the Arctic boundary layer. Heterogeneous reactions of Cl2 on aerosol particles and surface snowpack are suggested to play a key role in atmospheric Cl2 removal and possible BrCl production. Given the importance of the snowpack in the multiphase chemistry of the Arctic boundary layer, future measurements should focus on vertically-resolved measurements of NOx and reactive halogens, as well as simultaneous particulate and snow halide measurements, to further evaluate and isolate the halogen production and vertical propagation mechanisms through one-dimensional modeling.
      PubDate: 2017-10-20T20:15:29.943063-05:
      DOI: 10.1002/2017JD027175
  • Regional Climate Variability under Model Simulations of Solar
    • Authors: Katherine Dagon; Daniel P. Schrag
      Abstract: Solar geoengineering has been shown in modeling studies to successfully mitigate global mean surface temperature changes from greenhouse warming. Changes in land surface hydrology are complicated by the direct effect of carbon dioxide (CO2) on vegetation, which alters the flux of water from the land surface to the atmosphere. Here we investigate changes in boreal summer climate variability under solar geoengineering using multiple ensembles of model simulations. We find that spatially uniform solar geoengineering creates a strong meridional gradient in the Northern Hemisphere temperature response, with less consistent patterns in precipitation, evapotranspiration, and soil moisture. Using regional summertime temperature and precipitation results across 31-member ensembles, we show a decrease in the frequency of heat waves and consecutive dry days under solar geoengineering relative to a high-CO2 world. However in some regions solar geoengineering of this amount does not completely reduce summer heat extremes relative to present day climate. In western Russia and Siberia, an increase in heat waves is connected to a decrease in surface soil moisture that favors persistent high temperatures. Heat waves decrease in the central United States and the Sahel while the hydrologic response increases terrestrial water storage. Regional changes in soil moisture exhibit trends over time as the model adjusts to solar geoengineering, particularly in Siberia and the Sahel, leading to robust shifts in climate variance. These results suggest potential benefits and complications of large-scale uniform climate intervention schemes.
      PubDate: 2017-10-19T10:55:47.748269-05:
      DOI: 10.1002/2017JD027110
  • Factors That Modulate Properties of Primary Marine Aerosol Generated from
           Ambient Seawater on Ships at Sea
    • Authors: William C. Keene; Michael S. Long, Jeffrey S. Reid, Amanda A. Frossard, David J. Kieber, John R. Maben, Lynn M. Russell, Joanna D. Kinsey, Patricia K. Quinn, Timothy S. Bates
      Abstract: Model primary marine aerosol (mPMA) was produced by bubbling clean air through flowing natural seawater in a high-capacity generator deployed on ships in the eastern North Pacific and western North Atlantic Oceans. Physicochemical properties of seawater and mPMA were quantified to characterize factors that modulated production. Differences in surfactant organic matter (OM) and associated properties including surface tension sustained plumes with smaller bubble sizes, slower rise velocities, larger void fractions, and older surface ages in biologically productive relative to oligotrophic seawater. Production efficiencies for mPMA number (PEnum) and mass (PEmass) per unit air detrained from biologically productive seawater during daytime were greater and mass median diameters smaller than those in the same seawater at night and in oligotrophic seawater during day and night. PEmass decreased with increasing air detrainment rate suggesting that surface bubble rafts suppressed emission of jet droplets and associated mPMA mass. Relative to bubbles emitted at 60-cm depth, PEnum for bubbles emitted from 100-cm depth was approximately two times greater. mPMA OM enrichment factors (EFs) and mass fractions based on a coarse frit, fine frits, and a seawater jet exhibited similar size-dependent variability over a wide range in chlorophyll a concentrations. Results indicate that the physical production of PMA number and mass from the ocean surface varies systematically as interrelated functions of seawater type and, in biologically productive waters, time of day; bubble injection rate, depth, size, and surface age; and physical characteristics of the air-water interface whereas size-resolved OM EFs and mass fractions are relatively invariant.
      PubDate: 2017-10-19T10:55:32.332895-05:
      DOI: 10.1002/2017JD026872
  • Numerical modeling of the active layer thickness and permafrost thermal
           state across Qinghai-Tibetan Plateau
    • Authors: Yanhui Qin; Tonghua Wu, Lin Zhao, Xiaodong Wu, Ren Li, Changwei Xie, Qiangqiang Pang, Guojie Hu, Yongping Qiao, Guohui Zhao, Guangyue Liu, Xiaofan Zhu, Junming Hao
      Abstract: The dynamics of permafrost (including the permafrost thermal state and active layer thicknesses (ALT)) across the Qinghai-Tibetan Plateau (QTP) have not been well understood on a large scale. Here, we simulate the ALT and permafrost thermal state using the Geophysical Institute Permafrost Lab Version 2 (GIPL2) model across the QTP. Based on the single-point simulations, the model is upscaled to the entire QTP. The upscaled model is validated with five investigated regions (IRs), including Wenquan (WQIR), Gaize (GZIR), Aerjin (AEJIR), Xikunlun (XKLIR) and Qinghai-Tibetan highway (G109IR). The results show that the modified GIPL2 model improves the accuracy of the permafrost thermal state simulations. Due to our simulated results on the QTP, the average ALT is of 2.30 m (2.21 - 2.40 m). The ALT decreases with an increase in the altitude and decreases from the southeast to the northwest. The ALT is thin in the central QTP, but it is thick in the high-elevation mountain areas and some areas surrounding glaciers and lakes. The largest ALT is found in the border areas between permafrost and seasonally frozen ground regions. The simulated results of the MAGT (the mean annual ground temperature) indicate that most of the permafrost is sub-stable, which is sensitive to climate warming. The simulated results would be of great significance on assessing the impacts of permafrost dynamics on local hydrology, ecology, and engineering construction.
      PubDate: 2017-10-19T10:50:23.549304-05:
      DOI: 10.1002/2017JD026858
  • First Reprocessing of Southern Hemisphere Additional Ozonesondes (SHADOZ)
           Ozone Profiles (1998-2016). 2. Comparisons with Satellites and
           Ground-based Instruments
    • Authors: Anne M. Thompson; Jacquelyn C. Witte, Chance Sterling, Allen Jordan, Bryan J. Johnson, Samuel J. Oltmans, Masatomo Fujiwara, Holger Vömel, Marc Allaart, Ankie Piters, Gert J. R. Coetzee, Francoise Posny, Ernesto Corrales, Jorge Andres Diaz, Christian Félix, Ninong Komala, Nga Lai, Matakite Maata, Francis Mani, Zamuna Zainal, Shin-ya Ogino, Francisco Paredes, Tercio Luiz Bezerra Penha, Francisco Raimundo Silva, Sukarni Sallons-Mitro, Henry B. Selkirk, F. J. Schmidlin, Rene Stuebi, Kennedy Thiongo
      Abstract: The SHADOZ network was assembled to validate a new generation of ozone-monitoring satellites and to better characterize the vertical structure of tropical ozone in the troposphere and stratosphere. Beginning with nine stations in 1998, more than 7000 ozone and P-T-U profiles are available from 14 SHADOZ sites that have operated continuously for at least a decade. We analyze ozone profiles from the recently reprocessed SHADOZ dataset that is based on adjustments for inconsistencies caused by varying ozonesonde instruments and operating techniques. First, sonde-derived total ozone column amounts are compared to the overpasses from the EP/TOMS, OMI and OMPS satellites that cover 1998-2016. Second, characteristics of the stratospheric and tropospheric columns are examined along with ozone structure in the tropical tropopause layer (TTL). We find that: (1) Relative to our earlier evaluations of SHADOZ data, in 2003, 2007 and 2012, sonde-satellite total ozone column offsets at 12 stations are 2% or less, a significant improvement. (2) As in prior studies, the ten tropical SHADOZ stations, defined as within +19 degrees latitude, display statistically uniform stratospheric column ozone, 229 + 3.9 DU, and a tropospheric zonal wave-one pattern with a 14 DU mean amplitude. (3) The TTL ozone column, which is also zonally uniform, masks complex vertical structure; this argues against using satellites for lower stratospheric ozone trends. (4) Reprocessing has led to more uniform stratospheric column amounts across sites and reduced bias in stratospheric profiles. As a consequence the variability in total column ozone now averages 5%.
      PubDate: 2017-10-18T13:40:21.691816-05:
      DOI: 10.1002/2017JD027406
  • Location Accuracy Evaluation of ToA-Based Lightning Location Systems over
           Mountainous Terrain
    • Authors: Dongshuai Li; Marcos Rubinstein, Farhad Rachidi, Gerhard Diendorfer, Wolfgang Schulz, Gaopeng Lu
      Abstract: In this paper, we analyze the location error of Time of Arrival (ToA)-based lightning location systems (LLSs) caused by propagation effects over mountainous terrain around the Säntis tower located in the Swiss Alps. The study is based on a full-wave three-dimensional (3D) finite-difference time-domain (FDTD) approach using the topographic map including the Säntis Tower and the nearby sensors belonging to LLSs. It is found that the vertical electric fields are strongly affected by the presence of the mountainous terrain and the finite ground conductivity, and that the location error associated with the ToA technique depends strongly on the used onset time estimation technique. The evaluated location errors associated with amplitude thresholds of 10%, 20% and the time of the linear extrapolation of the tangent at maximum field derivative are found to be smallest (about 300 m or less). Finally, we assess the accuracy of two simplified methods (terrain-envelope method and tight-terrain-fit method) to account for the location error due to propagation over mountainous terrain. These two methods might represent an efficient alternative to estimate the additional time delay due to propagation over a non-flat terrain by using available topographic data. In addition, a possible real-time location error compensation algorithm using the elongated propagation path method to improve the location error of the LLSs in mountainous regions is presented and discussed.
      PubDate: 2017-10-18T13:35:20.634469-05:
      DOI: 10.1002/2017JD027520
  • Characteristics of upward lightning on the Beijing 325 m meteorology tower
           and corresponding thunderstorm conditions
    • Authors: Shanfeng Yuan; Rubin Jiang, Xiushu Qie, Dongfang Wang, Zhuling Sun, Mingyuan Liu
      Abstract: Comprehensive observation on lightning striking the 325-m meteorology tower has been conducted in Beijing, China, during the summer season from 2012 to 2016. For the 20 tower flashes captured by high-speed video camera, 19 (95%) cases were initiated from the tower (termed as upward lightning, UL), with duration of 42~844 ms. Most ULs (84.2%, 16/19) were triggered by nearby lightning, of which positive cloud-to-ground flashes (+CGs) account for 87.5% (14/16). Analysis on the characteristics of the causal or triggering lightning flashes suggests that the approaching of negative leader process through in-cloud horizontal channel is the vital condition for the initiation of upward leader. Generally, the ULs initiation follows the nearby discharges, while for a special case in our data set, the nearby positive return stroke was found to occur after the establishment of an upward channel from the tower. It is worth noting that this +CG led to a significant enhancement of the tower-initiating leader, with 8 subsequent return strokes and 2 M-component processes occurring to the tower thereafter. The radar echo of the corresponding thunderstorm indicates that the other-triggered UL (OTUL) tended to occur at the dissipation stage of the thunderstorm with relatively low cloud top height and weak radar echo, while the self-initiating UL (SIUL) was more likely to occur when the thunderstorm was in the mature stage and the tower was underneath the stratiform clouds behind the convective region. The meteorology condition for SIUL involved lower ambient temperature and higher wind speed, as compared to the OTUL.
      PubDate: 2017-10-18T13:30:20.527934-05:
      DOI: 10.1002/2017JD027198
  • No coincident nitrate enhancement events in polar ice cores following the
           largest known solar storms
    • Authors: F. Mekhaldi; J. R. McConnell, F. Adolphi, M. M. Arienzo, N. J. Chellman, O. J. Maselli, A. D. Moy, C. T. Plummer, M. Sig, R. Muscheler
      Abstract: Knowledge on the occurrence rate of extreme solar storms is strongly limited by the relatively recent advent of satellite monitoring of the sun. To extend our perspective of solar storms prior to the satellite era and because atmospheric ionization induced by SEPs can lead to the production of odd nitrogen, nitrate spikes in ice cores have been tentatively used to document both the occurrence and intensity of past SEP events. However, the reliability of the use of nitrate in ice records as a proxy for SEP events is strongly debated. This is partly due to equivocal detection of nitrate spikes in single ice cores and possible alternative sources, such as biomass burning plumes. Here we present new continuous high-resolution measurements of nitrate and of the biomass burning species ammonium and black carbon, from several Antarctic and Greenland ice cores. We investigate periods covering the two largest known SEP events of 775 and 994 CE as well as the Carrington event and the hard SEP event of February 1956. We report no coincident nitrate spikes associated with any of these benchmark events. We also demonstrate the low reproducibility of the nitrate signal in multiple ice cores and confirm the significant relationship between biomass burning plumes and nitrate spikes in individual ice cores. In the light of these new data, there is no line of evidence that supports the hypothesis that ice cores preserve or document detectable amounts of nitrate produced by SEPs, even for the most extreme events known to date.
      PubDate: 2017-10-14T14:55:26.78218-05:0
      DOI: 10.1002/2017JD027325
  • A framework for estimating the 30-m thermal-infrared broadband emissivity
           from Landsat surface-reflectance data
    • Authors: Jie Cheng; Hao Liu, Shunlin Liang, Aixiu Nie, Qiang Liu, Yamin Guo
      Abstract: The land surface thermal-infrared broadband emissivity (BBE) is a vital variable for estimating land surface radiation budgets (SRBs). We develop a framework for retrieving the 30-m BBE from Landsat surface reflectance data to estimate SRBs at finer scales and validate coarse-resolution data. In the developed framework, the land surface is classified as bare soils and vegetated surfaces to allow different algorithms to be used for the BBE estimation. We propose a downscaling algorithm that uses the empirical relationship between the ASTER BBE and Landsat surface reflectance at 90 m to retrieve the 30-m BBE over bare soils. A look-up table (LUT)-based algorithm is proposed for vegetated surfaces. The BBE is interpolated from a LUT that is constructed from the 4SAIL radiative transfer model with inputs of the leaf BBE, the soil background BBE and the leaf area index (LAI). Ground measurements that were collected at eleven relatively homogeneous sandy sites during three independent field campaigns are used to validate the proposed algorithm over bare soils. The average difference between the retrieved and field-measured BBEs is 0.012. We produce the land surface BBE of China in 2008 by using the developed framework and composited winter and summer seasonal BBE maps. The composited seasonal BBE maps are compared to the seasonal BBE maps derived from the ASTER emissivity product. The bias is within ±0.005 over bare soils and ranges from 0.012 to 0.019 over vegetated surfaces. Combined with the validated results in this study and published references, the comparison results demonstrate the good performance of the developed framework. This study provides a new perspective on estimating BBEs from sensors with only a thermal-infrared channel.
      PubDate: 2017-10-14T09:20:23.741007-05:
      DOI: 10.1002/2017JD027268
  • Temporal characteristics of CH4 vertical profiles observed in the West
           Siberian Lowland over Surgut from 1993 to 2015 and Novosibirsk from 1997
           to 2015
    • Authors: M. Sasakawa; T. Machida, K. Ishijima, M. Arshinov, P. K. Patra, A. Ito, S. Aoki, V. Petrov
      Abstract: We have carried out monthly flask sampling using aircraft, in the altitude range of 0-7 km, over the boreal wetlands in Surgut (61°N, 73°E; since 1993) and a pine forest near Novosibirsk (55°N, 83°E; since 1997), both located in the West Siberian Lowland (WSL). The temporal variation of methane (CH4) concentrations at all altitudes at both sites exhibited an increasing trend with stagnation during 2000-2006 as observed globally from ground-based networks. In addition to a winter maximum as seen at other remote sites in northern mid to high latitudes, another seasonal maximum was also observed in summer, particularly in the lower altitudes over the WSL, which could be attributed to emissions from the wetlands. Our measurements suggest that the vertical gradient at Surgut has been decreasing; the mean CH4 difference between 5.5 km and 1.0 km changed from 64±5 ppb during 1995-1999 to 37±3 ppb during 2009-2013 (mean ± standard error). No clear decline in the CH4 vertical gradient appeared at Novosibirsk. Simulations using an atmospheric chemistry-transport model captured the observed decrease in the vertical CH4 gradient at Surgut when CH4 emissions from Europe decreased but increased from the regions south of Siberia, e.g., East and South Asia. At Novosibirsk, the influence of the European emissions was relatively small. Our results also suggest that the regional emissions around the WSL did not change significantly over the period of our observations.
      PubDate: 2017-10-14T08:45:34.061873-05:
      DOI: 10.1002/2017JD026836
  • Distribution and variability of satellite-derived signals of isolated
           convection initiation events over central Eastern China
    • Authors: Yipeng Huang; Zhiyong Meng, Jing Li, Wanbiao Li, Lanqiang Bai, Murong Zhang, Xi Wang
      Abstract: This study combined measurements from the Chinese operational geostationary satellite Fengyun-2E (FY-2E) and ground-based weather radars to conduct a statistical survey of isolated convection initiation (CI) over central Eastern China (CEC). The convective environment in CEC is modulated by the complex topography and monsoon climate. From May to August in 2010, a total of 1630 isolated CI signals were derived from FY-2E using a semi-automated method. The formation of these satellite-derived CI signals peaks in the early afternoon and occurs with high frequency in areas with remarkable terrain inhomogeneity (e.g., mountain, water, and mountain–water areas). The high signal frequency areas shift from northwest CEC (dry, high altitude) in early summer to southeast CEC (humid, low altitude) in midsummer along with an increasing monthly mean frequency. The satellite-derived CI signals tend to have longer lead times (the time difference between satellite-derived signal formation and radar-based CI) in the late morning and afternoon than in the early morning and night. During the early morning and night, the distinction between cloud-top signatures and background terrestrial radiation becomes less apparent, resulting in delayed identification of the signals and thus short and even negative lead times. A decline in the lead time is observed from May to August, likely due to the increasing cloud growth rate and warm-rain processes. Results show increasing lead times with increasing landscape elevation, likely due to more warm-rain processes over the coastal sea and plain, along with a decreasing cloud growth rate from hill and mountain to the plateau.
      PubDate: 2017-10-14T06:00:22.537992-05:
      DOI: 10.1002/2017JD026946
  • A climatological study of short-period gravity waves and ripples at Davis
           Station, Antarctica (68 °S, 78 °E) during the (austral winter
           February-October) period 1999-2013
    • Authors: S. Rourke; F. J. Mulligan, W. J. R. French, D. J. Murphy
      Abstract: A scanning radiometer deployed at Davis Station, Antarctica (68°S, 78°E) has been recording infrared (1.10-1.65 μm) images of a small region (24 km × 24 km) of the zenith night sky once per minute each austral winter night since February 1999. These images have been processed to extract information on the passage of gravity waves (GWs) (horizontal wavelength, λh> 15 km) and ripples (λh ≤ 15 km) over the observing station. Phase speeds, periods, horizontal wavelengths, and predominant propagation directions have been deduced. Observed speeds were found to be highly correlated with horizontal wavelengths as has been reported in previous studies. Reverse ray-tracing of the detected GWs only enabled us to identify four distinct groups. On average only 15% of waves detected can be traced back to the troposphere, and a large proportion (~45%) were not successfully reverse traced substantially below the airglow layer. Two smaller groups were found to reach a termination condition for reverse ray-tracing at altitudes near 50 km and 75 km. Of those that reached the termination altitude in the troposphere (10 km), most of the end points fell within a radius of 300 km of the station, with a very pronounced concentration of wave initiation to the north west at approximately of the observing point. The predominant direction of propagation was southward, and they were observed throughout the year. Recent reports suggest the interaction of planetary waves with the background wind field as a potential source for these waves.
      PubDate: 2017-10-14T05:15:20.961112-05:
      DOI: 10.1002/2017JD026998
  • Modeling of X-ray images and energy spectra produced by stepping lightning
    • Authors: Wei Xu; Robert A. Marshall, Sebastien Celestin, Victor P. Pasko
      Abstract: Recent ground-based measurements at the International Center for Lightning Research and Testing (ICLRT) have greatly improved our knowledge of the energetics, fluence, and evolution of X-ray emissions during natural cloud-to-ground (CG) and rocket-triggered lightning flashes. In this paper, using Monte Carlo simulations and the response matrix of unshielded detectors in the Thunderstorm Energetic Radiation Array (TERA), we calculate the energy spectra of X-rays as would be detected by TERA and directly compare with the observational data during event MSE 10-01. The good agreement obtained between TERA measurements and theoretical calculations supports the mechanism of X-ray production by thermal runaway electrons during the negative corona flash stage of stepping lightning leaders. Modeling results also suggest that measurements of X-ray bursts can be used to estimate the approximate range of potential drop of lightning leaders. Moreover, the X-ray images produced during the leader stepping process in natural negative CG discharges, including both the evolution and morphological features, are theoretically quantified. We show that the compact emission pattern as recently observed in X-ray images is likely produced by X-rays originating from the source region and the diffuse emission pattern can be explained by the Compton scattering effects.
      PubDate: 2017-10-14T02:30:37.592918-05:
      DOI: 10.1002/2016JD026410
  • Observing the impact of Calbuco volcanic aerosols on South Polar ozone
           depletion in 2015
    • Authors: Kane A. Stone; Susan Solomon, Doug E. Kinnison, Michael C. Pitts, Lamont R. Poole, Michael J. Mills, Anja Schmidt, Ryan R. Neely, Diane Ivy, Michael J. Schwartz, Jean-Paul Vernier, Bryan J. Johnson, Matthew B. Tully, Andrew R. Klekociuk, Gert König-Langlo, Satoshi Hagiya
      Abstract: The Southern Hemisphere Antarctic stratosphere experienced two noteworthy events in 2015: a significant injection of sulfur from the Calbuco volcanic eruption in Chile in April, and a record-large Antarctic ozone hole in October and November. Here, we quantify Calbuco's influence on stratospheric ozone depletion in austral spring 2015 using observations and an earth system model. We analyze ozonesondes, as well as data from the Microwave Limb Sounder. We employ the Community Earth System Model, version 1, with the Whole Atmosphere Community Climate Model (CESM1(WACCM)) in a specified dynamics setup, which includes calculations of volcanic effects. The Cloud Aerosol Lidar with Orthogonal Polarization data indicate enhanced volcanic liquid sulfate 532 nm backscatter values as far poleward as 68°S during October and November (in broad agreement with WACCM). Comparison of the location of the enhanced aerosols to ozone data supports the view that aerosols played a major role in increasing the ozone hole size, especially at pressure levels between 150 and 100 hPa. Ozonesonde vertical ozone profiles from the sites of Syowa, South Pole, and Neumayer, display the lowest individual October or November measurements at 150 hPa since the 1991 Mt. Pinatubo eruption period, with Davis showing similarly low values, but no available 1990s data. The analysis suggests that under the cold conditions ideal for ozone depletion, stratospheric volcanic aerosol particles from the moderate-magnitude eruption of Calbuco in 2015 greatly enhanced austral ozone depletion, particularly at 55–68°S, where liquid binary sulfate aerosols have a large influence on ozone concentrations.
      PubDate: 2017-10-13T09:00:59.780055-05:
      DOI: 10.1002/2017JD026987
  • Using ARM Observations to Evaluate Climate Model Simulations of
           Land-Atmosphere Coupling on the U.S. Southern Great Plains
    • Authors: Thomas J. Phillips; Stephen A. Klein, Hsi-Yen Ma, Qi Tang, Shaocheng Xie, Ian N. Williams, Joseph A. Santanello, David R. Cook, Margaret S. Torn
      Abstract: Several independent measurements of warm-season soil moisture and surface atmospheric variables recorded at the ARM Southern Great Plains (SGP) research facility are used to estimate the terrestrial component of land-atmosphere coupling (LAC) strength, and its regional uncertainty. The observations reveal substantial variation in coupling strength, as estimated from three soil moisture measurements at a single site, as well as across six other sites having varied soil and land cover types. The observational estimates then serve as references for evaluating SGP terrestrial coupling strength in the Community Atmospheric Model coupled to the Community Land Model. These coupled model components are operated in both a free-running mode and in a controlled configuration, where the atmospheric and land states are reinitialized daily, so that they do not drift very far from observations. Although the controlled simulation deviates less from the observed surface climate than its free-running counterpart, the terrestrial LAC in both configurations is much stronger, and displays less spatial variability, than the SGP observational estimates. Preliminary investigation of vegetation leaf area index (LAI) substituted for soil moisture suggests that the overly strong coupling between model soil moisture and surface atmospheric variables is associated with too much evaporation from bare ground, and too little from the vegetation cover. These results imply that model surface characteristics such as LAI, as well as the physical parameterizations involved in the coupling of the land and atmospheric components, are likely to be important sources of the problematical LAC behaviors.
      PubDate: 2017-10-13T08:00:29.278891-05:
      DOI: 10.1002/2017JD027141
  • Ozone Variability and Anomalies Observed during SENEX and SEAC4RS
           Campaigns in 2013
    • Authors: Shi Kuang; Michael J. Newchurch, Anne M. Thompson, Ryan M. Stauffer, Bryan J. Johnson, Lihua Wang
      Abstract: Tropospheric ozone variability occurs because of multiple forcing factors including surface emission of ozone precursors, stratosphere-to-troposphere transport (STT), and meteorological conditions. Analyses of ozonesonde observations made in Huntsville, AL, during the peak ozone season (May to September) in 2013 indicate that ozone in the planetary boundary layer was significantly lower than the climatological average, especially in July and August when the Southeastern United States (SEUS) experienced unusually cool and wet weather. Because of a large influence of the lower stratosphere, however, upper-tropospheric ozone was mostly higher than climatology, especially from May to July. Tropospheric ozone anomalies were strongly anti-correlated (or correlated) with water vapor (or temperature) anomalies with a correlation coefficient mostly about 0.6 throughout the entire troposphere. The regression slopes between ozone and temperature anomalies for surface up to mid-troposphere are within 3.0-4.1 ppbv·K-1. The occurrence rates of tropospheric ozone laminae due to STT are ≥50% in May and June and about 30% in July, August and September suggesting that the stratospheric influence on free-tropospheric ozone could be significant during early summer. These STT laminae have a mean maximum ozone enhancement over the climatology of 52±33% (35±24 ppbv) with a mean minimum relative humidity of 2.3±1.7%.
      PubDate: 2017-10-12T22:45:45.994925-05:
      DOI: 10.1002/2017JD027139
  • The Saharan Air Layer as an Early Rainfall Season Suppressant in the
           Eastern Caribbean: The 2015 Puerto Rico Drought
    • Authors: Thomas L. Mote; Craig A. Ramseyer, Paul W. Miller
      Abstract: Eastern Puerto Rico and the surrounding Caribbean experienced a severe drought in 2015 that resulted in record low reservoir and river levels. Rainfall deficits in April and May, which represent the period when the drought began, were more severe in 2015 than recent droughts of record. While El Niño has been associated with drought in the Caribbean, onset of the 2015 drought was strongly associated with lower-than-average values of a recently developed tool used by weather forecasters in San Juan, the Gálvez-Davison Index (GDI), which is used to measure the potential for thunderstorm development and rainfall. Persistently low GDI values indicate strong and frequent intrusions of hot, dry air in the low- to mid-troposphere, suppressing convection, both locally and in development regions for tropical waves that impact Puerto Rico. The Saharan Air Layer (SAL) is largely responsible for this anomalously hot, dry air, which produced thermodynamically stable conditions and limited thunderstorms and rainfall. Moreover, higher-than-normal aerosol concentrations, typically associated with SAL intrusion over the Caribbean, were recorded in April and May 2015. A comparison to AVHRR aerosol optical thickness demonstrate that higher Caribbean aerosols in the early rainfall season, particularly June, are associated with decreased rainfall in eastern Puerto Rico. Results here demonstrate a direct link between the early and more pronounced SAL intrusions into the Caribbean and the suppression of the early rainfall season. More broadly, a reduction in the GDI and increase in the trade wind inversion was associated with reduced early season rainfall in the eastern Caribbean.
      PubDate: 2017-10-12T22:15:25.23623-05:0
      DOI: 10.1002/2017JD026911
  • Large uncertainties in urban-scale carbon emissions
    • Authors: C. K. Gately; L. R. Hutyra
      Abstract: Accurate estimates of fossil fuel carbon dioxide (FFCO2) emissions are a critical component of local, regional, and global climate agreements. Current global inventories of FFCO2 emissions do not directly quantify emissions at local scales, instead spatial proxies like population density, nighttime lights, and powerplant databases are used to downscale emissions from national totals. We have developed a high-resolution (hourly, 1km2) bottom-up Anthropogenic Carbon Emissions System (ACES) for FFCO2, based on local activity data for the year 2011 across the Northeastern U.S. We compare ACES with three widely used global inventories, finding significant differences at regional (20%) and city scales (50-250%). At a spatial resolution of 0.1°, inventories differ by over 100% for half of the grid cells in the domain, with the largest differences in urban areas and oil and gas production regions. Given recent US federal policy pull-backs regarding greenhouse gas emissions reductions, inventories like ACES are crucial for US actions, as the impetus for climate leadership has shifted to city and state governments. The development of a robust carbon monitoring system to track carbon fluxes is critical for emissions benchmarking and verification. We show that existing downscaled inventories are not suitable for urban emissions monitoring, as they do not consider important local activity patterns. The ACES methodology is designed for easy updating, making it suitable for emissions monitoring under most city, regional, and state greenhouse gas mitigation initiatives, in particular for the small and medium-sized cities that lack the resources to regularly perform their own bottom-up emissions inventories.
      PubDate: 2017-10-12T21:50:38.875595-05:
      DOI: 10.1002/2017JD027359
  • Onset of Stratospheric Ozone Recovery in the Antarctic ozone hole in
           assimilated daily total ozone columns
    • Authors: A. T. J. Laat; M. Weele, R. J. A
      Abstract: In this paper we evaluate the long-term changes in ozone depletion within the Antarctic ozone hole using a 37 years (1979-2015) of daily Ozone Mass Deficits (OMD) derived from assimilated total ozone column data. For each year an ‘average daily OMD’ is calculated over a 60-day preferential time period DOY (Day of Year) 220-280). Excluding years with a reduced Polar Stratospheric Cloud (PSC) volume (the so-called PSC-limited years), the 1979-2015 time series of spatially-integrated average daily OMD correlates very well with long-term changes in Equivalent Effective Stratospheric Chlorine (EESC; R2 = 0.89). We find a corresponding statistically highly significant post year-2000 decrease in OMD of -0.77 ± 0.17 MegaTon (Mt; trend significance of 9.8σ), with an associated post year-2000 change in OMD of approximately -30%, consistent with the post year-2000 change in EESC relative to 1980 EESC levels of approximately -30%. The post year-2000 trend significance is robust to the choice of start year.The spatial distribution of the average daily OMD trends reveals a vortex-core region (approximately covering the region [90°W – 0 ° – 90°E / 75°S – 85°S]) largely unaffected by dynamics with a post year-2000 trend significance of> 8σ, and a vortex-edge region in which the trend is locally strongly affected by vortex dynamics though not spatially integrated over the whole vortex-edge region (trend significance> 9σ). For the trend significance we do not find consistent evidence for long-term changes in wave driving, vortex mixing, pre-ozone hole conditions, or the applied assimilation method, playing a role.Our observation/assimilation-based analysis provides robust evidence of a post year-2000 statistically highly significant decrease in the average daily OMD that is consistent with the long-term decrease in ozone depleting substances since 2000 following international emission regulations.
      PubDate: 2017-10-12T21:20:35.162967-05:
      DOI: 10.1002/2016JD025723
  • Extraordinary features of the planetary wave propagation during the boreal
           winter 2013/2014 and the zonal wavenumber two predominance
    • Authors: Y. Harada; T. Hirooka
      Abstract: Observational features of the winter 2013/2014 are investigated using of the Japanese 55-year Reanalysis (JRA-55) data. This winter can be characterized by the continuous predominance of planetary waves of zonal wavenumber two (WN2) that did not cause major sudden stratospheric warming (SSW) events. It is found that the vertical component of the Eliassen–Palm flux of WN2 for the winter 2013/2014 is almost equal to the highest value of the winter 2008/2009. The longitudinal distribution of vertical components of Plumb wave activity flux (WAFz) for this winter shows marked downward propagation around 100°W and upward propagation around 60°E, both of which are the strongest of their type among the 56 winters since 1958/1959. The convergence of wave packets propagating from around 60°E contributes to the development and continuance of the quasi-barotropic Aleutian High, which is associated with the extension of negative extended refractive index (Ks) region. The extension of negative Ks region is related to the convergence or reflection of the wave packets emanating from tropospheric blocking highs developing in the North Pacific Ocean; the development and continuance of the quasi-barotropic Aleutian High is considered to be one of plausible reasons for the lack of major SSWs in the winter 2013/2014. In addition to these results, we revealed the significant contribution of smaller scale waves (with a zonal wavenumber of three or more) to the structure of localized wave packet propagation in the stratosphere.
      PubDate: 2017-10-12T20:30:35.852325-05:
      DOI: 10.1002/2017JD027053
  • Can we monitor snow properties on sea ice to investigate its role in
           tropospheric ozone depletion'
    • Authors: Florent Domine
      Abstract: In the lower troposphere over the Arctic Ocean, ozone is often destroyed in spring by chemical chain reactions involving the reactive bromine species Br and BrO. The role of surface snow in generating reactive bromine has been suspected but many details of the processes not understood. Using unique data such as BrO concentrations from instruments on buoys [Burd et al., 2017] observed that the snow melt onset date often coincides with the end of the reactive bromine season. They proposed that the decrease in snow specific surface area and/or the occurrence of liquid water in snow induced by melting dramatically slows the rate of surface reactions generating bromine, indicating that the physical state of the snow is critical for bromine generation. Their suggestion is discussed and a method to test it using novel instrumentation recently available is proposed.
      PubDate: 2017-10-10T12:20:24.765147-05:
      DOI: 10.1002/2017JD027676
  • Spatial variability of wet troposphere delays over inland water bodies
    • Authors: Ali Mehran; Elizabeth A. Clark, Dennis P. Lettenmaier
      Abstract: Satellite radar altimetry has enabled the study of water levels in large lakes and reservoirs at a global scale. The upcoming Surface Water and Ocean Topography (SWOT) satellite mission (scheduled launch 2020) will simultaneously measure water surface extent and elevation at an unprecedented accuracy and resolution. However, SWOT retrieval accuracy will be affected by a number of factors, including wet tropospheric delay – the delay in the signal's passage through the atmosphere due to atmospheric water content. In past applications, the wet tropospheric delay over large inland water bodies has been corrected using atmospheric moisture profiles based on atmospheric reanalysis data at relatively coarse (10s to 100s of km) spatial resolution. These products cannot resolve sub-grid variations in wet tropospheric delays at the spatial resolutions (of 1 Km and finer) that SWOT is intended to resolve. We calculate zenith wet tropospheric delays (ZWD) and their spatial variability from Weather Research and Forecasting (WRF) numerical weather prediction model simulations at 2.33-km spatial resolution over the Southwestern U.S. (SW), with attention in particular to Sam Rayburn, Ray Hubbard, and Elephant Butte Reservoirs which have width and length dimensions that are of order or larger than the WRF spatial resolution. We find that spatio-temporal variability of ZWD over the inland reservoirs depends on climatic conditions at the reservoir location, as well as distance from ocean, elevation, and surface area of the reservoir, but that the magnitude of subgrid variability (relative to analysis and reanalysis products) is generally less than 10 mm.
      PubDate: 2017-10-10T08:15:23.853633-05:
      DOI: 10.1002/2017JD026525
  • Deriving global OH abundance and atmospheric lifetimes for long-lived
           gases: A search for CH3CCl3 alternatives
    • Authors: Qing Liang; Martyn P. Chipperfield, Eric L. Fleming, N. Luke Abraham, Peter Braesicke, James B. Burkholder, John S. Daniel, Sandip Dhomse, Paul J. Fraser, Steven C. Hardiman, Charles H. Jackman, Douglas E. Kinnison, Paul B. Krummel, Stephen A. Montzka, Olaf Morgenstern, Archie McCulloch, Jens Mühle, Paul A. Newman, Vladimir L. Orkin, Giovanni Pitari, Ronald G. Prinn, Matthew Rigby, Eugene Rozanov, Andrea Stenke, Fiona Tummon, Guus J. M. Velders, Daniele Visioni, Ray F. Weiss
      Abstract: An accurate estimate of global hydroxyl radical (OH) abundance is important for projections of air quality, climate, and stratospheric ozone recovery. As the atmospheric mixing ratios of methyl chloroform (CH3CCl3, MCF), the commonly used OH reference gas, approaches zero, it is important to find alternative approaches to infer atmospheric OH abundance and variability. The lack of global bottom-up emission inventories is the primary obstacle in choosing a MCF-alternative. We illustrate that global emissions of long-lived trace gases can be inferred from their observed mixing ratio differences between the Northern Hemisphere (NH) and Southern Hemisphere (SH), given realistic estimates of their NH-SH exchange time, the emission partitioning between the two hemispheres, and the NH vs. SH OH abundance ratio. Using the observed long-term trend and emissions derived from the measured hemispheric gradient, the combination of HFC-32 (CH2F2), HFC-134a (CH2FCF3, HFC-152a (CH3CHF2), and HCFC-22 (CHClF2), instead of a single gas, will be useful as a MCF-alternative to infer global and hemispheric OH abundance and trace gas lifetimes. The primary assumption on which this multi-species approach relies is that the OH lifetimes can be estimated by scaling the thermal reaction rates of a reference gas at 272 K on global and hemispheric scales. Thus, the derived hemispheric and global OH estimates are forced to reconcile the observed trends and gradient for all four compounds simultaneously. However, currently, observations of these gases from the surface networks do not provide more accurate OH abundance estimate than that from MCF.
      PubDate: 2017-10-10T05:15:45.537636-05:
      DOI: 10.1002/2017JD026926
  • Rapid adjustments cause weak surface temperature response to increased
           black carbon concentrations
    • Authors: Camilla Weum Stjern; Bjørn Hallvard Samset, Gunnar Myhre, Piers M. Forster, Øivind Hodnebrog, Timothy Andrews, Olivier Boucher, Gregory Faluvegi, Trond Iversen, Matthew Kasoar, Viatcheslav Kharin, Alf Kirkevåg, Jean-François Lamarque, Dirk Olivié, Thomas Richardson, Dilshad Shawki, Drew Shindell, Christopher J. Smith, Toshihiko Takemura, Apostolos Voulgarakis
      Abstract: We investigate the climate response to increased concentrations of black carbon (BC), as part of the Precipitation Driver Response Model Intercomparison Project (PDRMIP). A tenfold increase in BC is simulated by 9 global coupled-climate models, producing a model-median effective radiative forcing (ERF) of 0.82 (ranging from 0.41 to 2.91) Wm-2, and a warming of 0.67 (0.16 to 1.66) K globally and 1.24 (0.26 to 4.31) K in the Arctic. A strong positive instantaneous radiative forcing (median of 2.10 Wm-2 based on five of the models) is countered by negative rapid adjustments (-0.64 Wm-2 for the same five models), which dampen the total surface temperature signal. Unlike other drivers of climate change, the response of temperature and cloud profiles to the BC forcing is dominated by rapid adjustments. Low-level cloud amounts increase for all models, while higher-level clouds are diminished. The rapid temperature response is particularly strong above 400 hPa, where increased atmospheric stabilization and reduced cloud cover contrast the response pattern of the other drivers. In conclusion, we find that this substantial increase in BC concentrations does have considerable impacts on important aspects of the climate system. However, some of these effects tend to offset one another, leaving a relatively small global warming of 0.47 K per Wm-2 – about 20 % lower than the response to a doubling of CO2. Translating the tenfold increase in BC to the present-day impact of anthropogenic BC (given the emissions used in this work) would leave a warming of merely 0.07 K.
      PubDate: 2017-10-10T01:15:49.326259-05:
      DOI: 10.1002/2017JD027326
  • Exploring a variable-resolution approach for simulating regional climate
           in the Rocky Mountain region using the VR-CESM
    • Authors: Chenglai Wu; Xiaohong Liu, Zhaohui Lin, Alan M. Rhoades, Paul A. Ullrich, Colin M. Zarzycki, Zheng Lu, Stefan R. Rahimi-Esfarjani
      Abstract: The reliability of climate simulations and projections, particularly in the regions with complex terrains, is greatly limited by the model resolution. In this study we evaluate the variable-resolution Community Earth System Model (VR-CESM) with a high-resolution (0.125°) refinement over the Rocky Mountain region. The VR-CESM results are compared with observations, as well as CESM simulation at a quasi-uniform 1° resolution (UNIF) and Canadian Regional Climate Model version 5 (CRCM5) simulation at a 0.11° resolution. We find that VR-CESM is effective at capturing the observed spatial patterns of temperature, precipitation, and snowpack in the Rocky Mountains with the performance comparable to CRCM5, while UNIF is unable to do so. VR-CESM and CRCM5 simulate better the seasonal variations of precipitation than UNIF, although VR-CESM still overestimates winter precipitation whereas CRCM5 and UNIF underestimate it. All simulations distribute more winter precipitation along the windward (west) flanks of mountain ridges with the greatest overestimation in VR-CESM. VR-CESM simulates much greater snow water equivalent peaks than CRCM5 and UNIF, although the peaks are still 10-40% less than observations. Moreover, the frequency of heavy precipitation events (daily precipitation ≥ 25 mm) in VR-CESM and CRCM5 is comparable to observations, whereas the same events in UNIF are an order of magnitude less frequent. In addition, VR-CESM captures the observed occurrence frequency and seasonal variation of rain-on-snow (ROS) days, and performs better than UNIF and CRCM5. These results demonstrate the VR-CESM's capability in regional climate modeling over the mountainous regions and its promising applications for climate change studies.
      PubDate: 2017-10-10T01:15:45.040596-05:
      DOI: 10.1002/2017JD027008
  • Raindrop Size Distribution Measurements at 4500 m on the Tibetan Plateau
           during TIPEX-III
    • Authors: Baojun Chen; Zhiqun Hu, Liping Liu, Guifu Zhang
      Abstract: As part of the third Tibetan Plateau Atmospheric Scientific Experiment (TIPEX-III) field campaign, raindrop size distribution (DSD) measurements were taken with a laser optical disdrometer in Naqu, China, at 4508 m above sea level (ASL) during the summer months of 2013, 2014, and 2015. The characteristics of DSDs for five different rain rates, for two rain types (convective and stratiform), and for daytime and nighttime rains were studied. The shapes of the averaged DSDs were similar for different rain rates, and the width increased with rainfall intensity. Little difference was found in stratiform DSDs between day and night, whereas convective DSDs exhibited a significant day-night difference. Daytime convective DSDs had larger mass-weighted mean diameters (Dm) and smaller generalized intercepts (NW) than the nighttime DSDs. The constrained relations between the intercept N0 and shape μ, slope Λ and μ, and NW and Dm of gamma DSDs were derived. We also derived empirical relations between Dm and the radar reflectivity factor in the Ku- and Ka-bands.
      PubDate: 2017-10-10T01:15:23.893938-05:
      DOI: 10.1002/2017JD027233
  • Does strong tropospheric forcing cause large-amplitude mesospheric gravity
           waves' - A DEEPWAVE Case Study
    • Authors: Martina Bramberger; Andreas Dörnbrack, Katrina Bossert, Benedikt Ehard, David C. Fritts, Bernd Kaifler, Christian Mallaun, Andrew Orr, P.-Dominique Pautet, Markus Rapp, Michael J. Taylor, Simon Vosper, Bifford Williams, Benjamin Witschas
      Abstract: On 4 July 2014, during the Deep Propagating Gravity Wave Experiment (DEEPWAVE), strong low-level horizontal winds of up to 35 m s−1 over the Southern Alps, New Zealand caused the excitation of gravity waves having the largest vertical energy fluxes of the whole campaign (38 W m−2). At the same time, large-amplitude mesospheric gravity waves were detected by the Temperature Lidar for Middle Atmospheric Research (TELMA) located in Lauder (45.0°  S, 169.7°  E), New Zealand. The coincidence of these two events lead to the question of whether the mesospheric gravity waves were generated by the strong tropospheric forcing. To answer this, an extensive data set is analysed, comprising: TELMA, in-situ aircraft measurements, radiosondes, wind lidar measurements aboard the DLR Falcon as well as Rayleigh lidar and advanced mesospheric temperature mapper (AMTM) measurements aboard the NSF/NCAR GV. These measurements are further complemented by limited area simulations using a numerical weather prediction model.This unique data set confirms that strong tropospheric forcing can cause large-amplitude gravity waves in the mesosphere, and that three essential ingredients are required to achieve this: First, nearly linear propagation across the tropopause, second, leakage through the stratospheric wind minimum, and third, amplification in the polar night jet. Stationary gravity waves were detected in all atmospheric layers up to the mesosphere with horizontal wavelengths between 20 and 100km. The complete coverage of our data set from troposphere to mesosphere proved to be valuable to identify the processes involved in deep gravity wave propagation.
      PubDate: 2017-10-08T19:20:43.817473-05:
      DOI: 10.1002/2017JD027371
  • Formaldehyde in the Tropical Western Pacific: Chemical sources and sinks,
           convective transport, and representation in CAM-Chem and the CCMI models
    • Authors: Daniel C. Anderson; Julie M. Nicely, Glenn M. Wolfe, Thomas F. Hanisco, Ross J. Salawitch, Timothy P. Canty, Russell R. Dickerson, Eric C. Apel, Sunil Baidar, Thomas J. Bannan, Nicola J. Blake, Dexian Chen, Barbara Dix, Rafael P. Fernandez, Samuel R. Hall, Rebecca S. Hornbrook, L. Gregory Huey, Beatrice Josse, Patrick Jöckel, Douglas E. Kinnison, Theodore K. Koenig, Michael LeBreton, Virginie Marécal, Olaf Morgenstern, Luke D. Oman, Laura L. Pan, Carl Percival, David Plummer, Laura E. Revell, Eugene Rozanov, Alfonso Saiz-Lopez, Andrea Stenke, Kengo Sudo, Simone Tilmes, Kirk Ullmann, Rainer Volkamer, Andrew J. Weinheimer, Guang Zeng
      Abstract: Formaldehyde (HCHO) directly affects the atmospheric oxidative capacity through its effects on HOx. In remote marine environments, such as the Tropical Western Pacific (TWP), it is particularly important to understand the processes controlling the abundance of HCHO because model output from these regions is used to correct satellite retrievals of HCHO. Here, we have used observations from the CONTRAST field campaign, conducted during January and February 2014, to evaluate our understanding of the processes controlling the distribution of HCHO in the TWP as well as its representation in chemical transport/climate models. Observed HCHO mixing ratios varied from ~500 pptv near the surface to ~75 pptv in the upper troposphere. Recent convective transport of near surface HCHO and its precursors, acetaldehyde and possibly methyl hydroperoxide, increased upper tropospheric HCHO mixing ratios by ~33% (22 pptv); this air contained roughly 60% less NO than more aged air. Output from the CAM-Chem chemistry transport model (2014 meteorology) as well as nine chemistry climate models from the Chemistry-Climate Model Initiative (free-running meteorology) are found to uniformly underestimate HCHO columns derived from in situ observations by between 4 and 50%. This underestimate of HCHO likely results from a near factor of two underestimate of NO in most models, which strongly suggests errors in NOx emissions inventories and/or in the model chemical mechanisms. Likewise, the lack of oceanic acetaldehyde emissions and potential errors in the model acetaldehyde chemistry lead to additional underestimates in modeled HCHO of up to 75 pptv (~15%) in the lower troposphere.
      PubDate: 2017-10-06T09:15:31.643374-05:
      DOI: 10.1002/2016JD026121
  • Identifying Meteorological Controls on Open and Closed Mesoscale Cellular
           Convection Associated with Marine Cold Air Outbreaks
    • Authors: Isabel L. McCoy; Robert Wood, Jennifer K. Fletcher
      Abstract: Mesoscale cellular convective (MCC) clouds occur in large-scale patterns over the ocean and have important radiative effects on the climate system. An examination of time-varying meteorological conditions associated with satellite-observed open and closed MCC clouds is conducted to illustrate the influence of large-scale meteorological conditions. Marine cold air outbreaks (MCAO) influence the development of open MCC clouds and the transition from closed to open MCC clouds. MCC neural network classifications on Moderate Resolution Imaging Spectroradiometer (MODIS) data for 2008 are collocated with Clouds and the Earth's Radiant Energy System (CERES) data and ERA-Interim reanalysis to determine the radiative effects of MCC clouds and their thermodynamic environments. Closed MCC clouds are found to have much higher albedo on average than open MCC clouds for the same cloud fraction. Three meteorological control metrics are tested: sea-air temperature difference (∆T), estimated inversion strength (EIS), and a MCAO index (M). These predictive metrics illustrate the importance of atmospheric surface forcing and static stability for open and closed MCC cloud formation. Predictive sigmoidal relations are found between M and MCC cloud frequency globally and regionally: negative for closed MCC cloud, positive for open MCC cloud. The open MCC cloud seasonal cycle is well-correlated with M, while the seasonality of closed MCC clouds is well-correlated with M in the mid-latitudes and EIS in the tropics and sub-tropics. M is found to best distinguish open and closed MCC clouds on average over shorter time scales. The possibility of a MCC cloud feedback is discussed.
      PubDate: 2017-10-06T06:10:41.983153-05:
      DOI: 10.1002/2017JD027031
  • Interaction of urban heating and local winds during the calm inter-monsoon
           seasons in the tropics
    • Authors: M. C. G. Ooi; A. Chan, K. Subramaniam, K. I. Morris, M. Y. Oozeer
      Abstract: Rapid urbanization of cities has greatly modified the thermal and dynamic profile in the urban boundary layer. This paper attempts to study the interaction of urban heating and the local topographic-induced flow circulation for a tropical coastal city, Greater Kuala Lumpur (GKL) in Malaysia. The role of sea and valley breezes-orientated synoptic flow (SBOS) on the interaction is determined by comparing two inter-monsoon periods. A state-of-art numerical model, Weather Research and Forecast Model (WRF-ARW) is used to identify the influence of urbanization through modification of urban surfaces. The model reasonably reproduces the vertical sounding data and near surface weather parameters. The diurnal urban heating pattern is attributed to three predominant factors: (i) weak under calm and clear sky condition (morning heating), (ii) weak under larger atmospheric moisture content (late afternoon convection), (iii) largest (1.4 °C) due to differential cooling rate of urban and rural surface at night. The interaction of urban thermals and upper level SBOS affects the effect of urbanization on local circulation during the day. The urban thermals reduce the weak opposing SBOS (< 2 ms-1) and enhances the inflow of moisture-rich sea breeze passage. This increases the intensity of downwind convective precipitation during late afternoon. On contrary, the strong opposing SBOS (> 2 ms-1) suppresses the vertical lifting of urban thermals and decelerates the sea breeze front. It is discovered that the interaction of urban heating and topographic-induced flow is inter-dependent while the synoptic flow plays a critical role in modifying both factors respectively.
      PubDate: 2017-10-06T05:55:36.331247-05:
      DOI: 10.1002/2017JD026690
  • Synoptic control of contrail cirrus life cycles and their modification due
           to reduced soot number emissions
    • Authors: A. Bier; U. Burkhardt, L. Bock
      Abstract: The atmospheric state, aircraft emissions and engine properties determine formation and initial properties of contrails. The synoptic situation controls microphysical and dynamical processes and causes a wide variability of contrail cirrus life cycles. A reduction of soot particle number emissions, resulting e.g. from the use of alternative fuels, strongly impacts initial ice crystal numbers and microphysical process rates of contrail cirrus. We use a climate model including a contrail cirrus scheme, ECHAM5-CCMod, studying process rates, properties and life cycles of contrail cirrus clusters within different synoptic situations. The impact of reduced soot number emissions is approximated by a reduction in the initial ice crystal number, exemplarily studied for 80%. Contrail cirrus microphysical and macrophysical properties can depend much more strongly on the synoptic situation than on the initial ice crystal number. They can attain a large cover, optical depth and ice water content in long-lived and large-scale ice-supersaturated areas, making them particularly climate relevant. In those synoptic situations, the accumulated ice crystal loss due to sedimentation is increased by around 15% and the volume of contrail cirrus, exceeding an optical depth of 0.02, and their short-wave radiative impact are strongly decreased due to reduced soot emissions. These reductions are of little consequence in short-lived and small-scale ice-supersaturated areas, where contrail cirrus stay optically very thin and attain a low cover. The synoptic situations in which long-lived and climate relevant contrail cirrus clusters can be found over the eastern USA occur in around 25% of cases.
      PubDate: 2017-10-06T04:30:47.104555-05:
      DOI: 10.1002/2017JD027011
  • Temperature Control of the Variability of Tropical Tropopause Layer Cirrus
    • Authors: Hsiu-Hui Tseng; Qiang Fu
      Abstract: This study examines the temperature control of variability of tropical tropopause layer (TTL) cirrus clouds (i.e., clouds with bases higher than 14.5 km) by using eight years (2006–2014) of observations from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) and Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC). It is found that the temporal variability of vertical structure of TTL cirrus cloud fraction averaged between 15°N and 15°S can be well explained by the vertical temperature gradient below ~17.5 km but by the local temperature above for both seasonal and interannual time scales. It is also found that the TTL cirrus cloud fraction at a given altitude is best correlated with the temperature at a higher altitude and this vertical displacement increases with a decrease of the cirrus altitude. It is shown that the TTL cirrus cloud fractions at all altitudes are significantly correlated with tropical cold point tropopause (CPT) temperature. The plausible mechanisms that might be responsible for the observed relations between TTL cirrus fraction and temperature-based variables are discussed, which include ice particle sediments, cooling associated with wave propagations, change of atmospheric stability, and vertical gradient of water vapor mixing ratio. We further examine the spatial co-variability of TTL total cirrus cloud fraction and CPT temperature. It is found that the El Niño Southern Oscillation (ENSO) and quasi-biennial oscillation (QBO) are the leading factors in controlling the spatial variability of the TTL cirrus clouds and temperatures.
      PubDate: 2017-10-05T01:30:29.429952-05:
      DOI: 10.1002/2017JD027093
  • The impact of föhn winds on surface energy balance during the 2010-11
           melt season over Larsen C Ice Shelf, Antarctica
    • Authors: J. C. King; A. Kirchgaessner, S. Bevan, A. D. Elvidge, P. Kuipers Munneke, A. Luckman, A. Orr, I. A. Renfrew, M. R. Broeke
      Abstract: We use model data from the Antarctic Mesoscale Prediction System (AMPS), measurements from automatic weather stations and satellite observations to investigate the association between surface energy balance (SEB), surface melt and the occurrence of föhn winds over Larsen C Ice Shelf (Antarctic Peninsula) over the period November 2010 – March 2011. Föhn conditions occurred for over 20% of the time during this period and are associated with increased air temperatures and decreased relative humidity (relative to non-föhn conditions) over the western part of the ice shelf. During föhn conditions, the downward turbulent flux of sensible heat and the downwelling shortwave radiation both increase. However, in AMPS, these warming tendencies are largely balanced by an increase in upward latent heat flux and a decrease in downwelling longwave radiation so the impact of föhn on the modelled net SEB is small. This balance is highly sensitive to the representation of surface energy fluxes in the model and limited validation data suggest that AMPS may underestimate the sensitivity of SEB and melt to föhn. There is broad agreement on the spatial pattern of melt between the model and satellite observations but disagreement in the frequency with which melt occurs. Satellite observations indicate localized regions of persistent melt along the foot of the Antarctic Peninsula mountains which are not simulated by the model. Furthermore, melt is observed to persist in these regions during extended periods when föhn does not occur, suggesting that other factors may be important in controlling melt in these regions.
      PubDate: 2017-10-04T23:55:33.824961-05:
      DOI: 10.1002/2017JD026809
  • On the Influence of Air-Mass Origin on Low-Cloud Properties in the
           South-East Atlantic
    • Authors: Julia Fuchs; Jan Cermak, Hendrik Andersen, Rainer Hollmann, Katharina Schwarz
      Abstract: This study investigates the impact of air-mass origin and dynamics on cloud property changes in the South-East Atlantic (SEA) during the biomass-burning season. The understanding of clouds and their determinants at different scales is important for constraining the Earth's radiative budget, and thus prominent in climate-system research. In this study, the thermodynamically stable SEA stratocumulus cover is observed not only as the result of local environmental conditions but also as connected to large-scale meteorology by the often neglected but important role of spatial origins of air masses entering this region. In order to assess to what extent cloud properties are impacted by aerosol concentration, air mass history, and meteorology, a HYSPLIT cluster analysis is conducted linking satellite observations of cloud properties (SEVIRI), information on aerosol species (MACC) and meteorological context (ERA-Interim reanalysis) to air-mass clusters. It is found that a characteristic pattern of air-mass origins connected to distinct synoptical conditions leads to marked cloud property changes in the southern part of the study area. Long-distance air masses are related to midlatitude weather disturbances that affect the cloud microphysics, especially in the southwestern subdomain of the study area. Changes in cloud effective radius are consistent with a boundary layer deepening and changes in LTS. In the southeastern subdomain cloud cover is controlled by a generally higher LTS, while air-mass origin plays a minor role. This study leads to a better understanding of the dynamical drivers behind observed stratocumulus cloud properties in the SEA and frames potentially interesting conditions for aerosol-cloud interactions.
      PubDate: 2017-10-04T09:50:39.46647-05:0
      DOI: 10.1002/2017JD027184
  • The 2016 southeastern US drought: an extreme departure from centennial
           wetting and cooling
    • Authors: A. Park Williams; Benjamin I. Cook, Jason E. Smerdon, Daniel A. Bishop, Richard Seager, Justin S. Mankin
      Abstract: The fall 2016 drought in the southeastern United States (SE US) appeared exceptional based on its widespread impacts, but the current monitoring framework that only extends from 1979-present does not readily facilitate evaluation of soil-moisture anomalies in a centennial context. A new method to extend monthly gridded soil-moisture estimates back to 1895 is developed, indicating that since 1895, October-November 2016 soil moisture (0-200 cm) in the SE US was likely the second lowest on record, behind 1954. This severe drought developed rapidly and was brought on by low September-November precipitation and record-high September-November daily maximum temperatures (Tmax). Record Tmax drove record-high atmospheric moisture demand, accounting for 28% of the October-November 2016 soil-moisture anomaly. Drought and heat in fall 2016 contrasted with 20th-century wetting and cooling in the region, but resembled conditions more common from 1895-1956. Dynamically, the exceptional drying in fall 2016 was driven by anomalous ridging over the central United States that reduced south-southwesterly moisture transports into the SE US by approximately 75%. These circulation anomalies were likely promoted by a moderate La Niña and warmth in the tropical Atlantic, but these processes accounted for very little of the SE US drying in fall 2016, implying a large role for internal atmospheric variability. The extended analysis back to 1895 indicates that SE US droughts as strong as the 2016 event are more likely than indicated from a shorter 60-year perspective, and continued multi-decadal swings in precipitation may combine with future warming to further enhance the likelihood of such events.
      PubDate: 2017-10-04T09:31:09.613877-05:
      DOI: 10.1002/2017JD027523
  • Seasonal Patterns of Dry Deposition at a High Elevation Site in the
           Colorado Rocky Mountains
    • Authors: Kaley M. Oldani; Natalie Mladenov, Mark W. Williams, Cari M. Campbell, David A. Lipson
      Abstract: In the Colorado Rocky Mountains, high elevation barren soils are deficient in carbon (C) and phosphorus (P) and enriched in nitrogen (N). The seasonal variability of dry deposition and its contributions to alpine elemental budgets are critical to understanding how dry deposition influences biogeochemical cycling in high elevation environments. In this two-year study, we evaluated dry and wet deposition inputs to the Niwot Ridge Long Term Ecological Research (NWT-LTER) site in the Colorado Rocky Mountains. The total organic C flux in wet + dry (including soluble and particulate C) deposition was>30 kg C ha-1 yr-1 and represents a substantial input for this C-limited environment. Our side-by-side comparison of dry deposition collectors with and without marble insert indicated that the insert improved retention of dry deposition by ~28%. Annual average dry deposition fluxes of water soluble organic carbon (4.25 kg C ha-1 yr-1) and other water soluble constituents, including ammonium (0.16 kg NH4+ha-1 yr-1), nitrate (1.99 kg NO3- ha-1 yr-1), phosphate (0.08 kg PO43- ha-1 yr-1), and sulfate (1.20 kg SO42- ha-1 yr-1), were comparable to those in wet deposition, with highest values measured in the summer. Backward trajectory analyses implicate air masses passing through the arid west and Four Corners, USA as dominant source areas for dry deposition, especially in spring months. Synchronous temporal patterns of deposition observed at the NWT-LTER site and a distant Rocky Mountain National Park CASTNET site indicate that seasonal dry deposition patterns are regional phenomena with important implications for the larger Rocky Mountain region.
      PubDate: 2017-09-30T08:30:21.662913-05:
      DOI: 10.1002/2016JD026416
  • Diurnal Cycle Variability of Surface Temperature Inferred from AIRS data.
    • Authors: A. Ruzmaikin; H. H.Aumann, Jae Lee, Joel Suskind
      Abstract: The Diurnal Cycle of the Earth surface temperature is investigated using the daily range of the satellite skin temperature data (DTR) provided by measurements of Atmospheric InfraRed Sounder (AIRS) in 2002-2015. The AIRS is on the Aqua satellite, which is in a polar orbit with two crossing times per day at every location on the Earth. Its measurements from the ascending (day) and descending (night) orbits can serve as a proxy for the diurnal cycle. The spatial pattern of the DTR of the skin temperature and its time variability for 14 years of the AIRS operation allows to evaluate the diurnal cycle change on the decadal time scale. Using the Empirical Mode Decomposition of the data time series it is found that the DTR of the surface (skin) temperature over the global Earth has a temporal small positive trend in the decade of the AIRS measurements indicating that the day temperatures grew slightly more rapidly than the night temperatures. A possible cause of the observed DTR increase is a decrease of the low cloud fraction at nighttime found for the same time period from the AIRS retrievals.
      PubDate: 2017-09-29T10:41:57.717687-05:
      DOI: 10.1002/2016JD026265
  • Uncertainty in predicting CCN activity of aged and primary aerosols
    • Authors: Fang Zhang; Jingye Ren, Jianfei Peng, Yuying Wang, Don Collins, Renyi Zhang, Yele Sun, Xin Yang, Zhanqing Li
      Abstract: Understanding particle CCN activity in diverse atmospheres is crucial when evaluating aerosol indirect effects. Here, aerosols measured at three sites in China were categorized as different types for attributing uncertainties in CCN prediction in terms of a comprehensive dataset including size-resolved CCN activity, size-resolved hygroscopic growth factor, and chemical composition. We show that CCN activity for aged aerosols is unexpectedly underestimated ~22% at a supersaturation (S) of 0.2% when using κ-Kohler theory with an assumption of an internal mixture with measured bulk composition that has typically resulted in an overestimate of the CCN activity in previous studies. We conclude that the underestimation stems from neglect of the effect of aging/coating on particle hygroscopicity, which is not considered properly in most current models. This effect enhanced the hygroscopicity parameter (κ) by between ~11% (polluted conditions) and 30% (clean days), as indicated in diurnal cycles of κ based on measurements by different instruments. In the urban Beijing atmosphere heavily influenced by fresh emissions, the CCN activity was overestimated by 45% at S=0.2%, likely because of inaccurate assumptions of particle mixing state and because of variability of chemical composition over the particle size range. For both fresh and aged aerosols, CCN prediction exhibits very limited sensitivity to κSOA, implying a critical role of other factors like mixing of aerosol components within and between particles in regulating CCN activity. Our findings could help improving CCN parameterization in climate models.
      PubDate: 2017-09-26T14:45:21.739055-05:
      DOI: 10.1002/2017JD027058
  • Revealing Layers of Pristine Oriented Crystals Embedded within Deep Ice
           Clouds using Differential Reflectivity and the Co-Polar Correlation
    • Authors: W. J. Keat; C. D. Westbrook
      Abstract: Pristine ice crystals typically have high aspect ratios (≫ 1), have a high density and tend to fall preferentially with their major axis aligned horizontally. Consequently, they can, in certain circumstances, be readily identified by measurements of differential reflectivity (ZDR), which is related to their average aspect ratio. However, because ZDR is reflectivity-weighted, its interpretation becomes ambiguous in the presence of even a few, larger aggregates or irregular polycrystals. An example of this is in mixed-phase regions that are embedded within deeper ice cloud. Currently, our understanding of the microphysical processes within these regions is hindered by a lack of good observations.In this paper, a novel technique is presented that removes this ambiguity using measurements from the 3 GHz Chilbolton Advanced Meteorological Radar in Southern England. By combining measurements of ZDR and the co-polar correlation coefficient (ρhv), we show that it is possible to retrieve both the relative contribution to the radar signal and “intrinsic" ZDR(ZDRIP) of the pristine oriented crystals, even in circumstances where their signal is being masked by the presence of aggregates. Results from two case studies indicate that enhancements in ZDR embedded within deep ice clouds are typically produced by pristine oriented crystals with ZDRIP values between 3 and 7 dB (equivalent to 5-9 dB at horizontal incidence) but with varying contributions to the radar reflectivity. Vertically pointing 35 GHz cloud radar Doppler spectra and in-situ particle images from the FAAM BAe-146 aircraft support the conceptual model used and are consistent with the retrieval interpretation.
      PubDate: 2017-09-25T10:12:49.021339-05:
      DOI: 10.1002/2017JD026754
  • Impacts of recent warming and the 2015/16 El Niño on tropical
           Peruvian ice fields
    • Authors: L. G. Thompson; M. E. Davis, E. Mosley-Thompson, E. Beaudon, S. E. Porter, S. Kutuzov, P.-N. Lin, V. N. Mikhalenko, K. R. Mountain
      Abstract: Data collected between 1974 and 2016 from snow pits and core samples from two Peruvian ice fields demonstrate the effect of the recent warming over the tropical Andes, augmented by El Niño, on the preservation of the climate record. As the 0oC isotherm is approaching the summit of the Quelccaya ice cap in the Andes of southern Peru (5670 masl), the distinctive seasonal δ18O oscillations in the fresh snow deposited within each thermal year are attenuated at depth due to melting and percolation through the firn. This has become increasingly pronounced over 43 years. In the Andes of northern Peru, the ice field on the col of Nevado Huascarán (6050 masl) has retained its seasonal δ18O variations at depth due to its higher elevation. During the 2015/16 El Niño, snow on Quelccaya and Huascarán was isotopically (δ18O) enriched and the net sum of accumulation over the previous year (NSA) was below the mean for non-El Niño years, particularly on Quelccaya (up to 64% below the mean) which was more pronounced than the NSA decrease during the comparable 1982/83 El Niño. Interannual large-scale oceanic and middle to upper level atmospheric temperatures influence δ18O in precipitation on both ice fields, although the influences are variably affected by strong El Niño-Southern Oscillation events, especially on Quelccaya. The rate of ice wastage along Quelccaya's margin was dramatically higher during 2015/16 compared with that of the previous 15 years, suggesting that warming from future El Niños may accelerate mass loss on Peruvian glaciers.
      PubDate: 2017-09-17T23:45:27.088704-05:
      DOI: 10.1002/2017JD026592
  • Seasonal scale dating of a shallow ice core from Greenland using oxygen
           isotope matching between data and simulation
    • Authors: Ryoto Furukawa; Ryu Uemura, Koji Fujita, Jesper Sjolte, Kei Yoshimura, Sumito Matoba, Yoshinori Iizuka
      Abstract: A precise age scale based on annual layer counting is essential for investigating past environmental changes from ice core records. However, sub-annual scale dating is hampered by the irregular intra-annual variabilities of oxygen isotope (δ18O) records. Here, we propose a dating method based on matching the δ18O variations between ice-core records and records simulated by isotope-enabled climate models. We applied this method to a new δ18O record from an ice core obtained from a dome site in southeast Greenland. The close similarity between the δ18O records from the ice core and models enables correlation and the production of a precise age scale, with an accuracy of a few months. A missing δ18O minimum in the 1995/1996 winter is an example of an indistinct δ18O seasonal cycle. Our analysis suggests that the missing δ18O minimum is likely caused by a combination of warm air temperature, weak moisture transport, and cool ocean temperature. Based on the age scale, the average accumulation rate from 1960 to 2014 is reconstructed as 1.02 m yr-1 in water equivalent. The annual accumulation rate shows an increasing trend with a slope of 3.6 mm year-1, which is mainly caused by the increase in the autumn accumulation rate of 2.6 mm year-1. This increase is likely linked to the enhanced hydrological cycle caused by the decrease in Arctic sea ice area. Unlike the strong seasonality of precipitation amount in the ERA re-analysis data in the southeast dome region, our reconstructed accumulation rate suggests a weak seasonality.
      PubDate: 2017-09-15T21:20:43.103828-05:
      DOI: 10.1002/2017JD026716
  • Systematical Evaluation of Satellite Precipitation Estimates over Central
           Asia using an Improved Error-component Procedure
    • Authors: Hao Guo; Anming Bao, Felix Ndayisaba, Tie Liu, Alishir Kurban, Philippe De Maeyer
      Abstract: Satellite precipitation estimates (SPEs) provide important alternative precipitation sources for various applications especially for regions where in-situ observations are limited or unavailable, like Central Asia. In this study, eight SPEs based on four different algorithms namely the Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis (TMPA) 3B42, Climate Prediction Center (CPC) morphing technique (CMORPH), Global Satellite Mapping of Precipitation (GSMaP) and Precipitation Estimation from Remotely Sensed Imagery Using Artificial Neural Networks (PERSIANN) are evaluated by using an improved evaluation system over Central Asia with respect to their performance in capturing precipitation occurrence and magnitude. Both satellite-only and gauge-corrected versions are assessed against gauge-gridded reference from June 2001 to May 2006. Main results show that all SPEs have difficulties in accurately estimating mountainous precipitation with great over/underestimation in both winter and summer. In winter, CMORPH products fail to capture events over ice/snow covered region. In summer, large overestimations dominated by positive hit bias and missed precipitation are found for all products in northern Central Asia. Interestingly, 3B42 and CMORPH products show great false alarm percentages (up to 90%) over Lake region, which is more significant in summer than in winter. Significant elevation-dependent errors exist in all products, especially for the high-altitude regions (>3000 m) with missed error and hit error being the two leading errors. Satellite-only products have large systematic and random errors, while the gauge-corrected products demonstrate significant improvements in reducing random errors. Generally, the gauge-corrected GSMaP performs better than others with good skills in reducing various errors.
      PubDate: 2017-09-15T21:00:38.651359-05:
      DOI: 10.1002/2017JD026877
  • Impacts of initial soil moisture and vegetation on the diurnal temperature
           range in arid and semiarid regions in China
    • Authors: Guanghui Yuan; Lei Zhang, Jiening Liang, Xianjie Cao, Qi Guo, Zhaohong Yang
      Abstract: To assess the impacts of initial soil moisture (SMOIS) and the vegetation fraction (Fg) on the diurnal temperature range (DTR) in arid and semiarid regions in China, three simulations using the weather research and forecasting (WRF) model are conducted by modifying the SMOIS, surface emissivity and Fg. SMOIS affects the daily maximum temperature (Tmax) and daily minimum temperature (Tmin) by altering the distribution of available energy between sensible and latent heat fluxes during the day and by altering the surface emissivity at night. Reduced soil wetness can increase both the Tmax and Tmin, but the effect on the DTR is determined by the relative strength of the effects on Tmax and Tmin. Observational data from the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) and the Shapotou Desert Research and Experimental Station (SPD) suggest that the magnitude of the SMOIS effect on the distribution of available energy during the day is larger than that on surface emissivity at night. In other words, SMOIS has a negative effect on the DTR. Changes in Fg modify the surface radiation and the energy budget. Due to the depth of the daytime convective boundary layer, the temperature in daytime is affected less than in nighttime by the radiation and energy budget. Increases in surface emissivity and decreases in soil heating resulting from increased Fg mainly decrease Tmin, thereby increasing the DTR. The effects of SMOIS and Fg on both Tmax and Tmin are the same, but the effects on DTR are the opposite.
      PubDate: 2017-09-10T21:50:20.671208-05:
      DOI: 10.1002/2017JD026790
  • Winter snow conditions on Arctic sea ice north of Svalbard during the
           Norwegian young sea ICE (N-ICE2015) expedition
    • Authors: Ioanna Merkouriadi; Jean-Charles Gallet, Glen E. Liston, Chris Polashenski, Robert M. Graham, Anja Rösel, Sebastian Gerland
      Abstract: Snow is a crucial component of the Arctic sea ice system. Its thickness and thermal properties control heat conduction and radiative fluxes across the ocean, ice and atmosphere interfaces. Hence, observations of the evolution of snow depth, density, thermal conductivity and stratigraphy, are crucial for the development of detailed snow numerical models predicting energy transfer through the snow pack. Snow depth is also a major uncertainty in predicting ice thickness using remote sensing algorithms. Here we examine the winter spatial and temporal evolution of snow physical properties on first-year (FYI) and second-year ice (SYI) in the Atlantic sector of the Arctic Ocean, during the Norwegian young sea ICE (N-ICE2015) expedition (January to March 2015). During N-ICE2015, the snow pack consisted of faceted grains (47%), depth hoar (28%) and wind slab (13%), indicating very different snow stratigraphy compared to what was observed in the Pacific sector of the Arctic Ocean during the SHEBA campaign (1997-1998). Average snow bulk density was 345 kg m-3 and it varied with ice type. Snow depth was 41±19 cm in January and 56±17 cm in February, which is significantly greater than earlier suggestions for this region. The snow water equivalent was 14.5±5.3 cm over first-year ice and 19±5.4 cm over second-year ice.
      PubDate: 2017-07-20T08:05:34.463292-05:
      DOI: 10.1002/2017JD026753
  • Vertical Thermodynamic Structure of the Troposphere during the Norwegian
           young sea ICE expedition (N-ICE2015)
    • Authors: Markus Kayser; Marion Maturilli, Robert M. Graham, Stephen R. Hudson, Annette Rinke, Lana Cohen, Joo-Hong Kim, Sang-Jong Park, Woosok Moon, Mats A. Granskog
      Abstract: The Norwegian young sea ICE (N-ICE2015) expedition was designed to investigate the atmosphere-snow-ice-ocean interactions in the young and thin sea-ice regime north of Svalbard. Radiosondes were launched twice daily during the expedition from January to June 2015. Here we use these upper-air measurements to study the multiple cyclonic events observed during N-ICE2015 with respect to changes in the vertical thermodynamic structure, moisture content and boundary layer characteristics. We provide statistics of temperature inversion characteristics, static stability and boundary layer extent. During winter, when radiative cooling is most effective, we find the strongest impact of synoptic cyclones. Changes to thermodynamic characteristics of the boundary layer are associated with transitions between the radiatively 'clear' and 'opaque' atmospheric states. In spring, radiative fluxes warm the surface leading to lifted temperature inversions and a statically unstable boundary layer. Further, we compare the N-ICE2015 static stability distributions to corresponding profiles from ERA-Interim reanalysis, from the closest land station in the Arctic North Atlantic sector, Ny-Ålesund, and to soundings from the SHEBA expedition 1997/1998. We find similar stability characteristics for N-ICE2015 and SHEBA throughout the troposphere, despite differences in location, sea-ice thickness and snow cover. For Ny-Ålesund, we observe similar characteristics above 1000 m, while the topography and ice-free fjord surrounding Ny-Ålesund generate great differences below. The long-term radiosonde record 1993-2014) from Ny-Ålesund indicates that during the N-ICE2015 spring period temperatures were close to the climatological mean, while the lowest 3000 m were 1 − 3∘C warmer than the climatology during winter.
      PubDate: 2017-05-02T09:55:40.429843-05:
      DOI: 10.1002/2016JD026089
  • Spring snow conditions on Arctic sea ice north of Svalbard, during the
           Norwegian young sea ICE (N-ICE2015) expedition
    • Authors: Jean-Charles Gallet; Ioanna Merkouriadi, Glen E. Liston, Chris Polashenski, Stephen Hudson, Anja Rösel, Sebastian Gerland
      Abstract: Snow is crucial over sea-ice due to its conflicting role in reflecting the incoming solar energy and reducing the heat transfer so that its temporal and spatial variability are important to estimate. During the N-ICE2015 campaign, snow physical properties and variability were examined, and results from April until mid-June 2015 are presented here. Overall, the snow thickness was about 20 cm higher than climatology for second year ice, with an average of 55 ± 27 cm and 32 ± 20 cm on first year ice. The average density was 350-400 Kg m-3 in spring, with higher values in June due to melting. Due to flooding in March, larger variability in snow water equivalent was observed. However, the snow structure was quite homogeneous in spring due to warmer weather and lower amount of storms passing over the field camp. The snow was mostly consisted of wind slab, faceted and depth hoar type crystals with occasional fresh snow. These observations highlight the more dynamic character of evolution of snow properties over sea-ice compared to previous observations, due to more variable sea-ice and weather conditions in this area. The snow pack was isothermal as early as June 10th with the first onset of melt clearly identified in early June. Based on our observations, we estimate than snow could be accurately represented by a three to four layers modeling approach, in order to better consider the high variability of snow thickness and density together with the rapid metamorphose of the snow in spring time.
      PubDate: 2017-04-06T13:55:42.084613-05:
      DOI: 10.1002/2016JD026035
  • Issue Information
    • Pages: 10,557 - 10,559
      Abstract: No abstract is available for this article.
      PubDate: 2017-11-15T16:11:00.716596-05:
      DOI: 10.1002/jgrd.53231
  • Projections of Future Precipitation Extremes Over Europe: A Multimodel
           Assessment of Climate Simulations
    • Authors: Jan Rajczak; Christoph Schär
      Pages: 10,773 - 10,800
      Abstract: Projections of precipitation and its extremes over the European continent are analyzed in an extensive multimodel ensemble of 12 and 50 km resolution EURO-CORDEX Regional Climate Models (RCMs) forced by RCP2.6, RCP4.5, and RCP8.5 (Representative Concentration Pathway) aerosol and greenhouse gas emission scenarios. A systematic intercomparison with ENSEMBLES RCMs is carried out, such that in total information is provided for an unprecedentedly large data set of 100 RCM simulations. An evaluation finds very reasonable skill for the EURO-CORDEX models in simulating temporal and geographical variations of (mean and heavy) precipitation at both horizontal resolutions. Heavy and extreme precipitation events are projected to intensify across most of Europe throughout the whole year. All considered models agree on a distinct intensification of extremes by often more than +20% in winter and fall and over central and northern Europe. A reduction of rainy days and mean precipitation in summer is simulated by a large majority of models in the Mediterranean area, but intermodel spread between the simulations is large. In central Europe and France during summer, models project decreases in precipitation but more intense heavy and extreme rainfalls. Comparison to previous RCM projections from ENSEMBLES reveals consistency but slight differences in summer, where reductions in southern European precipitation are not as pronounced as previously projected. The projected changes of the European hydrological cycle may have substantial impact on environmental and anthropogenic systems. In particular, the simulations indicate a rising probability of summertime drought in southern Europe and more frequent and intense heavy rainfall across all of Europe.
      PubDate: 2017-10-23T23:30:51.667429-05:
      DOI: 10.1002/2017JD027176
  • High-Resolution Regional Reanalysis in China: Evaluation of 1 Year
           Period Experiments
    • Authors: Qi Zhang; Yinong Pan, Shuyu Wang, Jianjun Xu, Jianping Tang
      Pages: 10,801 - 10,819
      Abstract: Globally, reanalysis data sets are widely used in assessing climate change, validating numerical models, and understanding the interactions between the components of a climate system. However, due to the relatively coarse resolution, most global reanalysis data sets are not suitable to apply at the local and regional scales directly with the inadequate descriptions of mesoscale systems and climatic extreme incidents such as mesoscale convective systems, squall lines, tropical cyclones, regional droughts, and heat waves. In this study, by using a data assimilation system of Gridpoint Statistical Interpolation, and a mesoscale atmospheric model of Weather Research and Forecast model, we build a regional reanalysis system. This is preliminary and the first experimental attempt to construct a high-resolution reanalysis for China main land. Four regional test bed data sets are generated for year 2013 via three widely used methods (classical dynamical downscaling, spectral nudging, and data assimilation) and a hybrid method with data assimilation coupled with spectral nudging. Temperature at 2 m, precipitation, and upper level atmospheric variables are evaluated by comparing against observations for one-year-long tests. It can be concluded that the regional reanalysis with assimilation and nudging methods can better produce the atmospheric variables from surface to upper levels, and regional extreme events such as heat waves, than the classical dynamical downscaling. Compared to the ERA-Interim global reanalysis, the hybrid nudging method performs slightly better in reproducing upper level temperature and low-level moisture over China, which improves regional reanalysis data quality.
      PubDate: 2017-10-23T23:29:23.154835-05:
      DOI: 10.1002/2017JD027476
  • Multisource Data-Based Integrated Agricultural Drought Monitoring in the
           Huai River Basin, China
    • Authors: Peng Sun; Qiang Zhang, Qingzhi Wen, Vijay P. Singh, Peijun Shi
      Pages: 10,751 - 10,772
      Abstract: Drought monitoring is critical for early warning of drought hazard. This study attempted to develop an integrated remote sensing drought monitoring index (IRSDI), based on meteorological data for 2003–2013 from 40 meteorological stations and soil moisture data from 16 observatory stations, as well as Moderate Resolution Imaging Spectroradiometer data using a linear trend detection method, and standardized precipitation evapotranspiration index. The objective was to investigate drought conditions across the Huai River basin in both space and time. Results indicate that (1) the proposed IRSDI monitors and describes drought conditions across the Huai River basin reasonably well in both space and time; (2) frequency of drought and severe drought are observed during April–May and July–September. The northeastern and eastern parts of Huai River basin are dominated by frequent droughts and intensified drought events. These regions are dominated by dry croplands, grasslands, and highly dense population and are hence more sensitive to drought hazards; (3) intensified droughts are detected during almost all months except January, August, October, and December. Besides, significant intensification of droughts is discerned mainly in eastern and western Huai River basin. The duration and regions dominated by intensified drought events would be a challenge for water resources management in view of agricultural and other activities in these regions in a changing climate.
      PubDate: 2017-10-19T11:09:20.202959-05:
      DOI: 10.1002/2017JD027186
  • The Record Los Angeles Heat Event of September 2010: 1.
           Synoptic-Scale-Meso-β-Scale Analyses of Interactive Planetary Wave
           Breaking, Terrain- and Coastal-Induced Circulations
    • Authors: Michael L. Kaplan; Jeffrey S. Tilley, Benjamin J. Hatchett, Craig M. Smith, Joshua M. Walston, Kacie N. Shourd, John M. Lewis
      Pages: 10,729 - 10,750
      Abstract: On 27 September 2010 the Los Angeles Civic Center reached its all-time record maximum temperature of 45°C before 1330 local daylight time with several other regional stations observing all-time record breaking heat early in that afternoon. This record event is associated with a general circulation pattern predisposed to hemispheric wave breaking. Three days before the event, wave breaking organizes complex terrain- and coastal-induced processes that lead to isentropic surface folding into the Los Angeles Basin. The first wave break occurs over the western two thirds of North America leading to trough elongation across the southwestern U.S. Collocated with this trough is an isentropic potential vorticity filament that is the locus of a thermally indirect circulation central to warming and associated thickness increases and ridging westward across the Great Basin. In response to this circulation, two subsynoptic wave breaks are triggered along the Pacific coast. The isentropic potential vorticity filament is coupled to the breaking waves and the interaction produces a subsynoptic low-pressure center and a deep vortex aloft over the southeastern California desert. This coupling leads to advection of an elevated mixed layer over Point Conception the night before the record-breaking heat that creates a coastally trapped low-pressure area southwest of Los Angeles. The two low-pressure centers create a low-level pressure gradient and east-southeasterly jet directed offshore over the Los Angeles Basin by sunrise on 27 September. This allows the advection of low-level warm air from the inland terrain toward the coastally trapped disturbance and descending circulation resulting in record heating.
      PubDate: 2017-10-19T11:07:55.381772-05:
      DOI: 10.1002/2017JD027162
  • The Importance of Soil-Type Contrast in Modulating August Precipitation
           Distribution Near the Edwards Plateau and Balcones Escarpment in Texas
    • Authors: Xiao-Ming Hu; Ming Xue, Renee A. McPherson
      Pages: 10,711 - 10,728
      Abstract: The Balcones Escarpment in central Texas is a sloped region between the Edwards Plateau and the coastal plain. The metropolitan areas located along the Balcones Escarpment (e.g., San Antonio, Austin, and Dallas-Fort Worth) are prone to heavy rain and devastating flood events. While the associated hydrological issues of the Balcones Escarpment have been extensively studied, the meteorological impacts of the Edwards Plateau and Balcones Escarpment are not well understood. The indeterminate impacts of the thermal and dynamic effects of the Edwards Plateau on August climatological precipitation are investigated in this study using the multisensor Stage IV precipitation data, high-resolution dynamic downscaling, and short-term sensitivity simulations. Analysis results indicate that the total August precipitation east of the Balcones Escarpment is suppressed and precipitation over the eastern part of the Edwards Plateau is enhanced. Locally initiated moist convection in the afternoon contributes most to the total precipitation during August in the region. The dynamic downscaling output captures the spatial pattern of afternoon precipitation, which is well aligned with the simulated upward motions. The clay-based soil types that dominate the Edwards Plateau have great potential to retain soil moisture and limit latent heat fluxes, consequently leading to higher sensible heat flux than over the plain to the east. As a result, vertical motion is induced, triggering the afternoon moist convection over the Edwards Plateau under favorable conditions. In comparison, the sloping terrain plays a smaller role in triggering the convection. Short-term sensitivity simulations for a clear day confirm and further prove such a diagnosis.
      PubDate: 2017-10-19T10:41:25.962343-05:
      DOI: 10.1002/2017JD027035
  • Correlations Between Secondary Cosmic Ray Rates and Strong Electric Fields
           at Lomnický štít
    • Authors: K. Kudela; J. Chum, M. Kollárik, R. Langer, I. Strhárský, J. Baše
      Pages: 10,700 - 10,710
      Abstract: Since March 2014, there is a continuous measurement of secondary cosmic rays by the detector system SEVAN (Space Environmental Viewing and Analysis Network) at Lomnický štít, altitude 2,634 m above sea level. Starting from June 2016, the count rates (1 s resolution) obtained from the three SEVAN detectors and from their coincidences are available, along with selected meteorological characteristics. Since 30 May 2016 the electric field measurements have been installed at the same site. Several events with clear increase of the count rate in the upper detector of SEVAN were observed during the thunderstorms until 17 September 2016. Examples of these measurements are presented and discussed. Barometric pressure correction and elimination of low-frequency variability from the signal allow to extract 2 min averaged increases from the data. It is shown that the 2 min averaged increases of count rates measured by SEVAN correspond with periods of high electric field (with higher probability during negative polarity) rather than with the individual discharges (lightning).
      PubDate: 2017-10-19T10:06:08.652737-05:
      DOI: 10.1002/2016JD026439
  • Heat Waves in China: Definitions, Leading Patterns, and Connections to
           Large-Scale Atmospheric Circulation and SSTs
    • Authors: Pinya Wang; Jianping Tang, Xuguang Sun, Shuyu Wang, Jian Wu, Xinning Dong, Juan Fang
      Pages: 10,679 - 10,699
      Abstract: Based on the daily maximum temperatures (Tmax) from 587 surface observation stations in China during 1959–2013, heat waves are detected using both absolute and relative definitions. The spatiotemporal variations of heat wave occurrence/duration/amplitude are compared between the two definitions. Considering the significant differences in regional climatology, relative threshold is more meaningful to detect the local extremes. By utilizing the empirical orthogonal function, the integral index heat wave total intensity is decomposed into three dominant modes: interdecadal (ID), interannual-tripole (IA-TR), and interannual-dipole (IA-DP) modes. The ID mode shows uniform anomalies over the whole China, with the maximum in north, and its corresponding time series depict notable interdecadal variations with a turning point around mid-1990s. The IA-DP mode exhibits opposite-signed anomalies over north and south China. The IA-TR mode shows an anomalous tripole pattern with negative anomalies over central China and positive anomalies over north and south China in its positive phase. Both the IA-DP and IA-TR patterns are more obvious since mid-1990s with mainly year-to-year variations before that. All the three modes are controlled by anomalous high-pressure systems, which are accompanied by local-scale dry land conditions. The diabatic heating associated with anomalous convective activities over tropical western Pacific triggers Rossby wave trains propagating northward along the East Asia, which causes abnormal heat waves through descending motion over the high-pressure nodes. In turn, the severe convections are generated by enhanced Walker circulation in the tropical Pacific due to warming and/or cooling sea surface temperature (SST) anomalies in the tropical western and eastern Pacific, respectively.
      PubDate: 2017-10-19T10:02:28.23427-05:0
      DOI: 10.1002/2017JD027180
  • Effects of Parameterized Orographic Drag on Weather Forecasting and
           Simulated Climatology Over East Asia During Boreal Summer
    • Authors: Hyun-Joo Choi; Suk-Jin Choi, Myung-Seo Koo, Jung-Eun Kim, Young Cheol Kwon, Song-You Hong
      Pages: 10,669 - 10,678
      Abstract: The impact of subgrid orographic drag on weather forecasting and simulated climatology over East Asia in boreal summer is examined using two parameterization schemes in a global forecast model. The schemes consider gravity wave drag (GWD) with and without lower-level wave breaking drag (LLWD) and flow-blocking drag (FBD). Simulation results from sensitivity experiments verify that the scheme with LLWD and FBD improves the intensity of a summertime continental high over the northern part of the Korean Peninsula, which is exaggerated with GWD only. This is because the enhanced lower tropospheric drag due to the effects of lower-level wave breaking and flow blocking slows down the wind flowing out of the high-pressure system in the lower troposphere. It is found that the decreased lower-level divergence induces a compensating weakening of middle- to upper-level convergence aloft. Extended experiments for medium-range forecasts for July 2013 and seasonal simulations for June to August of 2013–2015 are also conducted. Statistical skill scores for medium-range forecasting are improved not only in low-level winds but also in surface pressure when both LLWD and FBD are considered. A simulated climatology of summertime monsoon circulation in East Asia is also realistically reproduced.
      PubDate: 2017-10-19T10:00:58.153603-05:
      DOI: 10.1002/2017JD026696
  • Evaluation of a General Circulation Model by the CERES Flux-by-Cloud Type
    • Authors: Zachary A. Eitzen; Wenying Su, Kuan-Man Xu, Norman Loeb, Moguo Sun, David Doelling, Fred Rose, Alejandro Bodas-Salcedo
      Pages: 10,655 - 10,668
      Abstract: In this work, we use the Clouds and the Earth's Radiant Energy System (CERES) FluxByCloudTyp data product (FBCTObs), which calculates top-of-atmosphere (TOA) shortwave and longwave fluxes for cloud types defined by cloud optical depth (τ) and cloud top pressure (pc), and the CERES Flux-by-cloud type simulator (FBCTSim) to evaluate the HadGEM2-A model. FBCTSim is composed of a cloud generator that produces subcolumns with profiles of binary cloud fraction, a cloud property simulator that determines the cloud type (τ, pc) for each subcolumn, and a radiative transfer model that calculates TOA fluxes. The identification of duplicate subcolumns greatly reduces the number of radiative transfer calculations required. In the southern Great Plains region in January, February, and December 2008, FBCTSim shows that HadGEM2-A cloud tops are higher in altitude than in FBCTObs but also have higher values of outgoing longwave radiation (OLR) than in FBCTObs, leading to a compensating error that results in an average value of OLR that is close to observed. When FBCTSim is applied to the southeast Pacific stratocumulus region in JJA 2008, the cloud tops are primarily low in altitude; however, the clouds tend to be less numerous and have higher optical depths than are observed. In addition, the HadGEM2-A albedo is higher than that of FBCTObs for those cloud types that occur most frequently. FBCTSim is also applied to the entire 60°N to 60°S region, and it is found that there are both fewer clouds and higher albedos than observed for most cloud types, which represents a compensating error in terms of the shortwave radiative budget.
      PubDate: 2017-10-19T09:46:04.870689-05:
      DOI: 10.1002/2017JD027076
  • The Impacts of Meteorology on the Seasonal and Interannual Variabilities
           of Ozone Transport From North America to East Asia
    • Authors: Ye Zhu; Jane Liu, Tijian Wang, Bingliang Zhuang, Han Han, Hengmao Wang, Yi Chang, Ke Ding
      Pages: 10,612 - 10,636
      Abstract: The transport of North American (NA) ozone to East Asia is investigated through the analysis of a 20 year simulation (1987–2006) using a global chemical transport model (GEOS-Chem) and forward trajectories during the 1990s at three NA sites. NA ozone mainly influences northern East Asia (> 30°N), where NA ozone in the free troposphere peaks in spring and fall (~12 ppbv). At the surface, NA ozone ranges from 2 to 7 ppbv and peaks in winter, ~50% of which is from the NA boundary layer. The seasonality of the imported NA ozone reflects the combined effects of meteorology and chemistry. In summer, NA ozone can be diverted from reaching East Asia by strong downdrafts behind the European trough. In winter, the prevailing monsoon climate in East Asia can boost downdrafts of NA ozone to the surface. In spring and fall, the westerlies are stronger and shift farther south than in summer, bring more NA ozone to the East Asian (EA) free troposphere than in summer. The imported NA ozone at the EA surface also varies with interannual meteorology. This interannual variation is found to closely correlate to the East Asian winter monsoon (EAWM). The stronger the EAWM in a winter is, the stronger are the downdrafts bringing more NA ozone to the EA surface in that winter and the subsequent spring. Because the anthropogenic NA emissions have decreased since 1999, the year an emission inventory was used in the simulation, the simulated NA influence may serve as an upper limit.
      PubDate: 2017-10-19T09:41:48.621205-05:
      DOI: 10.1002/2017JD026761
  • Long-Term Warming Trends in Korea and Contribution of Urbanization: An
           Updated Assessment
    • Authors: Bo-Joung Park; Yeon-Hee Kim, Seung-Ki Min, Maeng-Ki Kim, Youngeun Choi, Kyung-On Boo, Sungbo Shim
      Pages: 10,637 - 10,654
      Abstract: This study conducted an updated analysis of the long-term temperature trends over South Korea and reassessed the contribution of the urbanization effect to the local warming trends. Linear trends were analyzed for three different periods over South Korea in order to consider possible inhomogeneity due to changes in the number of available stations: recent 103 years (1912–2014), 61 years (1954–2014), and 42 years (1973–2014). The local temperature has increased by 1.90°C, 1.35°C, and 0.99°C during the three periods, respectively, which are found 1.4–2.6 times larger than the global land mean trends. The countries located in the northern middle and high latitudes exhibit similar warming trends (about 1.5 times stronger than the global mean), suggesting a weak influence of urbanization on the local warming over South Korea. Urbanization contribution is assessed using two methods. First, results from “city minus rural” methods showed that 30–45% of the local warming trends during recent four decades are likely due to the urbanization effect, depending on station classification methods and analysis periods. Results from an “observation minus reanalysis” method using the Twentieth Century Reanalysis (20CR) data sets (v2 and v2c) indicated about 25–30% contribution of the urbanization effect to the local warming trend during the recent six decades. However, the urbanization contribution was estimated as low as 3–11% when considering the century-long period. Our results confirm large uncertainties in the estimation of urbanization contribution when using shorter-term periods and suggest that the urbanization contribution to the century-long warming trends could be much lower.
      PubDate: 2017-10-19T09:40:46.03438-05:0
      DOI: 10.1002/2017JD027167
  • On the pattern of CO2 radiative forcing and poleward energy transport
    • Authors: Yi Huang; Yan Xia, Xiaoxiao Tan
      Pages: 10,578 - 10,593
      Abstract: A set of general circulation model experiments are conducted to analyze how the poleward energy transport (PET) is related to the spatial pattern of CO2 radiative forcing. The effects of forcing pattern are affirmed by comparing the conventional doubling CO2 experiment, in which the forcing pattern is inhomogeneous, to a set of forcing homogenization experiments, in which the top of atmosphere (TOA), surface, or atmospheric forcing distribution is homogenized respectively. In addition, we separate and compare the effects of CO2 forcing to various feedbacks on atmospheric and oceanic PETs, by using a set of radiative kernels that we have developed for both TOA and surface radiation fluxes. The results here show that both the enhancement of atmospheric PET and weakening of oceanic PET during global warming are directly driven by the meridional gradients of the CO2 forcing. Interestingly, the overall feedback effect is to reinforce the forcing effect, mainly through the cloud feedback in the case of atmospheric PET and the albedo feedback in the case of the oceanic PET. Contrary to previous studies, we find that the water vapor feedback only has a weak effect on atmospheric PET. The Arctic warming amplification, which strongly affects atmospheric PET, is sensitive to the CO2 forcing pattern.
      PubDate: 2017-10-19T09:26:07.239122-05:
      DOI: 10.1002/2017JD027221
  • Simulated Diurnal Cycles and Seasonal Variability of Low-Level Jets in the
           Boundary Layer over Complex Terrain on the Coast of Southeast China
    • Authors: Min Shao; Qin Geng Wang, Jianjun Xu
      Pages: 10,594 - 10,611
      Abstract: The thermal and dynamical causes of boundary layer low-level jets (LLJs) over the southeast coast of China (a very complex terrain) are explored using a very high resolution diagnostic model California Meteorological (CALMET) along with the Advanced Research Weather Research and Forecasting (WRF-ARW) model, combined with observations. Both diurnal and seasonal variations in LLJs are investigated using simulations during two observational periods and four months in 2011. Two configurations, with different vertical and horizontal resolutions, are compared. The results show that the use of higher vertical and horizontal resolutions (including land cover/use) in the diagnostic model CALMET leads to large improvements in simulating boundary layer LLJs over complex terrain as compared with using lower vertical and horizontal resolutions in both WRF-ARW and CALMET models. The simulations using the diagnostic model CALMET better reproduced the observations, in that both LLJ events occur in the night (nocturnal LLJs) and in the afternoon (afternoon LLJs) are noticed. Compared to the nocturnal LLJs, the afternoon LLJs have larger wind speeds and occur at lower heights. The afternoon LLJ characteristics are closely associated with local thermodynamic circulations including the mountain-valley breeze and land-ocean breeze, which are regulated by the thermal contrasts between the ocean and mountains, and diurnal cycle of boundary layer friction. A comprehensive analysis of an afternoon LLJ and nocturnal LLJ indicates that there are large differences in wind field, vertical motions, and water vapor distributions between them. The local thermodynamic circulations also strongly affect vertical motions; even under a relatively stable atmosphere, the vertical motions during LLJs are stronger than the monthly average. Afternoon LLJ events, associated with the southeasterly land-ocean breeze in summer, occur more frequently than in winter; in contrast, nocturnal LLJ events occur in summer less frequently than in winter. The seasonal variation analysis shows that land-ocean breezes have significant effects on the wind speed, wind direction, and heights of the afternoon LLJs; local thermal contrast forcing is believed to be the main factor that affects the LLJs during the daytime especially in warm seasons.
      PubDate: 2017-10-19T08:51:14.396423-05:
      DOI: 10.1002/2017JD026685
  • Exploring the Links in Monthly to Decadal Variability of the Atmospheric
           Water Balance Over the Wettest Regions in ERA-20C
    • Authors: M. Nogueira
      Pages: 10,560 - 10,577
      Abstract: Monthly-to-decadal variability of the regional precipitation over Intertropical Convergence Zone and north-Atlantic and north-Pacific storm tracks was investigated using ERA-20C reanalysis. Satellite-based precipitation (P) and evaporation (E) climatological patterns were well reproduced by ERA-20C. Regional P and E monthly time series displayed ~20% differences, but these decreased rapidly with time scale (~10% at yearly time scales). Spectral analysis showed good scale-by-scale statistical agreement between ERA-20C and observations. Using ERA-Interim showed no improvement despite the much wider range of information assimilated (including satellites). Remarkably high Detrended Cross-Correlation Analysis coefficients (ρDCCA > 0.7 and often ρDCCA > 0.9) revealed tight links between the nonperiodic variability of P, moisture divergence (DIV), and pressure velocity (ω) at monthly-to-decadal time scales over all the wet regions. In contrast, ρDCCA was essentially nonsignificant between nonperiodic P and E or sea surface temperature (SST). Thus, the nonperiodic monthly-to-decadal variability of precipitation in these regions is almost fully controlled by dynamics and not by local E or SST (suggested by Clausius-Clapeyron relation). Analysis of regional nonperiodic standard deviations and power spectra (and respective spectral exponents, β) provided further robustness to this conclusion. Finally, clear transitions in β for P, DIV, and ω between tropical and storm track regions were found. The latter is dominated by transient storms, with energy accumulation at synoptic scales and β 
      PubDate: 2017-10-17T10:45:58.669785-05:
      DOI: 10.1002/2017JD027012
  • Refined Use of Satellite Aerosol Optical Depth Snapshots to Constrain
           Biomass Burning Emissions in the GOCART Model
    • Authors: Mariya Petrenko; Ralph Kahn, Mian Chin, James Limbacher
      Pages: 10,983 - 11,004
      Abstract: Simulations of biomass burning (BB) emissions in global chemistry and aerosol transport models depend on external inventories, which provide location and strength for BB aerosol sources. Our previous work shows that to first order, satellite snapshots of aerosol optical depth (AOD) near the emitted smoke plume can be used to constrain model-simulated AOD, and effectively, the smoke source strength. We now refine the satellite-snapshot method and investigate where applying simple multiplicative emission adjustment factors alone to the widely used Global Fire Emission Database version 3 emission inventory can achieve regional-scale consistency between Moderate Resolution Imaging Spectroradiometer (MODIS) AOD snapshots and the Goddard Chemistry Aerosol Radiation and Transport model. The model and satellite AOD are compared globally, over a set of BB cases observed by the MODIS instrument during the 2004, and 2006–2008 biomass burning seasons. Regional discrepancies between the model and satellite are diverse around the globe yet quite consistent within most ecosystems. We refine our approach to address physically based limitations of our earlier work (1) by expanding the number of fire cases from 124 to almost 900, (2) by using scaled reanalysis-model simulations to fill missing AOD retrievals in the MODIS observations, (3) by distinguishing the BB components of the total aerosol load from background aerosol in the near-source regions, and (4) by including emissions from fires too small to be identified explicitly in the satellite observations. The small-fire emission adjustment shows the complimentary nature of correcting for source strength and adding geographically distinct missing sources. Our analysis indicates that the method works best for fire cases where the BB fraction of total AOD is high, primarily evergreen or deciduous forests. In heavily polluted or agricultural burning regions, where smoke and background AOD values tend to be comparable, this approach encounters large uncertainties, and in some regions, other model- or measurement-related factors might contribute significantly to model-satellite discrepancies. This work sets the stage for a larger study within the Aerosol Comparison between Observations and Models (AeroCOM) multimodel biomass burning experiment. By comparing multiple model results using the refined technique presented here, we aim to separate BB inventory from model-specific contributions to the remaining discrepancies.
      PubDate: 2017-10-16T15:11:09.959767-05:
      DOI: 10.1002/2017JD026693
  • Ozone Design Values in Southern California's Air Basins: Temporal
           Evolution and U.S. Background Contribution
    • Authors: David D. Parrish; Lindsay M. Young, Mia H. Newman, Kenneth C. Aikin, Thomas B. Ryerson
      Pages: 11,166 - 11,182
      Abstract: California's ambient ozone concentrations have two principal contributions: U.S. background ozone and enhancements produced from anthropogenic precursor emissions; only the latter effectively respond to California emission controls. From 1980 to 2015 ozone has been monitored in eight air basins in Southern California. The temporal evolution of the largest measured concentrations, i.e., those that define the ozone design value (ODV) upon which the National Ambient Air Quality Standard (NAAQS) is based, is described very well by an exponential decrease on top of a positive offset. We identify this offset as the ODV due to the U.S. background ozone (i.e., the concentration that would be present if U.S. anthropogenic precursor emissions were reduced to zero) and is estimated to be 62.0 ± 1.9 ppb in six of the basins. California's emission control efforts have reduced the anthropogenic ozone enhancements by a factor of ~5 since 1980. However, assuming that the current rate of exponential decrease is maintained and that U.S. background ODV remains constant, projections of the past decrease suggest that ~35 years of additional emission control efforts will be required to reach the new NAAQS of 70 ppb in the Los Angeles area. The growing predominance of U.S. background ozone contributions has shifted the maximum ozone concentrations in all air basins from later to earlier in the summer. Comparisons indicate that currently accepted model estimates of U.S. background ozone concentrations in Southern California are somewhat underestimated; thus, reducing ozone in this region to the 2015 NAAQS may be more difficult than currently expected.
      PubDate: 2017-10-23T03:11:22.53602-05:0
      DOI: 10.1002/2016JD026329
  • Impact of In-Cloud Aqueous Processes on the Chemistry and Transport of
           Biogenic Volatile Organic Compounds
    • Authors: Yang Li; Mary C. Barth, Edward G. Patton, Allison L. Steiner
      Pages: 11,131 - 11,153
      Abstract: We investigate the impacts of cloud aqueous processes on the chemistry and transport of biogenic volatile organic compounds (BVOC) using the National Center for Atmospheric Research's large-eddy simulation code with an updated chemical mechanism that includes both gas- and aqueous-phase reactions. We simulate transport and chemistry for a meteorological case with a diurnal pattern of nonprecipitating cumulus clouds from the Baltimore-Washington area DISCOVER-AQ campaign. We evaluate two scenarios with and without aqueous-phase chemical reactions. In the cloud layer (2–3 km), the addition of aqueous phase reactions decreases HCHO by 18% over the domain due to its solubility and the fast depletion from aqueous reactions, resulting in a corresponding decrease in radical oxidants (e.g., 18% decrease in OH). The decrease of OH increases the mixing ratios of isoprene and methacrolein (MACR) (100% and 15%, respectively) in the cloud layer because the reaction rate is lower. Aqueous-phase reactions can modify the segregation between OH and BVOC by changing the sign of the segregation intensity, causing up to 55% reduction in the isoprene-OH reaction rate and 40% reduction for the MACR-OH reaction when clouds are present. Analysis of the isoprene-OH covariance budget shows the chemistry term is the primary driver of the strong segregation in clouds, triggered by the decrease in OH. All organic acids except acetic acid are formed only through aqueous-phase reactions. For acids formed in the aqueous phase, turbulence mixes these compounds on short time scales, with the near-surface mixing ratios of these acids reaching 20% of the mixing ratios in the cloud layer within 1 h of cloud formation.
      PubDate: 2017-10-19T11:05:50.413777-05:
      DOI: 10.1002/2017JD026688
  • Automatic Cloud-Type Classification Based On the Combined Use of a Sky
           Camera and a Ceilometer
    • Authors: J. Huertas-Tato; F. J. Rodríguez-Benítez, C. Arbizu-Barrena, R. Aler-Mur, I. Galvan-Leon, D. Pozo-Vázquez
      Pages: 11,045 - 11,061
      Abstract: A methodology, aimed to be fully operational, for automatic cloud classification based on the synergetic use of a sky camera and a ceilometer is presented. The random forest machine learning algorithm was used to train the classifier with 19 input features: 12 extracted from the sky camera images and 7 from the ceilometer. The method was developed and tested based on a set of 717 images collected at the radiometric stations of the Univ. of Jaén (Spain). Up to nine different types of clouds (plus clear sky) were considered (clear sky, cumulus, stratocumulus, nimbostratus, altocumulus, altostratus, stratus, cirrocumulus, cirrostratus, and cirrus) plus an additional category multicloud, aiming to account for the frequent cases in which the sky is covered by several cloud types. A total of eight experiments was conducted by (1) excluding/including the ceilometer information, (2) including/excluding the multicloud category, and (3) using six or nine different cloud types, aside from the clear-sky and multicloud category. The method provided accuracies ranging from 45% to 78%, being highly dependent on the use of the ceilometer information. This information showed to be particularly relevant for accurately classifying “cumuliform” clouds and to account for the multicloud category. In this regard, the camera information alone was found to be not suitable to deal with this category. Finally, while the use of the ceilometer provided an overall superior performance, some limitations were found, mainly related to the classification of clouds with similar cloud base height and geometric thickness.
      PubDate: 2017-10-19T10:41:52.445978-05:
      DOI: 10.1002/2017JD027131
  • Ground and Airborne U.K. Measurements of Nitryl Chloride: An Investigation
           of the Role of Cl Atom Oxidation at Weybourne Atmospheric Observatory
    • Authors: Thomas J. Bannan; Asan Bacak, Michael Le Breton, Michael Flynn, Bin Ouyang, Matthew McLeod, Rod Jones, Tamsin L. Malkin, Lisa K. Whalley, Dwayne E. Heard, Brian Bandy, M. Anwar H. Khan, Dudley E. Shallcross, Carl J. Percival
      Pages: 11,154 - 11,165
      Abstract: Nitryl chloride (ClNO2) measurements from the Weybourne Atmospheric Observatory (WAO) are reported from March to April 2013 using a quadruple chemical ionization mass spectrometer with the I− ionization scheme. WAO is a rural coastal site with generally low NOx concentrations, a type of location poorly studied for ClNO2 production. Concentrations of ClNO2 exceeded that of the limit of detection (0.8 ppt) on each night of the campaign, as did concentrations of N2O5, which was also measured simultaneously with the Cambridge Broadband Cavity Enhanced Absorption Spectrometer. A peak concentration of 65 ppt of ClNO2 is reported here. Vertical profiles of ClNO2 from early- to middle-morning flights in close proximity to WAO are also reported, showing elevated concentrations at low altitude. The photolysis of observed ClNO2 and a box model utilizing the Master Chemical Mechanism modified to include chlorine chemistry was used to calculate Cl atom concentrations. This model utilized numerous VOCs from the second Tropospheric Organic Chemistry project in 2004, at the same location and time of year. From this the relative importance of the oxidation of three groups of measured VOCs (alkanes, alkenes, and alkynes) by OH radicals, Cl atoms, and O3 is compared. Cl atom oxidation was deemed generally insignificant at this time and location for total oxidation due to the much lower concentration of ClNO2 observed, even following the night of greatest ClNO2 production.
      PubDate: 2017-10-19T10:40:35.000399-05:
      DOI: 10.1002/2017JD026624
  • Development of Algorithm for Discriminating Hydrometeor Particle Types
           With a Synergistic Use of CloudSat and CALIPSO
    • Authors: M. Kikuchi; H. Okamoto, K. Sato, K. Suzuki, G. Cesana, Y. Hagihara, N. Takahashi, T. Hayasaka, R. Oki
      Pages: 11,022 - 11,044
      Abstract: We developed a method for classifying hydrometeor particle types, including cloud and precipitation phase and ice crystal habit, by a synergistic use of CloudSat/Cloud Profiling Radar (CPR) and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO)/Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP). We investigated how the cloud phase and ice crystal habit characterized by CALIOP globally relate with radar reflectivity and temperature. The global relationship thus identified was employed to develop an algorithm for hydrometeor type classification with CPR alone. The CPR-based type classification was then combined with CALIPSO-based type characterization to give CPR-CALIOP synergy classification. A unique aspect of this algorithm is to exploit and combine the lidar's sensitivity to thin ice clouds and the radar's ability to penetrate light precipitation to offer more complete picture of vertically resolved hydrometeor type classification than has been provided by previous studies. Given the complementary nature of radar and lidar detections of hydrometeors, our algorithm delivers 13 hydrometeor types: warm water, supercooled water, randomly oriented ice crystal (3D-ice), horizontally oriented plate (2D-plate), 3D-ice + 2D-plate, liquid drizzle, mixed-phase drizzle, rain, snow, mixed-phase cloud, water + liquid drizzle, water + rain, and unknown. The global statistics of three-dimensional occurrence frequency of each hydrometeor type revealed that 3D-ice contributes the most to the total cloud occurrence frequency (53.8%), followed by supercooled water (14.3%), 2D-plate (9.2%), rain (5.9%), warm water (5.7%), snow (4.8%), mixed-phase drizzle (2.3%), and the remaining types (4.0%). This hydrometeor type classification provides observation-based insight for climate model diagnostics in representation of cloud phase and their microphysical characteristics.
      PubDate: 2017-10-19T09:26:41.740437-05:
      DOI: 10.1002/2017JD027113
  • High-Resolution Large Eddy Simulation of Snow Accumulation in Alpine
    • Authors: Vincent Vionnet; Eric Martin, Valéry Masson, Christine Lac, Florence Naaim Bouvet, Gilbert Guyomarc'h
      Pages: 11,005 - 11,021
      Abstract: Snow accumulation in alpine terrain is controlled by three main processes that act at different spatial scales: (i) orographic snowfall, (ii) preferential deposition of snowfall, and (iii) wind-induced snow transport of deposited snow. The relative importance of these processes largely remains uncertain at small scale (10–100 m). This study presents how high-resolution coupled snowpack/atmosphere simulations help quantifying the effects of these processes. The simulation system consists of the detailed snowpack model Crocus and the atmospheric model Meso-NH used in Large Eddy Simulation mode. Dedicated routines allow the coupled system to explicitly simulate wind-induced snow transport. Our case study is a snowfall event that occurred in February 2011 in the French Alps. Three nested domains at 450, 150 and 50 m grid spacing allow the model to simulate the complex 3D precipitation and wind fields down to fine scale. We firstly assess the ability of the coupled model to reproduce meteorological conditions during the event (wind speed and direction, snowfall amount, and blowing snow fluxes). The spatial variability of snowfall and snow accumulation is then considered. At 50 m grid spacing, snowfall presents local maxima associated with the formation of rimed snow aggregates and graupel in regions of sustained updrafts. Variograms show that the resultant spatial variability of snowfall is lower than the variability of snow accumulation when considering snow transport. Despite an overestimation of simulated blowing fluxes, our results suggest that wind-induced snow transport is the main source of spatial variability of snow accumulation in our case study.
      PubDate: 2017-10-19T09:21:15.621022-05:
      DOI: 10.1002/2017JD026947
  • Meteoric Smoke Deposition in the Polar Regions: A Comparison of
           Measurements With Global Atmospheric Models
    • Authors: James S. A. Brooke; Wuhu Feng, Juan Diego Carrillo-Sánchez, Graham W. Mann, Alexander D. James, Charles G. Bardeen, John M. C. Plane
      Pages: 11,112 - 11,130
      Abstract: The accumulation rate of meteoric smoke particles (MSPs) in ice cores—determined from the trace elements Ir and Pt, and superparamagnetic Fe particles—is significantly higher than expected from the measured vertical fluxes of Na and Fe atoms in the upper mesosphere and the surface deposition of cosmic spherules. The Whole Atmosphere Community Climate Model with the Community Aerosol and Radiation Model for Atmospheres has been used to simulate MSP production, transport, and deposition, using a global MSP input of 7.9 t d−1 based on these other measurements. The modeled MSP deposition rates are smaller than the measurements by factors of ~32 in Greenland and ~12 in Antarctica, even after reanalysis of the Ir/Pt ice core data with inclusion of a volcanic source. Variations of the model deposition scheme and use of the United Kingdom Chemistry and Aerosols model do not improve the agreement. Direct removal of MSP-nucleated polar stratospheric cloud particles to the surface gives much better agreement, but would result in an unfeasibly high rate of nitrate deposition. The unablated fraction of cosmic dust (~35 t d−1) would provide sufficient Ir and Pt to account for the Antarctic measurements, but the relatively small flux of these large (>3 μm) particles would lead to greater variability in the ice core measurements than is observed, although this would be partly offset if significant fragmentation of cosmic dust particles occurred during atmospheric entry. Future directions to resolve these discrepancies between models and measurements are also discussed.
      PubDate: 2017-10-16T15:05:40.708053-05:
      DOI: 10.1002/2017JD027143
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