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Journal Cover Journal of Geophysical Research : Atmospheres
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   ISSN (Online) 2169-8996
   Published by AGU Homepage  [17 journals]
  • Impacts of Horizontal Propagation of Orographic Gravity Waves on the Wave
           Drag in the Stratosphere and Lower Mesosphere
    • Authors: Xin Xu; Yuan Wang, Ming Xue, Kefeng Zhu
      Abstract: The impact of horizontal propagation of mountain waves on the orographic gravity wave drag (OGWD) in the stratosphere and lower mesosphere of Northern Hemisphere is evaluated for the first time. Using a fine-resolution (1-arc minute) terrain and 2.5°×2.5° ECMWF ERA-interim reanalysis data during 2011-2016, two sets of OGWD are calculated offline according to a traditional parameterization scheme (without horizontal propagation) and a newly proposed scheme (with horizontal propagation). In both cases, the zonal-mean OGWD show similar spatial patterns and undergo a notable seasonal variation. In winter, the OGWD is mainly distributed in the upper stratosphere and lower mesosphere of mid-high latitudes, whereas the summertime OGWD is confined in the lower stratosphere. Comparison between the two sets of OGWD reveal that the horizontal propagation of mountain waves tends to decrease (increase) the OGWD in the lower stratosphere (mid-upper stratosphere and lower mesosphere). Consequently, including the horizontal propagation of mountain waves in the parameterization of OGWD can reduce the excessive OGWD in the lower stratosphere and strengthen the insufficient gravity wave forcing in the mesosphere, which are the known problems of traditional OGWD schemes. The impact of horizontal propagation is more prominent in winter than in summer, with the OGWD in western Tibetan Plateau, Rocky Mountains and Greenland notably affected.
      PubDate: 2017-10-15T22:15:22.721959-05:
      DOI: 10.1002/2017JD027528
  • 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
  • Chemical Characteristics of Organic Aerosols in Shanghai: A Study by
           Ultra-High-Performance Liquid Chromatography Coupled with Orbitrap Mass
    • Authors: Xinke Wang; Nathalie Hayeck, Martin Brüggemann, Lei Yao, Hangfei Chen, Ci Zhang, Corinne Emmelin, Jianmin Chen, Christian George, Lin Wang
      Abstract: PM2.5 filter samples were collected in July and October 2014 and January and April 2015 in urban Shanghai, and analyzed using ultra-high-performance liquid chromatography (UHPLC) coupled to Orbitrap mass spectrometry (MS). The measured chromatogram-mass spectra were processed by a non-target screening approach to identify significant signals. In total, 810-1510 chemical formulas of organic compounds in the negative polarity (ESI-) and 860-1790 in the positive polarity (ESI+), respectively, were determined. The chemical characteristics of organic aerosols (OAs) in Shanghai varied among different months and between daytime and nighttime. In the January samples, organics were generally richer in terms of both number and abundance, whereas those in the July samples were far lower. More CHO- (compounds containing only carbon, hydrogen, and oxygen and detected in ESI-) and CHOS- (sulfur-containing organics) were found in the daytime samples, suggesting a photochemical source, whereas CHONS- (nitrogen- and sulfur-containing organics) were more abundant in the nighttime samples, due to nocturnal nitrate radical chemistry. A significant number of mono- and polycyclic aromatic compounds, and nitrogen- and sulfur-containing heterocyclic compounds were detected in all samples, indicating that biomass burning and fossil fuel combustion made important contributions to the OAs in urban Shanghai. Additionally, precursor-product pair analysis indicates that the epoxide pathway is an important formation route for organosulfates observed in Shanghai. Moreover, a similar analysis suggests that 35-57% of nitrogen-containing compounds detected in ESI+ could be formed through reactions between ammonia and carbonyls. Our study presents a comprehensive overview of OAs in urban Shanghai, which helps to understand their characteristics and sources.
      PubDate: 2017-10-14T07:55:26.954445-05:
      DOI: 10.1002/2017JD026930
  • 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
  • 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
      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 non-precipitating 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 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 one hour of cloud formation.
      PubDate: 2017-10-08T19:35:29.4687-05:00
      DOI: 10.1002/2017JD026688
  • 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
  • The Record Los Angeles Heat Event of September 2010 Part I: Synoptic –
           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
      Abstract: On 27 September 2010 the Los Angeles Civic Center reached its all-time record maximum temperature of 45 °C before 1330 LDT 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 and the IPV 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 the 27th. This allows the advection of low-level warm air from the inland terrain towards the coastally-trapped disturbance and descending circulation resulting in record heating.
      PubDate: 2017-10-04T09:50:51.53034-05:0
      DOI: 10.1002/2017JD027162
  • 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
  • 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
      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 MODIS data using a linear trend detection method, and standardized precipitation evapotranspiration index (SPEI). 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: 2017-10-04T09:40:24.876294-05:
      DOI: 10.1002/2017JD027186
  • 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
      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 9 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 multi-cloud, aiming to account for the frequent cases in which the sky is covered by several cloud types. A total of 8 experiments were conducted by: 1) excluding/including the ceilometer information; 2) including/excluding the multi-cloud category and 3) using 6 or 9 different cloud types, aside from the clear sky and multi-cloud 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 multi-cloud category. At 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: 2017-10-04T09:35:22.5224-05:00
      DOI: 10.1002/2017JD027131
  • 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
  • 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, Mike 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
      Abstract: Nitryl Chloride (ClNO2) measurements from the Weybourne Atmospheric Observatory (WAO) are reported from March and April 2013 using a quadruple chemical ionisation mass spectrometer (CIMS) with the I- ionisation 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 (BBCEAS). A peak concentration of 65 ppt of ClNO2 is reported here. Vertical profiles of ClNO2 from early- to mid-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 utilising the Master Chemical Mechanism modified to include chlorine chemistry was used to calculate Cl atom concentrations. This model utilised numerous VOCs from the second Tropospheric ORganic CHemistry project (TORCH 2) 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: 2017-09-30T07:55:20.182619-05:
      DOI: 10.1002/2017JD026624
  • 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
      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 multi-sensor 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: 2017-09-30T07:00:21.679835-05:
      DOI: 10.1002/2017JD027035
  • Chemistry of Volatile Organic Compounds in the Los Angeles Basin:
           Nighttime Removal of Alkenes and Determination of Emission Ratios
    • Authors: J. A. Gouw; J. B. Gilman, S.-W. Kim, B. M. Lerner, G. Isaacman-VanWertz, B. C. McDonald, C. Warneke, W. C. Kuster, B. L. Lefer, S. M. Griffith, S. Dusanter, P. S. Stevens, J. Stutz
      Abstract: We reanalyze a data set of hydrocarbons in ambient air obtained by gas chromatography-mass spectrometry at a surface site in Pasadena in the Los Angeles basin during the NOAA California Nexus study (CalNex) in 2010. The number of hydrocarbon compounds quantified from the chromatograms is expanded through the use of new peak-fitting data analysis software. We also re-examine hydrocarbon removal processes. For alkanes, small alkenes and aromatics, the removal is determined by the reaction with hydroxyl (OH) radicals. For several highly reactive alkenes, the nighttime removal by ozone and nitrate (NO3) radicals is also significant. We discuss how this nighttime removal affects the determination of emission ratios vs. carbon monoxide (CO) and show that previous estimates based on nighttime correlations with CO were too low. We analyze model output from the chemistry-transport model WRF-Chem for hydrocarbons and radicals at the Pasadena location to evaluate our methods for determining emission ratios from the measurements. We find that our methods agree with the modeled emission ratios for the domain centered on Pasadena, and that the modeled emission ratios vary by 23% across the wider South Coast basin. We compare the alkene emission ratios with published results from ambient measurements, and from tunnel and dynamometer studies of motor vehicle emissions. We find that with few exceptions the composition of alkene emissions determined from the measurements in Pasadena closely resembles that of motor vehicle emissions.
      PubDate: 2017-09-30T06:50:28.346226-05:
      DOI: 10.1002/2017JD027459
  • Evaluation of WRF Simulations With Different Selections of Subgrid
           Orographic Drag Over the Tibetan Plateau
    • Authors: X. Zhou; A. Beljaars, Y. Wang, B. Huang, C. Lin, Y. Chen, H. Wu
      Abstract: Weather Research and Forecasting (WRF) simulations with different selections of subgrid orographic drag over the Tibetan Plateau have been evaluated with observation and ERA-Interim reanalysis. Results show that the subgrid orographic drag schemes, especially the turbulent orographic form drag (TOFD) scheme, efficiently reduce the 10 m wind speed bias and RMS error with respect to station measurements. With the combination of gravity wave, flow blocking and TOFD schemes, wind speed is simulated more realistically than with the individual schemes only. Improvements are also seen in the 2 m air temperature and surface pressure. The gravity wave drag, flow blocking drag, and TOFD schemes combined have the smallest station mean bias (−2.05°C in 2 m air temperature and 1.27 hPa in surface pressure) and RMS error (3.59°C in 2 m air temperature and 2.37 hPa in surface pressure). Meanwhile, the TOFD scheme contributes more to the improvements than the gravity wave drag and flow blocking schemes. The improvements are more pronounced at low levels of the atmosphere than at high levels due to the stronger drag enhancement on the low-level flow. The reduced near-surface cold bias and high-pressure bias over the Tibetan Plateau are the result of changes in the low-level wind components associated with the geostrophic balance. The enhanced drag directly leads to weakened westerlies but also enhances the a-geostrophic flow in this case reducing (enhancing) the northerlies (southerlies), which bring more warm air across the Himalaya Mountain ranges from South Asia (bring less cold air from the north) to the interior Tibetan Plateau.
      PubDate: 2017-09-29T23:16:09.795343-05:
      DOI: 10.1002/2017JD027212
  • High-Resolution Regional Reanalysis in China: Evaluation of One-year
           Period Experiments
    • Authors: Qi Zhang; Yinong Pan, Shuyu Wang, Jianjun Xu, Jianping Tang
      Abstract: Globally, reanalysis datasets 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 datasets 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 (MCS), squall lines, tropical cyclones, regional droughts and heat waves. In this study, by using a data assimilation system of Gridpoint Statistical Interpolation (GSI), and a mesoscale atmospheric model of WRF (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 testbed datasets are generated for year 2013 via three widely used methods (classical dynamical downscaling, spectral nudging, data assimilation) and a hybrid method with data assimilation coupled with spectral nudging. Temperature at 2m, 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: 2017-09-29T20:45:25.03017-05:0
      DOI: 10.1002/2017JD027476
  • 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
  • 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
      Abstract: Since March 2014, there is a continuous measurement of secondary cosmic rays (CRs) by the detector system SEVAN (Space Environmental Viewing and Analysis Network) at Lomnický štít, altitude 2634 masl. Starting from June 2016, the count rates (1s resolution) obtained from the three SEVAN detectors and from their coincidences are available, along with selected meteorological characteristics. Since May 30, 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 September 17, 2016. Examples of these measurements are presented and discussed. Barometric pressure correction and elimination of low frequency variability from the signal allows 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: 2017-09-26T21:05:37.189046-05:
      DOI: 10.1002/2016JD026439
  • Modeling C1-C4 alkyl nitrate photochemistry and their impacts on O3
           production in urban and suburban environments of Hong Kong
    • Authors: X. P. Lyu; H. Guo, N. Wang, I. J. Simpson, H. R. Cheng, L. W. Zeng, S. M. Saunders, S. H. M. Lam, S. Meinardi, D. R. Blake
      Abstract: As intermediate products of photochemical reactions, alkyl nitrates (RONO2) regulate ozone (O3) formation. In this study, a photochemical box model (PBM) incorporating master chemical mechanism (MCM) well reproduced the observed RONO2 at an urban and a mountainous site, with index of agreement (IOA) in the range of 0.66-0.73. 0.0003 was identified to be the most appropriate branching ratio for C1 RONO2, with the error less than 50%. Although levels of the parent hydrocarbons and nitric oxide (NO) were significantly higher at the urban site than the mountainous site, the production of C2-C3 RONO2 was comparable to or even lower than at the mountainous site, due to the lower concentrations of oxidative radicals in the urban environment. Based on the profiles of air pollutants at the mountainous site, the formation of C2-C4 RONO2 was limited by NOx (VOCs) when TVOCs/NOx was higher (lower) than 10.0±0.4 ppbv/ppbv. This dividing ratio decreased (p
      PubDate: 2017-09-26T17:00:30.69231-05:0
      DOI: 10.1002/2017JD027315
  • 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
  • Heatwaves 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
      Abstract: Based on the daily maximum temperatures (Tmax) from 587 surface observation stations in China during 1959-2013, heatwaves are detected using both absolute and relative definitions. The spatiotemporal variations of heatwave 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 (EOF), the integral index heatwave total intensity (HWTI) 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 heatwaves 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 anomalies in the tropical western and eastern Pacific, respectively.
      PubDate: 2017-09-26T10:35:33.131043-05:
      DOI: 10.1002/2017JD027180
  • The Influence of Recurrent Modes of Climate Variability on the Occurrence
           of Monthly Temperature Extremes over South America
    • Authors: Paul C. Loikith; Judah Detzer, Carlos R. Mechoso, Kyo Lee, Armineh Barkhordarian
      Abstract: The associations between extreme temperature months and four prominent modes of recurrent climate variability are examined over South America. Associations are computed as the percent of extreme temperature months concurrent with the upper and lower quartiles of the El Niño Southern Oscillation (ENSO), the Atlantic Niño, the Pacific Decadal Oscillation (PDO), and the Southern Annular Mode (SAM) index distributions, stratified by season. The relationship is strongest for ENSO, with nearly every extreme temperature month concurrent with the upper or lower quartiles of its distribution in portions of northwestern South America during some seasons. The likelihood of extreme warm temperatures is enhanced over parts of northern South America when the Atlantic Niño index is in the upper quartile, while cold extremes are often association with the lowest quartile. Concurrent precipitation anomalies may contribute to these relations. The PDO shows weak associations during December, January, and February, while in June, July, and August its relationship with extreme warm temperatures closely matches that of ENSO. This may be due to the positive relationship between the PDO and ENSO, rather than the PDO acting as an independent physical mechanism. Over Patagonia, the SAM is highly influential during spring and fall, with warm and cold extremes being associated with positive and negative phases of the SAM respectively. Composites of sea level pressure anomalies for extreme temperature months over Patagonia suggest an important role of local synoptic scale weather variability in addition to a favorable SAM for the occurrence of these extremes.
      PubDate: 2017-09-26T10:05:45.930169-05:
      DOI: 10.1002/2017JD027561
  • 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
      Abstract: In this work, we use the CERES FluxByCloudTyp data product (FBCTObs), which calculates 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 comprised 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 (JFD) 2008, FBCTSim shows that HadGEM2-A cloud tops are higher in altitude than in FBCTObs, but also have higher values of 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: 2017-09-25T10:12:51.64427-05:0
      DOI: 10.1002/2017JD027076
  • 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
  • Development of a Polar Stratospheric Cloud Model within the Community
           Earth System Model: Assessment of 2010 Antarctic Winter
    • Authors: Yunqian Zhu; Owen B. Toon, Alyn Lambert, Douglas E. Kinnison, Charles Bardeen, Michael C. Pitts
      Abstract: To simulate polar stratospheric clouds (PSCs) during the Antarctic winter of 2010, we have developed a PSC model within the Community Earth System Model framework that includes detailed microphysics of sulfuric aerosols and three types of PSCs: supercooled ternary solution (STS), nitric acid trihydrate (NAT) and ice. Our model includes two major NAT formation mechanisms both of which are essential to reproduce the PSC and gas phase chemical features in the 2010 Antarctic winter. Homogeneous nucleation of NAT from STS produces NAT particles with sizes near 8 μm, which are important to properly simulate denitrification and the gas phase HNO3 observed by the Microwave Limb Sounder (MLS). Heterogeneous nucleation of NAT on ice particles or ice particles on NAT and subsequent evaporation of the ice produces NAT particles with sizes from sub-micrometers to a few micrometers. These particles account for the large backscattering ratio from NAT observed by the CALIPSO satellite, especially in the mid-winter season. Adding temperature fluctuations from gravity waves is important to produce larger number density and higher backscattering ratio from ice and NAT particles. However, our model needs a better representation of waves to improve the backscattering ratio and gas phase HNO3 compared with observations. Our model also includes homogeneous nucleation of ice from STS and heterogeneous nucleation of ice on NAT. The model reproduces the gas phase H2O during the winter within the uncertainty of the MLS observations.
      PubDate: 2017-09-25T10:12:45.28301-05:0
      DOI: 10.1002/2017JD027003
  • 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
      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 mid- 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: 2017-09-25T10:12:23.760385-05:
      DOI: 10.1002/2017JD026696
  • The turbulent structure of the Arctic summer boundary layer during ASCOS
    • Authors: Ian M. Brooks; Michael Tjernström, P. Ola G. Persson, Matthew D. Shupe, Rebecca A. Atkinson, Guylaine Canut, Cathryn E. Birch, Thorsten Mauritsen, Joseph Sedlar, Barbara J. Brooks
      Abstract: The mostly ice-covered Arctic Ocean is dominated by low-level liquid- or mixed-phase clouds. Turbulence within stratocumulus is primarily driven by cloud-top cooling that induces convective instability. Using a suite of in situ and remote sensing instruments we characterize turbulent mixing in Arctic stratocumulus, and for the first time we estimate profiles of the gradient Richardson number at relatively high resolution in both time (10 minutes) and altitude (10 m). It is found that the mixing occurs both within the cloud, as expected, and by wind shear instability near the surface. About 75 percent of the time these two layers are separated by a stably stratified inversion at 100–200 m altitude. Exceptions are associated with low cloud bases that allow the cloud-driven turbulence to reach the surface. The results imply that turbulent coupling between the surface and the cloud is sporadic or intermittent.
      PubDate: 2017-09-22T12:40:23.910046-05:
      DOI: 10.1002/2017JD027234
  • 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
      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 mid- 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 20CR reanalysis datasets (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: 2017-09-22T10:58:41.171049-05:
      DOI: 10.1002/2017JD027167
  • Modulation of Soil Initial State on WRF Model Performance over China
    • Authors: Haile Xue; Qinjian Jin, Bingqi Yi, Gretchen L. Mullendore, Xiaohui Zheng, Hongchun Jin
      Abstract: The soil state (e.g. temperature and moisture) in a mesoscale numerical prediction model is typically initialized by reanalysis or analysis data that may be subject to large bias. Such bias may lead to unrealistic land–atmosphere interactions. This study shows that the Climate Forecast System Reanalysis (CFSR) dramatically underestimates soil temperature and overestimates soil moisture over most parts of China in the first (0-10cm) and second (10-25cm) soil layers compared to in situ observations in July, 2013. A correction based on the global optimal dual kriging is employed to correct CFSR bias in soil temperature and moisture using in situ observations. To investigate the impacts of the corrected soil state on model forecasts, two numerical model simulations—a control run with CFSR soil state and a disturbed run with the corrected soil state—were conducted using the Weather Research and Forecasting model. All the simulations are initiated four times per day and run 48 hours. Model results show that the corrected soil state, i.e. warmer and drier surface over the most parts of China, can enhance evaporation over wet regions, which changes the overlying atmospheric temperature and moisture. The changes of the lifting condensation level, level of free convection, and water transport due to corrected soil state favor precipitation over wet regions, while prohibiting precipitation over dry regions. Moreover, diagnoses indicate that the remote moisture flux convergence plays a dominant role in the precipitation changes over the wet regions.
      PubDate: 2017-09-21T17:05:59.791675-05:
      DOI: 10.1002/2017JD027023
  • Using observations and source specific model tracers to characterize
           pollutant transport during FRAPPÉ and DISCOVER-AQ
    • Authors: G. G. Pfister; P. Reddy, M. C. Barth, F. F. Flocke, A. Fried, S. C. Herndon, B. C. Sive, J. T. Sullivan, A. M. Thompson, T. I. Yacovitch, A. J. Weinheimer, A. Wisthaler
      Abstract: Transport is a key parameter in air quality research and plays a dominant role in the Colorado Northern Front Range Metropolitan Area (NFRMA), where terrain induced flows and recirculation patterns can lead to vigorous mixing of different emission sources. To assess different transport processes and their connection to air quality in the NFRMA during the FRAPPÉ and DISCOVER-AQ campaigns in summer 2014, we use the Weather Research and Forecasting Model with inert tracers. Overall, the model represents well the measured winds and the inert tracers are in good agreement with observations of comparable trace gas concentrations. The model tracers support the analysis of surface wind and ozone measurements and allow for the analysis of transport patterns and interactions of emissions. A main focus of this study is on characterizing pollution transport from the NFRMA to the mountains by mountain-valley flows and the potential for recirculating pollution back into the NFRMA. One such event on 12 August 2014 was well captured by the aircraft and is studied in more detail. The model represents the flow conditions and demonstrates that during upslope events, frequently there is a separation of air masses that are heavily influenced by oil and gas emissions to the North and dominated by urban emissions to the South. This case study provides evidence that NFRMA pollution not only can impact the nearby Foothills and mountain areas to the East of the Continental Divide, but that pollution can "spill over" into the valleys to the West of the Continental Divide.
      PubDate: 2017-09-21T17:00:29.177516-05:
      DOI: 10.1002/2017JD027257
  • An assessment of drift correction alternatives for CMIP5 decadal
    • Authors: Dipayan Choudhury; Alexander Sen Gupta, Ashish Sharma, Rajeshwar Mehrotra, Bellie Sivakumar
      Abstract: Drift correction is an important step before using the outputs of decadal prediction experiments and has seen considerable research. However, most drift-correction studies consider a relatively small sample of variables and models. Here, we present a systematic application of the existing drift correction strategies for decadal predictions of various sea surface temperature (SST)-based metrics from a suite of five state-of-the-art climate models (CanCM4i1, GFDL-CM2.1, HadCM3i2&i3, MIROC5 and MPI-ESM-LR). The best method of drift correction for each metric and model is reported. Preliminary analysis suggests that there is no single method of drift correction that consistently performs best. Initial condition-based drift correction (ICDC) provides the lowest errors in most regions for MIROC5 and the two HadCM3 models, while the trend-based drift correction (TrDC) produces lowest errors for CanCM4i1, GFDL-CM2.1 and MPI-ESM-LR over the largest share of the area. There is no merit in using a k-nearest neighbour (k-nn) approach for these drift-correction methods. Further, in almost all cases, the multi-model ensemble (MME) outperforms the individual models, and thus the study conclusively suggests using forecasts based on multi-model averages. We also show some additional benefit to be gained by drift correcting each model/metric using their best correction method prior to model averaging and suggest that the results presented here would help potential users expend time and resources judiciously while dealing with outputs from these experiments.
      PubDate: 2017-09-21T16:46:20.501756-05:
      DOI: 10.1002/2017JD026900
  • Radio-frequency electromagnetic radiation from streamer collisions
    • Authors: A. Luque
      Abstract: We present a full electromagnetic model of streamer propagation where the Maxwell equations are solved self-consistently together with electron transport and reactions including photo-ionization. We apply this model to the collision of counter-propagating streamers in gaps tens of centimeters wide and with large potential differences of hundreds of kilovolts. Our results show that streamer collisions emit electromagnetic pulses that, at atmospheric pressure, dominate the radiofrequency spectrum of an extended corona in the range from about 100MHz to a few gigahertz. We also investigate the fast penetration, after a collision, of electromagnetic fields into the streamer heads and show that these fields are capable of accelerating electrons up to about 100keV. By substantiating the link between X-rays and high-frequency radio emissions and by describing a mechanism for the early acceleration of run-away electrons, our results support the hypothesis that streamer collisions are essential precursors of high-energy processes in electric discharges.
      PubDate: 2017-09-20T05:55:34.302539-05:
      DOI: 10.1002/2017JD027157
  • AERONET-based nonspherical dust optical models and effects on the VIIRS
           Deep Blue/SOAR over-water aerosol product
    • Authors: Jaehwa Lee; N. Christina Hsu, Andrew M. Sayer, Corey Bettenhausen, Ping Yang
      Abstract: Aerosol Robotic Network (AERONET)-based nonspherical dust optical models are developed and applied to the Satellite Ocean Aerosol Retrieval (SOAR) algorithm as part of the Version 1 Visible Infrared Imaging Radiometer Suite (VIIRS) NASA ‘Deep Blue’ aerosol data product suite. The optical models are created using Version 2 AERONET inversion data at six distinct sites influenced frequently by dust aerosols from different source regions. The same spheroid shape distribution as used in the AERONET inversion algorithm is assumed to account for the nonspherical characteristics of mineral dust, which ensures the consistency between the bulk scattering properties of the developed optical models with the AERONET-retrieved microphysical and optical properties. For the Version 1 SOAR aerosol product, the dust optical models representative for Capo Verde site are used, considering the strong influence of Saharan dust over the global ocean in terms of amount and spatial coverage. Comparisons of the VIIRS-retrieved aerosol optical properties against AERONET direct-Sun observations at three island/coastal sites suggest that the use of nonspherical dust optical models significantly improves the retrievals of aerosol optical depth (AOD) and Ångström exponent by mitigating the well-known artifact of scattering angle dependence of the variables observed when incorrectly assuming spherical dust. The resulting removal of these artifacts results in a more natural spatial pattern of AOD along the transport path of Saharan dust to the Atlantic Ocean; i.e., AOD decreases with increasing distance transported, whereas the spherical assumption leads to a strong wave pattern due to the spurious scattering angle dependence of AOD.
      PubDate: 2017-09-17T23:55:20.748196-05:
      DOI: 10.1002/2017JD027258
  • 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
  • Hydrological drought in the Anthropocene: impacts of local water
           extraction and reservoir regulation in the US
    • Authors: Wenhua Wan; Jianshi Zhao, Hong-Yi Li, Ashok Mishra, L. Ruby Leung, Mohamad Hejazi, Wei Wang, Hui Lu, Zhiqun Deng, Yonas Demissisie, Hao Wang
      Abstract: Hydrological drought is a substantial negative deviation from normal hydrologic conditions and is influenced by climate and human activities such as water management. By perturbing the streamflow regime, climate change and water management may significantly alter drought characteristics in the future. Here we utilize a high-resolution integrated modeling framework that represents water management in terms of both local surface water extraction and reservoir regulation, and use the Standardized Streamflow Index (SSI) to quantify hydrological drought. We explore the impacts of water management on hydrological drought over the contiguous US in a warming climate with and without emissions mitigation. Despite the uncertainty of climate change impacts, local surface water extraction consistently intensifies drought that dominates at the regional to national scale. However, reservoir regulation alleviates drought by enhancing summer flow downstream of reservoirs. The relative dominance of drought intensification or relief is largely determined by the water demand, with drought intensification dominating in regions with intense water demand such as the Great Plains and California, while drought relief dominates in regions with low water demand. At the national level, water management increases the spatial extent of extreme drought despite some alleviations of moderate to severe drought. In an emissions mitigation scenario with increased irrigation demand for bioenergy production, water management intensifies drought more than the business-as-usual scenario at the national level, so the impacts of emissions mitigation must be evaluated by considering its benefit in reducing warming and evapotranspiration against its effects on increasing water demand and intensifying drought.
      PubDate: 2017-09-17T23:25:43.05456-05:0
      DOI: 10.1002/2017JD026899
  • A universal Ts − VI triangle method for the continuous retrieval of
           evaporative fraction from MODIS products
    • Authors: Wenbin Zhu; Shaofeng Jia, Aifeng Lv
      Abstract: The triangle method based on the spatial relationship between remotely sensed land surface temperature (Ts) and vegetation index (VI) has been widely used for the estimates of evaporative fraction (EF). In the present study, a universal triangle method was proposed by transforming the Ts − VI feature space from a regional scale to a pixel scale. The retrieval of EF is only related to the boundary conditions at pixel scale, regardless of the Ts − VI configuration over the spatial domain. The boundary conditions of each pixel are composed of the theoretical dry edge determined by the surface energy balance principle and the wet edge determined by the average air temperature of open water. The universal triangle method was validated using the EF observations collected by the Energy Balance Bowen Ratio systems in the Southern Great Plains of the United States of America (USA). Two parameterization schemes of EF were used to demonstrate their applicability with Terra MODerate-resolution Imaging Spectroradiometer (MODIS) products over the whole year 2004. The results of this study show that the accuracy produced by both of these two parameterization schemes is comparable to that produced by the traditional triangle method, although the universal triangle method seems specifically suited to the parameterization scheme proposed in our previous research. The independence of the universal triangle method from the Ts − VI feature space makes it possible to conduct a continuous monitoring of evapotranspiration and soil moisture. That is just the ability the traditional triangle method does not possess.
      PubDate: 2017-09-15T22:05:43.416555-05:
      DOI: 10.1002/2017JD026964
  • Observed co-variations of aerosol optical depth and cloud cover in
           extratropical cyclones
    • Authors: Catherine M. Naud; Derek J. Posselt, Susan C. Heever
      Abstract: Using NASA Moderate Resolution Imaging Spectroradiometer aerosol optical depth and total cloud cover retrievals, CloudSat-CALIPSO cloud profiles, and a database of extratropical cyclones and frontal boundary locations, relationships between changes in aerosol optical depth and cloud cover in extratropical cyclones occurring over northern hemisphere oceans are examined. A reanalysis dataset is used to constrain column water vapor and ascent strength in the cyclones in an attempt to distinguish their impact on cloud cover from the effect of changes in aerosol loading. The results suggest that high aerosol optical depth cyclones exhibit larger mid- and high- level cloud cover than their low aerosol optical depth counterparts. However, the opposite behavior is found for low-level cloud cover. These relationships are found to depend on the large-scale environment, in particular the column water vapor and vertical motion. Despite the inability of the observations to provide a causal physical link between aerosol load and cloud cover, these results can help to constrain and evaluate model simulations.
      PubDate: 2017-09-15T21:40:31.082443-05:
      DOI: 10.1002/2017JD027240
  • 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
  • Simulation of optical properties, direct and indirect radiative effects of
           smoke aerosols over marine stratocumulus clouds during summer 2008 in
           California with the regional climate model RegCM.
    • Authors: M. Mallet; F. Solmon, L. Roblou, F. Peers, S. Turquety, F. Waquet, H. Jethva, O. Torres
      Abstract: The regional climate model RegCM has been modified to better account for the climatic effects of biomass burning particles. Smoke aerosols are represented by new tracers with consistent radiative and hygroscopic properties to simulate the direct radiative forcing (DRF) and a new parameterization has been integrated for relating the droplet number concentration to the aerosol concentration for marine stratocumulus clouds (Sc). RegCM has been tested during the summer 2008 over California, when extreme concentration of smoke, together with the presence of Sc, is observed. This work indicates that significant AOD (~1-2 at 550 nm) are related to the intense 2008 fires. Compared to OMI and MODIS, the regional pattern of RegCM AOD is well represented although the magnitude is lower than satellite observations. Comparisons with POLDER Above-Clouds Aerosol Optical Depth (ACAOD) show the ability of RegCM to simulate realistic ACAOD during the transport of smoke above the Pacific Ocean. The simulated single scattering albedo (SSA) is ~0.90 (at 550 nm) near biomass-burning sources, consistent with OMI and POLDER, and smoke leads to shortwave heating rates ~1.5-2 °K day−1. RegCM is not able to correctly resolve the daily patterns in cloud properties notably due to its coarse horizontal resolutions. However, the changes in the sign of the DRF at TOA (negative to positive) from clear-sky to all-sky conditions is well simulated. Finally, the « aerosol-cloud » parameterization allows simulating an increase of the cloud optical depth (COD) for significant concentrations, leading to large perturbations of radiative fluxes at TOA.
      PubDate: 2017-09-15T12:10:39.328699-05:
      DOI: 10.1002/2017JD026905
  • Planetary wave characteristics in the lower atmosphere over Xianghe
           (117.00°E, 39.77°N), China revealed by the Beijing MST radar and MERRA
    • Authors: Chunming Huang; Shaodong Zhang, Gang Chen, Siyu Zhang, Kaiming Huang
      Abstract: Using observation data from the Beijing MST radar from December 2013 to November 2014, together with the MERRA data, the dominant planetary waves (PWs) in the lower atmosphere over Xianghe (117.00°E, 39.77°N), i.e., quasi-16-day and quasi-10-day oscillations, were identified and investigated. These two kinds of PWs displayed similar seasonal and height variations, indicating they may have similar generation sources and dissipation processes. For both of them, near the tropospheric jet, significant zonal amplitudes could be observed in winter and spring months; quasi-constant phase or partial vertical wavelength larger than 100 km was present in the zonal wind in December, March and April, indicating they were quasi vertical stasnding waves near the tropospheric jet. The calculated refractive indexes of these two PWs were significantly negative in the lower troposphere (3.5-5 km) and near the tropopause (15-20 km), and the resulted strong wave evanescence or even wave reflection could explain the observed quasi standing structure of these two PWs and height variations of their wind amplitudes. Their estimated zonal wavenumbers in every month both showed the prevailing eastward propagation. Furthermore, we investigated the impact of PWs on the background wind by E-P fluxes and divergences, which indicates that both the quasi-16-day and the quasi-10-day PWs, especially the latter, may contribute significantly to the construction and maintenance of the tropospheric jet. We also found that the tropospheric jet magnitude and height were both intensively modulated by the quasi-16-day and quasi-10-day PWs.
      PubDate: 2017-09-14T09:55:21.297787-05:
      DOI: 10.1002/2017JD027029
  • Concurrent temporal and spatial trends in sulfate and organic mass
           concentrations measured in the IMPROVE monitoring program
    • Authors: William C. Malm; Bret A. Schichtel, Jenny L. Hand, Jeffrey L. Collett, Jr
      Abstract: Recent modeling and field studies have highlighted a relationship between sulfate concentrations and secondarily formed organic aerosols related to isoprene and other volatile biogenic gaseous emissions. The relationship between these biogenic emissions and sulfate is thought to be primarily associated with the effect of sulfate on aerosol acidity, increased aerosol water at high relative humidities, and aerosol volume. The Interagency Monitoring of Protected Visual Environments (IMPROVE) program provides aerosol concentration levels of sulfate (SO4) and organic carbon (OC) at 136 monitoring sites in rural and remote areas of the United States over time periods of between 15 and 28 years. This dataset allows for an examination of relationships between these variables over time and space. The relative decreases in SO4 and OC were similar over most of the eastern United States, even though concentrations varied dramatically from one region to another. The analysis implied that for every unit decrease in SO4 there was about a 0.29 decrease in organic aerosol mass (OA = 1.8 × OC). This translated to a 2 μg/m3 decrease in biogenically derived secondary organic aerosol over 15 years in the southeastern United States. The analysis further implied that 35% and 27% in 2001 and 2015, respectively, of average total OA may be biogenically derived secondary organic aerosols and that there was a small but significant decrease in OA not linked to changes in SO4 concentrations. The analysis yields a constraint on ambient SO4–OC relationships that should help to refine and improve regional-scale chemical transport models.
      PubDate: 2017-09-14T09:50:25.2131-05:00
      DOI: 10.1002/2017JD026865
  • 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
      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 (WACCM) with the Community Aerosol and Radiation Model for Atmospheres (CARMA) 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 (UKCA) 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: 2017-09-14T09:45:31.447759-05:
      DOI: 10.1002/2017JD027143
  • Sub-Tropical Dust Storms and Downslope Wind Events
    • Authors: Ashok Kumar Pokharel; Michael L. Kaplan, Stephanie Fiedler
      Abstract: We performed detailed meso-scale observational analyses and WRF model simulations to study the terrain-induced downslope winds that generated dust-emitting winds at the beginning of three strong sub-tropical dust storms in three distinctly different regions of North Africa and the Arabian Peninsula. We revisit the Harmattan dust storm of March 2, 2004, the Saudi dust storm of March 9, 2009, and the Bodélé Depression dust storm of December 8, 2011 and use high-resolution WRF modeling to assess the dynamical processes during the onset of the storms in more depth. Our results highlight the generation of terrain-induced downslope winds in response to the transition of the atmospheric flow from a sub-critical to super-critical state in all three cases. These events precede the unbalanced adjustment processes in the lee of the mountain ranges that produced larger scale dust aerosol mobilization and transport. We see that only the higher resolution data sets can resolve the mesoscale processes, which are mainly responsible for creating strong low level terrain-induced downslope winds leading to the initial dust storms.
      PubDate: 2017-09-14T09:45:27.001922-05:
      DOI: 10.1002/2017JD026942
  • 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
      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 (Petrenko et al., 2012) 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 if applying simple multiplicative emission adjustment factors alone to the widely used Global Fire Emission Database version 3 (GFEDv3) emission inventory can achieve regional-scale consistency between MODIS AOD snapshots and the Goddard Chemistry Aerosol Radiation and Transport (GOCART) 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 Inter-comparisons between Observations and Models (AeroCom) multi-model 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: 2017-09-14T09:40:38.400555-05:
      DOI: 10.1002/2017JD026693
  • The Mass-Dimensional Properties of Cirrus Clouds During TC4
    • Authors: Jeana Mascio; Zhuocan Xu, Gerald G. Mace
      Abstract: Remote sensing retrievals and ice microphysical parameterizations in Global Climate Models typically use assumptions about the distribution of ice mass as a function of particle size using mass-dimensional (m-D) relationships. This study investigates the ice crystal m-D properties of tropical anvil cirrus during the Tropical Composition, Cloud and Climate Coupling Experiment (TC4) to better document the distribution of ice mass with size in this particular class of tropical ice clouds. Two optimal estimation (OE) algorithms (XIWC and MZ) are used to estimate the m-D relationship for each particle size distribution (PSD) collected in situ. The XIWC algorithm minimizes the difference between measured ice water content (IWC) and PSD calculated IWC, while the MZ algorithm minimizes the difference between measured radar reflectivity factors and those calculated from the in situ PSDs. Results from these algorithms are compared to previous studies to establish consistency of the methodologies. The XIWC results show that both parameters in the m-D relationship increase with temperature. Changes in m-D with temperature have substantial implications for remote sensing retrievals. With the prefactor varying by a factor of five and the exponent varying by some 16% over a typical range of ice cloud temperatures, forward modeling errors in radar reflectivity could be typically in excess of 5 dB, further suggesting that retrievals of IWC and precipitation rates from radar measurements in ice clouds be in error by factors easily exceeding 3.
      PubDate: 2017-09-14T09:40:26.807663-05:
      DOI: 10.1002/2017JD026787
  • 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
      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 1980-2015 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 suggests 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: 2017-09-14T09:39:51.283071-05:
      DOI: 10.1002/2016JD026329
  • Evaluation of MODIS Deep Blue aerosol algorithm in desert region of East
           Asia: ground validation and inter-comparison
    • Authors: Minghui Tao; Liangfu Chen, Zifeng Wang, Jun Wang, Huizheng Che, Xiaoguang Xu, Wencai Wang, Jinhua Tao, Hao Zhu, Can Hou
      Abstract: The abundant dust particles from widespread deserts in East Asia play a significant role in regional climate and air quality. In this study, we provide a comprehensive evaluation of the widely used Moderate Resolution Imaging Spectroradiometer (MODIS) Deep Blue (DB) aerosol retrievals in desert regions of East Asia using ground-based observations over eight sites of the China Aerosol Remote Sensing Network (CARSNET). Different from their well-characterized performance in urban and cropland areas around the globe, DB aerosol optical depth (AOD) retrievals exhibit underestimation across the deserts in East Asia. We found 38%-96% of satellite values fall out of an expected-error envelope of ±(0.05+20%AODCARSNET), with the worst performance in Taklimakan Desert. In particular, DB retrievals erroneously give a nearly-constant low values of 0.05 in Taklimakan Desert when AOD is below 0.5, which doesn’t match with variation of moderate dust plumes. Comparison with Multi-angle Imaging SpectroRadiometer (MISR) AOD shows that a similar underestimation is prevalent over the extensive deserts. Inversion of sky light measurements show that single scattering albedos of the yellow dust in East Asia are mostly below 0.9 at 440 nm, much lower than the “whiter” and “redder” dust models applied in the DB algorithm. On the other hand, overestimation of surface reflectance dominantly contributes to the significant low constant AOD values in MODIS DB retrievals in Taklimakan Desert. These large biases, however, can be substantially reduced by considering unique characteristics of aerosols and surface over the arid regions in East Asia.
      PubDate: 2017-09-11T22:30:35.191111-05:
      DOI: 10.1002/2017JD026976
  • Application of a PCA-based Fast Radiative Transfer Model to XCO2
           Retrievals in the Shortwave Infrared
    • Authors: P. Somkuti; H. Boesch, V. Natraj, P. Kopparla
      Abstract: In this work, we extend the PCA-based approach to accelerate radiative transfer (RT) calculations by accounting for the spectral variation of aerosol properties. Using linear error analysis, the errors induced by this fast RT method are quantified for a large number of simulated Greenhouse Gases Observing Satellite (GOSAT) measurements (N≈30,000). The computational speed-up of the approach is typically two orders of magnitude compared to a line-by-line discrete ordinates calculation with 16 streams, while the radiance residuals do not exceed 0.01% for the most part compared to the same baseline calculations. We find that the errors due to the PCA-based approach tend to be less than ±0.06 ppm for both land and ocean scenes when two or more empirical orthogonal functions (EOFs) are used. One advantage of this method is that it maintains the high accuracy over a large range of aerosol optical depths. This technique shows great potential to be used in operational retrievals for GOSAT and other remote sensing missions.
      PubDate: 2017-09-11T13:35:51.841681-05:
      DOI: 10.1002/2017JD027013
  • Influence of solar and lunar tides on the mesopause region as observed in
           Polar Mesosphere Summer Echoes characteristics
    • Authors: P. Dalin; S. Kirkwood, N. Pertsev, V. Perminov
      Abstract: Long-term observations of Polar Mesosphere Summer Echoes (PMSE) from 2002 to 2012 are investigated with the aim to statistically study the effects of solar thermal migrating and lunar gravitational tides on aerosol layers and their environment at altitudes 80-90 km. The solar and lunar tidal periodicities are clearly present in PMSE data. For the first time, both amplitudes and phases of solar and lunar tides are estimated using PMSE data from the ESRAD radar located at Esrange (Sweden). The diurnal, semidiurnal and terdiurnal solar migrating tides show pronounced periodicities in the PMSE strength and wind velocity components. Lunar tides demonstrate clear oscillations in the PMSE strength and wind velocities as well. “Canonical” lunar gravitational tides, corresponding to the lunar gravitational potential, produce rather large amplitudes and are comparable to the solar thermal tides, whereas “non-canonical” lunar oscillations have minor effects on PMSE layers, but are still statistically significant. The influence of diurnal/semidiurnal tides and monthly/semimonthly tidal components is studied separately. Our estimations of solar thermal and lunar tidal amplitudes are in good agreement with those of previous model and experimental studies. A new mechanism of quadratic demodulation of the solar semidiurnal and lunar semidiurnal tides is shown to be valid at the summer mesopause and can explain periodical PMSE oscillations due to the lunar synodic semimonthly tide with period of 14.77 days. Two harmonics with periods of 27.0 and 13.5 days supposedly representing the solar rotation cycle are also clearly present in PMSE data.
      PubDate: 2017-09-11T09:46:15.035954-05:
      DOI: 10.1002/2017JD026509
  • 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
      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 further 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 simulations, the simulated NA influence may serve as an upper limit.
      PubDate: 2017-09-11T04:25:27.633774-05:
      DOI: 10.1002/2017JD026761
  • Troposphere-stratosphere temperature trends derived from satellite data
           compared with ensemble simulations from WACCM
    • Authors: William J. Randel; Lorenzo Polvani, Fei Wu, Douglas E. Kinnison, Cheng-Zhi Zou, Carl Mears
      Abstract: Decadal-scale trends in tropospheric and stratospheric temperatures derived from satellite measurements over 1979-2014 are compared with ensemble simulations from the Whole Atmosphere Community Climate Model (WACCM). The model is forced with observed sea surface temperatures (SSTs), changes in greenhouse gases (GHG) and ozone depleting substances (ODS), plus solar and volcanic effects, and results from five WACCM realizations (with slightly different initial conditions) are analyzed. We focus on the vertical structure of tropospheric warming and stratospheric cooling increasing with height, the latitudinal and seasonal dependence of trends, and on the temporal evolution of stratospheric temperatures in response to stratospheric ozone depletion and partial recovery. The model captures the observed trend structure in most respects, and the ensemble of simulations provides quantitative estimates of the impact of internal variability on trend estimates. In regions of low variability (e.g. over low latitudes) the ensemble mean trends agree with the observations, while in regions of high variability (e.g. the polar stratosphere) the observations mostly fall within the range of realizations. Temperature response to evolving stratospheric ozone is evaluated by computing separate trends over 1979-1997 (ozone depletion) and 1998-2014 (partial recovery). Robust changes in temperature trends between these periods occur in the global upper stratosphere and in the Antarctic spring lower stratosphere, with consistent behavior between model and observations. Observed lower stratospheric temperatures in the Antarctic show statistically significant warming after 1998, reflecting recently reported healing of the ozone hole.
      PubDate: 2017-09-11T00:50:28.90747-05:0
      DOI: 10.1002/2017JD027158
  • Estimating daily evapotranspiration from remotely sensed instantaneous
           observations with simplified derivations of a theoretical model
    • Authors: Ronglin Tang; Zhao-Liang Li
      Abstract: Surface evapotranspiration (ET) is one of the key components in global hydrological cycle and energy budget on the earth. This paper designs a theoretical relationship between daily and instantaneous ETs with a multiplication of multiple fractions through a mathematical derivation of the physics-based Penman-Monteith equation and further develops five methods for converting remotely sensed instantaneous ET to daily values, one of which is equivalent to the conventional constant evaporative fraction (EF) method. The five methods are then evaluated and intercompared using long-term ground-based eddy covariance system-measured half-hourly latent heat flux (LE) and three groups of MODIS-based instantaneous LE datasets collected from April 2009 to late October 2011 at the Yucheng station. Overall, the constant decoupling factor (Ω) method, the constant surface resistance (Rc) method, and the constant ratio of surface resistance to aerodynamic resistance (Rc/Ra) method could produce daily LE estimates that are in reasonably good agreement with the ground-based EC measurements whereas the constant EF method and the constant Priestley-Taylor parameter (α) method underestimate the daily LE with larger biases and root mean square errors. The former three methods are of more solid physical foundation and can effectively capture the effect of temporally variable meteorological factors on the diurnal pattern of surface ET. They provide good alternatives to the nowadays commonly applied methods for the conversion of remotely sensed instantaneous ET to daily values.
      PubDate: 2017-09-11T00:50:26.134813-05:
      DOI: 10.1002/2017JD027094
    • Authors: Timothy J. Lang
      Abstract: The seasonal and diurnal cycles of ocean vector winds in the domain of the South China Sea are characterized and compared using RapidScat and the Cross-Calibrated Multi-Platform (CCMP) datasets. Broad agreement in seasonal flow patterns exists between these datasets during the year 2015. Both observe the dramatic reversal from wintertime trade winds (November-April) to westerly flow associated with the summer monsoon (May-October). These seasonal changes have strong but not equivalent effects on mean wind divergence patterns in both datasets. Specifically near the Philippines, the datasets agree on several aspects of the seasonal mean and diurnal cycle of near-surface vector winds and divergence. In particular, RapidScat and CCMP agree that daytime onshore and nocturnal offshore flow patterns affect the diurnal cycle of winds up to ~200 km west of Luzon, Philippines. Observed disagreements over the diurnal cycle are explainable by measurement uncertainty, as well as shortcomings in both datasets.
      PubDate: 2017-09-10T23:50:47.375823-05:
      DOI: 10.1002/2017JD027516
  • Evaluating a space-based indicator of surface ozone-NOx-VOC sensitivity
           over mid-latitude source regions and application to decadal trends
    • Authors: Xiaomeng Jin; Arlene M. Fiore, Lee T. Murray, Lukas C. Valin, Lok N. Lamsal, Bryan Duncan, K. Folkert Boersma, Isabelle De Smedt, Gonzalo Gonzalez Abad, Kelly Chance, Gail S. Tonnesen
      Abstract: Determining effective strategies for mitigating surface ozone (O3) pollution requires knowledge of the relative ambient concentrations of its precursors, NOx and VOCs. The space-based tropospheric column ratio of formaldehyde to NO2 (FNR) has been used as an indicator to identify NOx-limited versus NOx-saturated O3 formation regimes. However, quantitative use of this indicator ratio is subject to three major uncertainties: 1) the split between NOx-limited and NOx-saturated conditions may shift; 2) the ratio of the vertically integrated column may not represent the near-surface environment; 3) satellite products contain errors. We use the GEOS-Chem global chemical transport model to evaluate the quantitative utility of FNR observed from the Ozone Monitoring Instrument over three northern mid-latitude source regions. We find that FNR in the model surface layer is a robust predictor of the simulated near-surface O3 production regime. Extending this surface-based predictor to a column-based FNR requires accounting for differences in the HCHO and NO2 vertical profiles. We compare four combinations of two OMI HCHO and NO2 retrievals with modeled FNR. The spatial and temporal correlations between the modeled and satellite-derived FNR vary with the choice of NO2 product, while the mean offset depends on the choice of HCHO product. Space-based FNR indicates that the spring transition to NOx-limited regimes has shifted at least a month earlier over major cities (e.g. New York, London, Seoul) between 2005 and 2015. This increase in NOx sensitivity implies that NOx emission controls will improve O3 air quality more now than it would have a decade ago.
      PubDate: 2017-09-10T22:20:27.333432-05:
      DOI: 10.1002/2017JD026720
  • 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
  • Optical properties of aerosols and implications for radiative effects in
           Beijing during the Asia-Pacific Economic Cooperation (APEC) Summit 2014
    • Authors: Yaqing Zhou; Qiyuan Wang, Rujin Huang, Suixin Liu, Xuexi Tie, Xiaoli Su, Xinyi Niu, Zhuzi Zhao, Haiyan Ni, Meng Wang, Yonggang Zhang, Junji Cao
      Abstract: An intensive measurement campaign was conducted in Beijing during the Asia-Pacific Economic Cooperation (APEC) Summit 2014 to investigate the effectiveness of stringent emission controls on aerosol optical properties and direct radiative forcing (DRF). Average values of PM2.5, light scattering (bscat), and light absorption (babs) coefficients decreased by 40, 64, and 56%, respectively, during the APEC-control period compared with non-control periods. For the APEC-control period, the PM2.5 mass scattering and absorption efficiencies were both smaller than the non-control period by a factor of ~2. Calculations based on a revised IMPROVE method and linear regression showed that sulfate, nitrate, organic matter, elemental carbon, and fine soil contributed comparably to the light extinction coefficient (bext) in both periods, but the bext values were 27–64% lower during the APEC period. A positive matrix factorization receptor model showed that bext from two secondary aerosol sources, biomass burning, traffic-related emissions, and coal burning decreased by 26–87% during the APEC-control period. The average DRF calculated from the Tropospheric Ultraviolet and Visible radiation model was -11.9 and -4.6 W m-2 at the surface during the non- and APEC-control periods, respectively, suggesting an overall cooling effect. The reduction of DRF from each emission source ranged from~30–80% during the APEC-control period. The results suggest that the pollution control measures implemented for APEC substantially reduced air pollution and could help mitigate the cooling effects of aerosols at the surface in Beijing.
      PubDate: 2017-09-10T21:25:19.989956-05:
      DOI: 10.1002/2017JD026997
  • Exploring the links in monthly to decadal variability of the atmospheric
           water balance over the wettest regions in ERA-20C
    • Authors: M. Nogueira
      Abstract: Monthly-to-decadal variability of the regional precipitation over ITCZ 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 non-periodic 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 non-significant between non-periodic P and E or sea-surface temperature (SST). Thus, the non-periodic 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 non-periodic 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 are dominated by transient storms, with energy accumulation at synoptic-scales and β
      PubDate: 2017-09-10T21:00:27.939896-05:
      DOI: 10.1002/2017JD027012
  • 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
      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 thirteen 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 useful observation-based information for climate model diagnostics in representation of cloud phase and their microphysical characteristics.
      PubDate: 2017-09-10T20:50:24.698645-05:
      DOI: 10.1002/2017JD027113
  • A TRMM/GPM Retrieval of the Total Mean Generator Current for the Global
           Electric Circuit
    • Authors: Michael Peterson; Wiebke Deierling, Chuntao Liu, Douglas Mach, Christina Kalb
      Abstract: A specialized satellite version of the passive microwave electric field retrieval algorithm (Peterson et al., 2015) is applied to observations from the Tropical Rainfall Measuring Mission (TRMM) and Global Precipitation Measurement (GPM) satellites to estimate the generator current for the Global Electric Circuit (GEC) and compute its temporal variability. By integrating retrieved Wilson currents from electrified clouds across the globe, we estimate a total mean current of between 1.4 kA (assuming the 7% fraction of electrified clouds producing downward currents measured by the ER-2 is representative) to 1.6 kA (assuming all electrified clouds contribute to the GEC). These current estimates come from all types of convective weather without preference, including Electrified Shower Clouds (ESCs).The diurnal distribution of the retrieved generator current is in excellent agreement with the Carnegie curve (RMS difference: 1.7%). The temporal variability of the total mean generator current ranges from 110% on semi-annual timescales (29% on an annual timescale) to 7.5% on decadal timescales with notable responses to the Madden-Julian Oscillation and El Nino Southern Oscillation. The geographical distribution of current includes significant contributions from oceanic regions in addition to the land-based tropical chimneys. The relative importance of the Americas and Asia chimneys compared to Africa is consistent with the best modern ground-based observations and further highlights the importance of ESCs for the GEC.
      PubDate: 2017-09-07T20:55:34.196816-05:
      DOI: 10.1002/2016JD026336
  • Deducing climatic elasticity to assess projected climate change impacts on
           streamflow change across China
    • Authors: Jianyu Liu; Qiang Zhang, Yongqiang Zhang, Xi Chen, Jianfeng Li, Santosh K. Aryal
      Abstract: Climatic elasticity has been widely applied to assess streamflow responses to climate changes. To fully assess impacts of climate under global warming on streamflow and reduce the error and uncertainty from various control variables, we develop a four-parameter (precipitation, catchment characteristics n, maximum and minimum temperature) climatic elasticity method named PnT, based on the widely used Budyko framework and simplified Makkink equation. We use this method to carry out the first comprehensive evaluation of the streamflow response to potential climate change for 372 widely spread catchments in China. The PnT climatic elasticity was firstly evaluated for a period 1980-2000, and then used to evaluate streamflow change response to climate change based on 12 global climate models (GCMs) under Representative Concentration Pathway 2.6 (RCP2.6) and RCP 8.5 emission scenarios. The results show that: (1) the PnT climatic elasticity method is reliable; (2) projected increasing streamflow takes place in more than 60% of the selected catchments, with mean increments of 9% and 15.4% under RCP2.6 and RCP8.5 respectively; (3) uncertainties in the projected streamflow are considerable in several regions, such as the Pearl River and Yellow River, with more than 40% of the selected catchments showing inconsistent change directions. Our results can help Chinese policy makers to manage and plan water resources more effectively, and the PnT climatic elasticity should be applied to other parts of the world.
      PubDate: 2017-09-04T21:07:25.984097-05:
      DOI: 10.1002/2017JD026701
  • Improvements to GPS airborne radio occultation in the lower troposphere
           through implementation of the phase matching (PM) method
    • Authors: K.-N. Wang; J. L. Garrison, J. S. Haase, B. J. Murphy
      Abstract: Airborne radio occultation (ARO) is a remote sensing technique for atmospheric sounding using Global Positioning System (GPS) signals received by an airborne instrument. The atmospheric refractivity profile, which depends on pressure, temperature, and water vapor, can be retrieved by measuring the signal delay due to the refractive medium through which the signal traverses. The ARO system was developed to make repeated observations within an individual meteorological event such as a tropical storm, regardless of the presence of clouds and precipitation, and complements existing observation techniques such as dropsondes and satellite remote sensing.RO systems can suffer multipath ray propagation in the lower troposphere if there are strong refractivity gradients, for example, due to a highly variable moisture distribution or a sharp boundary layer, interfering with continuous carrier phase tracking as well as complicating retrievals. The phase matching method has now been adapted for ARO and is shown to reduce negative biases in the refractivity retrieval by providing robust retrievals of bending angle in the presence of multipath. The retrieval results are presented for a flight campaign in September 2010 for Hurricane Karl in the Caribbean Sea. The accuracy is assessed through comparison with the European Center for Medium Range Weather Forecasting (ECMWF) Interim Reanalysis (ERA-I). The fractional difference in refractivity can be maintained at a standard deviation of 2% from flight level down to a height of 2 km. The PM method decreases the negative refractivity bias by as much as 4% over the classical geometrical optics retrieval method.
      PubDate: 2017-09-04T20:55:47.991736-05:
      DOI: 10.1002/2017JD026568
  • 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
      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 CALMET along with the advanced 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: 2017-09-04T20:35:40.830684-05:
      DOI: 10.1002/2017JD026685
  • Observation-based study on aerosol optical depth and particle size in
           partly cloudy regions
    • Authors: T. Várnai; A. Marshak, T. F. Eck
      Abstract: This study seeks to help better understand aerosol-cloud interactions by examining statistical relationships between aerosol properties and nearby low-altitude cloudiness using satellite data. The analysis of a global dataset of MODIS (Moderate Resolution Imaging Spectroradiometer) observations reveals that the positive correlation between cloudiness and aerosol optical depth (AOD) reported in earlier studies is strong throughout the globe and during both winter and summer. Typically, AOD is 30-50% higher on cloudier-than-average days than on less cloudy days. A combination of satellite observations and MERRA-2 global reanalysis data reveals that the correlation between cloud cover and AOD is strong for all aerosol types considered: sulfate, dust, carbon, and sea salt.The observations also indicate that in the presence of nearby clouds, aerosol size distributions tend to shift toward smaller particles over large regions of the Earth. This is consistent with a greater cloud-related increase in the AOD of fine mode than of coarse mode particles. The greater increase in fine mode AOD implies that the cloudiness-AOD correlation does not come predominantly from cloud detection uncertainties. Additionally, the results show that aerosol particle size increases near clouds even in regions where it decreases with increasing cloudiness. This suggests that the decrease with cloudiness comes mainly from changes in large-scale environment, rather than from clouds increasing the number or the size of fine mode aerosols. Finally, combining different aerosol retrieval algorithms demonstrated that quality assessment flags based on local variability can help identifying when the observed aerosol populations are affected by surrounding clouds.
      PubDate: 2017-09-04T20:16:36.411373-05:
      DOI: 10.1002/2017JD027028
  • The Impact of Satellite Derived Land Surface Temperatures on Numerical
           Weather Prediction Analyses and Forecasts
    • Authors: B. Candy; R. W. Saunders, D. Ghent, C. E. Bulgin
      Abstract: Land surface temperature (LST) observations from a variety of satellite instruments operating in the infrared have been compared to estimates of surface temperature from the Met Office operational numerical weather prediction (NWP) model. The comparisons show that during the day the NWP model can under predict the surface temperature by up to 10 K in certain regions such as the Sahel and Southern Africa. By contrast at night the differences are generally smaller. Matchups have also been performed between satellite LSTs and observations from an in situ radiometer located in Southern England within a region of mixed land use. These matchups demonstrate good agreement at night and suggest that the satellite uncertainties in LST are less than 2 K. The Met Office surface analysis scheme has been adapted to utilize nighttime LST observations. Experiments using these analyses in an NWP model have shown a benefit to the resulting forecasts of near surface air temperature, particularly over Africa.
      PubDate: 2017-09-04T20:05:59.253469-05:
      DOI: 10.1002/2016JD026417
  • Aerosol Properties Observed in the Subtropical North Pacific Boundary
    • Authors: T. M. Royalty; B. N. Phillips, K. W. Dawson, R. Reed, N. Meskhidze, M. D. Petters
      Abstract: The impact of anthropogenic aerosol on climate forcing remains uncertain largely due to inadequate representation of natural aerosols in climate models. The marine boundary layer (MBL) might serve as a model location to study natural aerosol processes. Yet, source and sink mechanisms controlling the MBL aerosol number, size distribution, chemical composition, and hygroscopic properties remain poorly constrained. Here, aerosol size distribution and water uptake measurements were made aboard the R/V Hi’ialakai from 27 June to 3 July 2016 in the subtropical North Pacific Ocean. Size distributions were predominantly bimodal with an average integrated number concentration of 197±98 cm-3. Hygroscopic growth factors were measured using the tandem differential mobility analyzer technique for dry 48, 96, and 144 nm particles. Mode kappa values for these were 0.57±0.12, 0.51±0.09, and 0.52±0.08, respectively. To better understand remote MBL aerosol sources, a new algorithm was developed which decomposes hygroscopicity distributions into three classes: carbon-containing particles, sulfate-like particles, and sodium-containing particles. Results from this algorithm showed low and steady sodium-containing particle concentrations while the sulfate-like and carbon-containing particle concentrations varied during the cruise. According to the classification scheme, carbon-containing particles contributed at least 3-7%, sulfate-like particles contributed at most 77-88% and sodium-containing particles at least contributed 9-16% to the total aerosol number concentration. Size distribution and hygroscopicity data, in conjunction with airmass back-trajectory analysis, suggested that the aerosol budget in the subtropical North Pacific MBL may be controlled by aerosol entrainment from the free troposphere.
      PubDate: 2017-09-04T20:01:53.802083-05:
      DOI: 10.1002/2017JD026897
  • Three-Dimensional Modeling of Two Interacting Streamers
    • Authors: Feng Shi; Ningyu Liu, Joseph R. Dwyer
      Abstract: Interacting streamers are commonly found in natural plasma discharges, such as lightning, jets and sprites, but are poorly understood at present. In this paper, we report a three-dimensional modeling study of two interacting streamers simultaneously initiated from identical plasma columns in a uniform ambient field. Simulation results show that the streamers no longer propagate in the direction of the ambient field due to strong repulsion between them. The results also show that the peak field in the streamer head determines the direction of streamer propagation, and that the two streamers propagate in separate directions. We further conjecture that they will be unlikely to merge later on. Finally, the interacting streamers still grow exponentially, with a growth rate similar to an isolated streamer under the same condition.
      PubDate: 2017-09-04T19:31:32.844062-05:
      DOI: 10.1002/2017JD026935
  • Statistical Characteristics of Convective Clouds over the Western Ghats
           Derived from Weather Radar Observations
    • Authors: Bhowmik Utsav; Sachin Deshpande, Subrata Das, Govindan Pandithurai
      Abstract: X-band radar observations at Mandhardev (18.04°N, 73.85°E) are used to investigate statistics of convective clouds over the Western Ghats during monsoon season (June-September, 2014). Convective storms (cells) are identified using an objective-tracking method to examine their spatio-temporal variability, thus quantifying the time continuous aspects of convective cloud population over the region for the first time. An increased frequency of storm location and initiation along the windward mountains compared to coastal and lee side highlights orographic response to southwesterly flow, with superimposed diurnal cycle. An eastward progression of convective activity from upstream the barrier through windward slopes of mountains over to the lee side is observed. Storm area, height and duration follow lognormal distributions; wherein, small-sized storms contribute more to total population and unimodal distribution of 35-dBZ top heights (peaking at 5.5 km) depicts the dominance of shallow convection. Storms exhibit a pronounced diurnal cycle with a peak in afternoon hours, while the convective area maximum is delayed by several hours to that of precipitation flux. Cell lifetime and propagation show that cells move with slow speeds and have mean duration of 46 minutes. They align east-west nearly parallel to mountain ridges and their direction of movement is steered mostly by large-scale winds at lower levels. Based on top heights, convective cells are further classified into cumulus, congestus and deep clouds. In general, congestus (deep) cells are most abundant in the windward (leeward) side. A lead-lag relationship between congestus and deep cells indicates midtroposphere-moistening by congestus cells prior to deep convection.
      PubDate: 2017-09-04T18:38:43.558881-05:
      DOI: 10.1002/2016JD026183
  • An assessment of ground-level and free-tropospheric ozone over California
           and Nevada
    • Authors: E.L. Yates; M.S. Johnson, L.T. Iraci, J.-M. Ryoo, R.B. Pierce, P.D. Cullis, W. Gore, M.A. Ives, B.J. Johnson, T. Leblanc, J.E. Marrero, C.W. Sterling, T. Tanaka
      Abstract: Increasing free tropospheric ozone (O3), combined with the high elevation and often deep boundary layers at western US surface stations, poses challenges in attaining the more stringent 70 ppb O3 National Ambient Air Quality Standard. As such, use of observational data to identify sources and mechanisms that contribute to surface O3 is increasingly important. This work analyzes surface and vertical O3 observations over California and Nevada from 1995 to 2015. Over this period, the number of high O3 events (95th percentile) at US EPA CASTNET sites has decreased during summer, as a result of decreasing US emissions. In contrast, an increase in springtime 5th percentile O3 indicates a general increase of baseline O3. During 2012 there was a peak in exceedances and in the average spring-summer O3 mixing ratios at CASTNET sites. GEOS-Chem results show that the surface O3 attributable to transport from the upper troposphere and stratosphere were increased in 2013 compared to 2012, highlighting the importance of measurements aloft. Vertical O3 measurements from aircraft, ozonesondes and lidar show distinct seasonal trends, with a high percentage of elevated O3 laminae (O3>70 ppb, 3-8 km) during spring and summer. Analysis of the timing of high O3 surface events and correlation between surface and vertical O3 data is used to discuss varying sources of western US surface O3.
      PubDate: 2017-08-31T13:45:34.419532-05:
      DOI: 10.1002/2016JD026266
  • Reactive uptake of sulfur dioxide and ozone on volcanic glass and ash at
           ambient temperature
    • Authors: Elena C. Maters; Pierre Delmelle, Michel J. Rossi, Paul M. Ayris
      Abstract: AbstractThe atmospheric impacts of volcanic ash from explosive eruptions are rarely considered alongside those of volcanogenic gases/aerosols. While airborne particles provide solid surfaces for chemical reactions with trace gases in the atmosphere, the reactivity of airborne ash has seldom been investigated. Here we determine the total uptake capacity (NiM) and initial uptake coefficient (γM) for sulfur dioxide (SO2) and ozone (O3) on a compositional array of volcanic ash and glass powders at ~25 °C in a Knudsen flow reactor. The measured ranges of NiSO2 and γSO2 (1011-1013 molecules cm-2 and 10-3-10-2) and NiO3 and γO3 (1012-1013 molecules cm-2 and 10-3-10-2) are comparable to values reported for mineral dust. Differences in ash and glass reactivity towards SO2 and O3 may relate to varying abundances of, respectively, basic and reducing sites on these materials. The typically lower SO2 and O3 uptake on ash compared to glass likely results from prior exposure of ash surfaces to acidic and oxidizing conditions within the volcanic eruption plume/cloud. While sequential uptake experiments overall suggest that these gases do not compete for reactive surface sites, SO2 uptake forming adsorbed S(IV) species may enhance the capacity for subsequent O3 uptake via redox reaction forming adsorbed S(VI) species. Our findings imply that ash emissions may represent a hitherto neglected sink for atmospheric SO2 and O3.
      PubDate: 2017-08-31T12:50:27.867966-05:
      DOI: 10.1002/2017JD026993
  • Projections of future precipitation extremes over Europe: a multi-model
           assessment of climate simulations.
    • Authors: Jan Rajczak; Christoph Schär
      Abstract: Projections of precipitation and its extremes over the European continent are analyzed in an extensive multi-model ensemble of 12 and 50 km resolution EURO-CORDEX regional climate simulations (RCMs) forced by RCP2.6, RCP4.5 and RCP8.5 aerosol and greenhouse gas emission scenarios. A systematic inter-comparison with ENSEMBLES RCMs is carried out, such that in total information is provided for an unprecedentedly large dataset 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 inter-model 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 summer-time drought in southern Europe and more frequent and intense heavy rainfall across all of Europe.
      PubDate: 2017-08-31T11:25:29.919599-05:
      DOI: 10.1002/2017JD027176
  • Variability of Stratospheric Reactive Nitrogen and Ozone Related to the
    • Authors: M. Park; W. J. Randel, D. E. Kinnison, A. E. Bourassa, D. A. Degenstein, C. Z. Roth, C. A. McLinden, C. E. Sioris, N. J. Livesey, M. L. Santee
      Abstract: The stratospheric quasi-biennial oscillation (QBO) dominates interannual variability of dynamical variables and trace constituents in the tropical stratosphere, and provides a natural experiment to test circulation-chemistry interactions. This work quantifies the relationships among ozone (O3), reactive nitrogen (NOy) and source gas N2O, and their links to the QBO, based on satellite constituent measurements and meteorological data spanning 2005-2014 (over four QBO cycles). Data include O3, HNO3 and N2O from the Aura Microwave Limb Sounder (MLS), and an NOx proxy derived from Optical Spectrograph and InfraRed Imager System (OSIRIS) NO2 measurements combined with a photochemical box model (= NOx*). Results are compared to simulations from the WACCM4 model incorporating a QBO circulation nudged to assimilated winds. Cross-correlations and composites with respect to the QBO phase show coherent 180-degree out-of-phase relationships between NOy and N2O throughout the stratosphere, with the NOx/HNO3 ratio increasing with altitude. The anomalies in NOy species propagate coherently downward with the QBO. Ozone is anti-correlated with reactive nitrogen in the middle stratosphere above ~ 28 km due to NOx control of ozone catalytic loss cycles. Quantitative comparisons of nitrogen partitioning and O3 sensitivity to NOx show good overall agreement between satellite observations and model results (suggesting closure of the NOy budget), although the model results show larger (up to ~20 %) N2O, NOx and O3 variations near ~35 km compared to observations. These analyses serve to assess the consistency of diverse satellite-based data sets, and also to evaluate nitrogen partitioning and NOx-dependent ozone chemistry in the global model.
      PubDate: 2017-08-31T10:45:28.323583-05:
      DOI: 10.1002/2017JD027061
  • A Comparison of the Ocean Microbarom Recorded on the Ground and in the
    • Authors: D. C. Bowman; J. M. Lees
      Abstract: The ocean microbarom is an acoustic signal generated via nonlinear interaction of ocean surface waves. It can propagate for thousands of kilometers and represents a significant infrasonic noise source for ground infrasound stations across the globe. However, wind noise often compromises detections at ground stations. Furthermore, the microbarom may travel in elevated acoustic ducts that do not transmit enough energy for detections on ground stations. Here, the presence of the ocean microbarom on two high altitude balloon flights is investigated. A spectral peak consistent with the microbarom was observed on sensors in the stratosphere but not on those deployed on the ground near the flight path of the balloon. This is probably due to an elevated acoustic duct and/or a superior signal to noise ratio in the stratosphere. Thus, microbarom activity quantified solely with ground based sensors may underestimate the occurrence of the phenomenon. However, high levels of interference from flight system electronics and/other other payloads may have obscured other microbarom episodes during the balloon deployments.
      PubDate: 2017-08-31T08:50:01.811455-05:
      DOI: 10.1002/2017JD026474
  • Diurnal convection–wind coupling in the Bay of Bengal
    • Authors: Thomas Kilpatrick; Shang-Ping Xie, Tomoe Nasuno
      Abstract: Satellite observations of infrared brightness temperature and rainfall have shown offshore propagation of diurnal rainfall signals in some coastal areas of the tropics, suggesting that diurnal rainfall is coupled to land–sea breeze circulations. Here we utilize satellite observations of surface winds and rainfall to show the offshore co-propagation of land breeze and diurnal rainfall signals for 300–400km from the east coast of India into the Bay of Bengal. The wind observations are from the 2003 QuikSCAT–SeaWinds “tandem mission” and from 17 years of the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI); the rainfall observations are from the TRMM 3B42 product and from TMI. The surface wind convergence maximum leads the rainfall maximum by 1–2h in the western part of the bay, implying that the land breeze forces the diurnal cycle of rainfall. The phase speed of the offshore propagation is approximately 18ms−1, consistent with a deep hydrostatic gravity wave forced by diurnal heating over India. Comparisons with a cloud system-resolving atmospheric model and the ERA-Interim reanalysis indicate that the models realistically simulate the surface land breeze, but greatly underestimate the amplitude of the rainfall diurnal cycle. The satellite observations presented in this study therefore provide a benchmark for model representation of this important atmosphere–ocean–land surface interaction.
      PubDate: 2017-08-30T22:50:36.397539-05:
      DOI: 10.1002/2017JD027271
  • Influence of super-parameterization and a higher-order turbulence closure
           on rainfall bias over Amazonia in Community Atmosphere Model version 5
    • Authors: Kai Zhang; Rong Fu, Muhammad J. Shaikh, Steven Ghan, Minghuai Wang, L. Ruby Leung, Robert E. Dickinson, Jose Marengo
      Abstract: We evaluate the Community Atmosphere Model Version 5 (CAM5) with a higher-order turbulence closure scheme, named Cloud Layers Unified By Binomials (CLUBB), and a Multiscale Modeling Framework, referred as the “super-parameterization” (SP) with two different microphysics configurations to investigate their influences on rainfall simulations over Southern Amazonia. The two different microphysics configurations in SP are the one-moment cloud microphysics without aerosol treatment (SP1) and two-moment cloud microphysics coupled with aerosol treatment (SP2). Results show that both SP2 and CLUBB effectively reduce the low biases of rainfall, mainly during the wet season, and reduce low biases of humidity in the lower troposphere with further reduced shallow clouds and increased surface solar flux. These changes increase moist static energy in the lower atmosphere, contribute to stronger convection and more rainfall. SP2 appears to realistically capture the observed increase of relative humidity prior to deep convection and it significantly increases rainfall in the afternoon; CLUBB significantly delays the afternoon peak rainfall and produces more precipitation in the early morning, due to more gradual transition between shallow and deep convection. In CAM5 and CAM5 with CLUBB, occurrence of more deep convection appears to be a result of stronger heating rather than higher relative humidity.
      PubDate: 2017-08-29T19:50:30.05667-05:0
      DOI: 10.1002/2017JD026576
  • Evaluation of ENTLN performance characteristics based on the ground-truth
           natural and rocket-triggered lightning data acquired in Florida
    • Authors: Y. Zhu; V. A. Rakov, M. D. Tran, M. G. Stock, S. Heckman, C. Liu, C. D. Sloop, D. M. Jordan, M. A. Uman, J. A. Caicedo, D. A. Kotovsky, R. A. Wilkes, F. L. Carvalho, T. Ngin, W. R. Gamerota, J. T. Pilkey, B. M. Hare
      Abstract: The performance characteristics of the Earth Networks Total Lightning Network (ENTLN) were evaluated by using as ground-truth natural cloud-to-ground (CG) lightning data acquired at the Lightning Observatory in Gainesville (LOG) and rocket-triggered lightning data obtained at Camp Blanding (CB), Florida, in 2014 and 2015. Two ENTLN processors (data processing algorithms) were evaluated. The old processor (P2014) was put into use in June 2014 and the new one (P2015) has been operational since August 2015. Based on the natural-CG-lightning dataset (219 flashes containing 608 strokes), the flash detection efficiency (DE), flash classification accuracy (CA), stroke DE, and stroke CA for the new processor were found to be 99%, 97%, 96%, and 91%, respectively, and the corresponding values for the old processor were 99%, 91%, 97%, and 68%. The stroke DE and stroke CA for first strokes are higher than those for subsequent strokes. Based on the rocket-triggered lightning dataset (36 CG flashes containing 175 strokes), the flash DE, flash CA, stroke DE, and stroke CA for the new processor were found to be 100%, 97%, 97%, and 86%, respectively, while the corresponding values for the old processor were 100%, 92%, 97%, and 42%. The median values of location error and absolute peak current estimation error were 215 m and 15% for the new processor, and 205 m and 15% for the old processor. For both natural and triggered CG lightning, strokes with higher peak currents were more likely to be both detected and correctly classified by the ENTLN.
      PubDate: 2017-08-29T19:35:25.551905-05:
      DOI: 10.1002/2017JD027270
  • Determination of desert soil apparent thermal diffusivity using a
           conduction-convection algorithm
    • Authors: Zhiqiu Gao; Bing Tong, Robert Horton, Ali Mamtimin, Yubin Li, Linlin Wang
      Abstract: Surface soil temperatures impact land-atmosphere interactions in desert environments. Soil apparent thermal diffusivity (k) is a crucial physical parameter affecting soil temperature. Previous studies using the conduction-convection algorithm reported k values of desert soils for only a few days. The main objective of this study is to determine the daily and monthly variations of desert k for a range of water contents over a ten-month period. The k values were estimated with a conduction-convection algorithm using soil temperature measured at the 0.00 m and 0.20 m depths from January 1 to October 11, 2011 at the Tazhong station in the Taklimakan Desert of China. Generally, the daily values of k ranged from 1.46 × 10-7m2  s-1 to 5.88 × 10-7m2  s-1, and the ten month average k value was 2.5(±0.8) × 10-7m2  s-1 for the 0.00 m to 0.20 m soil layer. The k values varied significantly with soil water content. The apparent convection parameter (W), which is the sum of the vertical gradient of k and apparent water flux density, was also determined. Comparison of the magnitudes of W and k gradients indicated that little water movement occurred during the dry months, some water infiltrated downward during the wet months, and some water moved upwards in response to evaporation following the wet months. These findings confirmed that the conduction-convection algorithm described the general pattern of soil water movement. The presented daily and monthly values of k can be used as soil parameters when modeling land-atmosphere interactions in the Taklimakan desert.
      PubDate: 2017-08-29T17:11:19.713801-05:
      DOI: 10.1002/2017JD027290
  • 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
      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 (LES) 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: 2017-08-29T09:56:08.605088-05:
      DOI: 10.1002/2017JD026947
  • The East Asian Atmospheric Water Cycle and Monsoon Circulation in the Met
           Office Unified Model.
    • Authors: Jose M. Rodriguez; Sean F. Milton, Charline Marzin
      Abstract: In this study the low-level monsoon circulation and observed sources of moisture responsible for the maintenance and seasonal evolution of the East Asian Monsoon are examined, studying the detailed water budget components. These observational estimates are contrasted with the Met Office Unified Model (MetUM) climate simulation performance in capturing the circulation and water cycle at a variety of model horizontal resolutions and in fully coupled ocean-atmosphere simulations. We study the role of large-scale circulation in determining the hydrological cycle by analysing key systematic errors in the model simulations. MetUM climate simulations exhibit robust circulation errors, including a weakening of the summer west Pacific subtropical high, which leads to an underestimation of the south-westerly monsoon flow over the region. Precipitation and implied diabatic heating biases in the South Asian monsoon and Maritime Continent region are shown, via nudging sensitivity experiments, to have an impact on the East Asian monsoon circulation. By inference, the improvement of these tropical biases with increased model horizontal resolution is hypothesised to be a factor in improvements seen over East Asia with increased resolution. Results from the annual cycle of the hydrological budget components in 5 domains show a good agreement between MetUM simulations and ERA-Interim reanalysis in northern and Tibetan domains. In simulations, the contribution from moisture convergence is larger than in re-analysis and they display less precipitation recycling over land. The errors are closely linked to monsoon circulation biases.
      PubDate: 2017-08-29T09:50:52.046197-05:
      DOI: 10.1002/2016JD025460
  • The isotopic composition of near-surface water vapor at the Maïdo
           Observatory (Reunion Island, Southwestern Indian Ocean) documents the
           controls of the humidity of the subtropical troposphere
    • Authors: Etienne Guilpart; Françoise Vimeux, Stéphanie Evan, Jérôme Brioude, Jean-Marc Metzger, Christelle Barthe, Camille Risi, Olivier Cattani
      Abstract: We present a 1-year long record of the isotopic composition of near-surface water vapor (δ18Ov) at the Maïdo atmospheric observatory (Reunion Island, Indian Ocean, 22°S, 55°E) from November 1st, 2014 to October 31st, 2015, using Wavelength-Scanned Cavity Ring Down Spectroscopy. Except during cyclone periods where δ18Ov is highly depleted (-20.5 ‰), a significant diurnal variability can be seen on both δ18Ov and qv with enriched (depleted) water vapor (mean δ18Ov is -13.4 ‰ (-16.6 ‰)) and moist (dry) conditions (mean qv is 9.7 g/kg (6.4 g/kg)) during daytime (nighttime). We show that δ18Ov diurnal cycle arises from mixing processes for 65 % of cases with two distinct sources of water vapor. We suggest that δ18Ov diurnal cycle is controlled by an interplay of thermally driven land-sea breezes and upslope-downslope flows, bringing maritime air to the observatory during daytime whereas at night, the observatory is above the atmospheric boundary layer and samples free tropospheric air. Interestingly, δ18Ov record also shows that some nights (15 %) are extremely depleted (mean δ18Ov is -21.4 ‰). They are among the driest of the record (mean qv is 2.9 g/kg). Based on different modeling studies, we suggest that extreme nocturnal isotopic depletions are caused by large-scale atmospheric transport and subsidence of dry air masses from the upper troposphere to the surface, induced by the subtropical westerly jet.
      PubDate: 2017-08-29T09:45:37.419922-05:
      DOI: 10.1002/2017JD026791
  • Contrasting features of monsoon precipitation around the Meghalaya Plateau
           under westerly and easterly regimes
    • Authors: Hatsuki Fujinami; Tomonori Sato, Hironari Kanamori, Fumie Murata
      Abstract: Precipitation features around the Meghalaya Plateau, northeast India, during summer are investigated using a 17-year (1998–2014) high-spatial-resolution TRMM precipitation radar dataset. Precipitation around the plateau fell into two distinct regimes based on the low-level wind direction that fluctuates on intraseasonal timescales over Bangladesh, windward of the plateau: a westerly regime (WR) and an easterly regime (ER). Under the WR, strong low-level onshore southwesterlies across Bangladesh encounter the plateau, and localized strong low-level southerlies running parallel to the Arakan Mountains (i.e., the barrier jet) also blow toward the plateau, concentrating convective unstable air onto its southern slopes. The low-level wind fields and large-scale upper-level divergent fields promote frequent and intense orographic rainfall along the southern slopes due to forced uplift, generating high precipitation. In contrast, under the ER, strong southeasterlies that blow along the Gangetic Plain without encountering the plateau and subsidence inhibit upward motion around the plateau, resulting in low precipitation. Diurnal variations in precipitation significantly affect the daily precipitation around the plateau under both regimes. High rainfall frequency persists over the southern slopes between 2100 and 1200 LT of the next day under the WR, whereas modest rainfall frequency occurs between 0000 and 0600 LT under the ER, with a daytime minimum and nocturnal maximum in both regimes. The atmospheric boundary layer processes over Bangladesh regulate the wind speed and vertical structure of the low-level wind toward the plateau, with deceleration during daytime and acceleration at night (i.e., nocturnal jet) that result in the nocturnal rainfall maximum.
      PubDate: 2017-08-29T09:45:26.320894-05:
      DOI: 10.1002/2016JD026116
  • Large wind shears and their implications for diffusion in regions with
           enhanced static stability: the mesopause and the tropopause
    • Authors: H.-L. Liu
      Abstract: The NCAR Whole Atmosphere Community Climate Model (WACCM), with a quasi-uniform horizontal resolution of ∼25km and a vertical resolution of 0.1 scale height, produces large vertical shear of horizontal wind with peaks around the mesopause and the tropical and midlatitude tropopause. In these regions, the static stability also reaches peak values and therefore allows large vertical shears before the onset of dynamical instability. The wind shear peaks near the mesopause and the tropopause from the simulation compare well with those identified in observations, including the magnitude, latitudinal dependence, and large shear statistics. By analyzing the probability density functions of the wind shears and their dependence on the zonal scales, it is found that smaller scale processes, likely gravity waves, contribute significantly to the large shears, and may play a dominant role in producing the largest shears. Climatological tidal waves have secondary contribution to the large winds and shears, but spectral analysis suggests that they can modulate wind shear perturbations by gravity waves in the mesosphere and lower thermosphere. Implications for tracer transport and mixing in these regions are explored by estimating diffusion coefficients based on the root mean square winds, shears and corresponding spatial scales from model results.
      PubDate: 2017-08-25T16:10:50.499692-05:
      DOI: 10.1002/2017JD026748
  • Surface air relative humidities spuriously exceeding 100% in CMIP5 model
           output and their impact on future projections
    • Authors: Kimmo Ruosteenoja; Kirsti Jylhä, Jouni Räisänen, Antti Mäkelä
      Abstract: In 17 out of the 29 CMIP5 climate models examined in this work, near-surface air relative humidity (RH) frequently exceeded 100 % with respect to ice in polar areas in winter. The degree of supersaturation varied considerably across the models, and the same evidently applies to the causes of the phenomenon. Consultations with the modelling groups revealed three categories of explanations for supersaturation occurrence: specification of RH with respect to ice rather than liquid water; inconsistencies in the determination of specific humidity and air temperature for the near-surface level; and the nonlinearity of saturation specific humidity as a function of temperature. Modelled global warming tended to reduce the artificial supersaturations, inducing a spurious negative trend in the future RH change. For example, over East Antarctica under RCP8.5, the multi-model mean RH would decrease by about 10 % by the end of the ongoing century. Truncation of overly high RHs to a maximum value of 100 % cut the RH response close to zero. In Siberia and northern North America, truncation even reversed the sign of the response. The institutes responsible for the CMIP6 model experiments should be aware of the supersaturation issue, and the algorithms used to produce near-surface RH should be developed to eliminate the problem before publishing the RH output data.
      PubDate: 2017-08-24T09:45:24.761452-05:
      DOI: 10.1002/2017JD026909
  • Processes maintaining tropopause sharpness in numerical models
    • Authors: L. Saffin; S. L. Gray, J. Methven, K. D. Williams
      Abstract: Recent work has shown that the sharpness of the extratropical tropopause declines with lead time in numerical weather prediction models, indicating an imbalance between processes acting to sharpen and smooth the tropopause. In this study the systematic effects of processes contributing to the tropopause sharpness are investigated using daily initialised forecasts run with the Met Office Unified Model over a three-month winter period. Artificial tracers, each forced by the potential vorticity tendency due to a different model process, are used to separate the effects of such processes. The advection scheme is shown to result in an exponential decay of tropopause sharpness towards a finite value at short lead times with a timescale of 20-24 hours. The systematic effect of non-conservative processes is to sharpen the tropopause, consistent with previous case studies. The decay of tropopause sharpness due to the advection scheme is stronger than the sharpening effect of non-conservative processes leading to a systematic decline in tropopause sharpness with forecast lead time. The systematic forecast errors in tropopause-level potential vorticity are comparable to the integrated tendencies of the parametrized physical processes suggesting that the systematic error in tropopause sharpness could be significantly reduced through realistic adjustments to the model parametrization schemes.
      PubDate: 2017-08-24T09:41:32.832418-05:
      DOI: 10.1002/2017JD026879
  • Effect of Heterogeneity and Shape on Optical Properties of Urban Dust
           Based on 3-Dimensional Modeling of Individual Particles
    • Authors: Joseph M. Conny; Diana L. Ortiz-Montalvo
      Abstract: We show the effect of composition heterogeneity and shape on the optical properties of urban dust particles based on the 3-dimensional spatial and optical modeling of individual particles. Using scanning electron microscopy/x-ray spectroscopy (SEM/EDX) and focused ion-beam (FIB) tomography, spatial models of particles collected in Los Angeles and Seattle accounted for surface features, inclusions, and voids, as well as overall composition and shape. Using voxel data from the spatial models and the discrete dipole approximation method, we report extinction efficiency, asymmetry parameter, and single scattering albedo (SSA). Test models of the particles involved 1) the particle's actual morphology as a single homogeneous phase, and 2) simple geometric shapes (spheres, cubes, and tetrahedra) depicting composition homogeneity or heterogeneity (with multiple spheres). Test models were compared with a reference model, which included the particle's actual morphology and heterogeneity based on SEM/EDX and FIB tomography. Results show particle shape to be a more important factor for determining extinction efficiency than accounting for individual phases in a particle, regardless of whether absorption or scattering dominated. In addition to homogeneous models with the particles' actual morphology, tetrahedral geometric models provided better extinction accuracy than spherical or cubic models. For iron-containing heterogeneous particles, the asymmetry parameter as well as SSA varied with the composition of the iron-containing phase, even if the phase was
      PubDate: 2017-08-24T07:35:22.154761-05:
      DOI: 10.1002/2017JD026488
  • Using radiocarbon to constrain black and organic carbon aerosol sources in
           Salt Lake City
    • Authors: Gergana O. Mouteva; James T. Randerson, Simon M. Fahrni, Susan E. Bush, James R. Ehleringer, Xiaomei Xu, Guaciara M. Santos, Roman Kuprov, Bret A. Schichtel, Claudia I. Czimczik
      Abstract: Black carbon (BC) and organic carbon (OC) aerosols are important components of fine particulate matter (PM2.5) in polluted urban environments. Quantifying the contribution of fossil fuel and biomass combustion to BC and OC concentrations is critical for developing and validating effective air quality control measures and climate change mitigation policy. We used radiocarbon (14C) to measure fossil and contemporary biomass contributions to BC and OC at three locations in Salt Lake City, Utah, USA during 2012-2014, including during winter inversion events. Aerosol filters were analyzed with the Swiss_4S thermal-optical protocol to isolate BC. We measured fraction modern (fM) of BC and total carbon in PM2.5 with accelerator mass spectrometry and derived the fM of OC using isotope mass balance. Combined with 14C information of endmember composition, our dataset of 31 14C aerosol measurements provided a baseline of the fossil and contemporary biomass components of carbonaceous aerosol. We show that fossil fuels were the dominant source of carbonaceous aerosol during winter, contributing 88% (80–98%) of BC and 58% (48–69%) of OC. While the concentration of both BC and OC increased during inversion events, the relative source contributions did not change. The sources of BC also did not vary throughout the year, while OC had a considerably higher contemporary biomass component in summer 62% (49-76%) and was more variable. Our results suggest that in order to reduce PM2.5 levels in Salt Lake City to meet national standards, a more stringent policy targeting mobile fossil fuel sources may be necessary.
      PubDate: 2017-08-24T06:55:19.351153-05:
      DOI: 10.1002/2017JD026519
  • Western North Pacific tropical cyclone model tracks in present and future
    • Authors: Jennifer Nakamura; Suzana J. Camargo, Adam H. Sobel, Naomi Henderson, Kerry A. Emanuel, Arun Kumar, Timothy E. LaRow, Hiroyuki Murakami, Malcolm J. Roberts, Enrico Scoccimarro, Pier Luigi Vidale, Hui Wang, Michael F. Wehner, Ming Zhao
      Abstract: Western North Pacific tropical cyclone (TC) model tracks are analyzed in two large multi-model ensembles, spanning a large variety of models and multiple future climate scenarios. Two methodologies are used to synthesize the properties of TC tracks in this large dataset: cluster analysis and mass moments ellipses. First, the models' TC tracks are compared to observed TC tracks' characteristics and a subset of the models is chosen for analysis, based on the tracks' similarity to observations and sample size. Potential changes in track types in a warming climate are identified by comparing the kernel smoothed probability distributions of various track variables in historical and future scenarios using a Kolmogorov-Smirnov significance test. Two track changes are identified. The first is a statistically significant increase in the North-South expansion, which can also be viewed as a poleward shift, as TC tracks are prevented from expanding equatorward due to the weak Coriolis force near the Equator. The second change is an eastward shift in the storm tracks that occur near the central Pacific in one of the multi-model ensembles, indicating a possible increase in the occurrence of storms near Hawaii in a warming climate. The dependence of the results on which model and future scenario are considered emphasizes the necessity of including multiple models and scenarios when considering future changes in TC characteristics.
      PubDate: 2017-08-24T05:45:23.434057-05:
      DOI: 10.1002/2017JD027007
  • Why are mixed-phase altocumulus clouds poorly predicted by large-scale
           models' Part I: Physical processes
    • Authors: Andrew I. Barrett; Robin J. Hogan, Richard M. Forbes
      Abstract: Mixed-phase layer clouds are radiatively important and their correct representation in numerical models of the atmosphere is needed for both weather forecasts and climate prediction. In particular mid-level mixed-phase layer clouds (altocumulus) are often poorly predicted. Here, the representation of altocumulus cloud in five operational models and the ERA-Interim reanalysis is evaluated using ground-based remote sensors. All models are found to underestimate the supercooled liquid water content by at least a factor of two. The models with the most sophisticated microphysics (separate prognostic variables for liquid and ice) had least supercooled liquid of all models, though they could simulate the correct liquid-over-ice structure of individual clouds. To investigate the reasons for the lack of predicted supercooled liquid water, a single column model (EMPIRE) was developed incorporating the relevant physical processes for altocumulus cloud. The supercooled liquid water was found to be most sensitive to factors that significantly affect the glaciation rate, including aspects of the ice microphysics formulation, as well as the model vertical resolution. Using observations to improve the ice particle size distribution formulation and the parametrization of ice cloud fraction also lead to a significant increase in supercooled liquid water in the simulated clouds. The study highlights the main parameterized processes that need careful attention in large-scale models in order to adequately represent the liquid phase in mixed-phase layer clouds. In Part II, the reason for the sensitivity to vertical resolution is investigated and a new parameterization for models with coarse vertical resolution is proposed.
      PubDate: 2017-08-18T09:20:22.4695-05:00
      DOI: 10.1002/2016JD026321
  • Why are mixed-phase altocumulus clouds poorly predicted by large-scale
    • Authors: Andrew I. Barrett; Robin J. Hogan, Richard M. Forbes
      Abstract: Single-column model simulations of mixed-phase altocumulus clouds were shown to have a strong vertical-resolution sensitivity in Part I of this paper. Coarse resolution models were unable to simulate the long-lived supercooled-liquid-layer at cloud top, typically only 200-m thick. In this paper, the sensitivity to vertical resolution is investigated using idealized simulations. Vertical gradients of ice water mixing ratio and temperature near cloud top are found to be inadequately represented at coarse-resolution. The vertical discretization using grid-box mean values, rather than the full vertical profile, leads to biased calculations of mixed-phase microphysical process rates and affects the diagnosis of thin liquid-water layers. As a result the liquid-water layer becomes quickly glaciated and altocumulus cloud lifetime is underestimated. Similar impacts are expected for mixed-phase boundary layer clouds commonly observed at high latitudes.A novel parameterization is introduced that accounts for the vertical gradients of ice water mixing ratio and temperature in the microphysics calculations and the diagnosis of liquid near cloud top. It substantially improves the representation of altocumulus layers in coarse vertical-resolution single-column model simulations and reduces the bias identified in Part I. The new parameterization removes the large underestimate in supercooled water content caused by the resolution sensitivity for temperatures warmer than −30∘C. Given the radiative importance of mixed-phase altocumulus clouds, their underestimation by numerical weather prediction models and their potential to act as a negative climate feedback, there is a need to re-evaluate the global climate sensitivity by implementing the findings in these two papers in a climate model.
      PubDate: 2017-08-17T18:25:43.764508-05:
      DOI: 10.1002/2016JD026322
  • A novel methodology to determine volcanic aerosols optical properties in
           the UV and NIR and Ångström parameters using sun-photometry
    • Authors: P. Sellitto; G. Salerno, A. La Spina, T. Caltabiano, L. Terray, P.-J. Gauthier, P. Briole
      Abstract: Remote volcanic aerosol optical depth (AOD) observations of Mt. Etna summit and distal bulk plume have been carried out between 14 and 20 July 2016 in the framework of the EPL-RADIO project. Ultraviolet (UV) and near-infrared (NIR) AODs were measured using a Microtops-II “Ozone Monitor” (MIIOM) sun-photometer, using a Langley plot (LP) instrumental calibration routine. Ozone-corrected UV AODs at 320 nm are derived for the first time with a Microtops, thus extending the exploitable spectral band range of portable photometers to shorter wavelengths. The new UV AODs have theoretical uncertainties
      PubDate: 2017-08-10T13:20:25.751978-05:
      DOI: 10.1002/2017JD026723
  • Measurements of light-absorbing particles in snow across the Arctic, North
           America, and China: effects on surface albedo
    • Authors: Cheng Dang; Stephen G. Warren, Qiang Fu, Sarah J. Doherty, Matthew Sturm
      Abstract: Using field observation, we perform radiative transfer calculations on snowpacks in the Arctic, China, and North America to quantify the impact of light-absorbing particles (LAPs) on snow albedo and its sensitivity to different factors. For new snow, the regional-averaged albedo reductions caused by all LAPs in the Arctic, North America, and China are 0.009, 0.012, and 0.077, respectively, of which the albedo reductions caused by black carbon (BC) alone are 0.005, 0.005, and 0.031, corresponding to a positive radiative forcing of 0.06, 0.3, and 3 Wm-2. The albedo reduction for old melting snow is larger than that of new snow by a factor of 2, for the same particulate concentrations; this leads to 3 – 8 times larger radiative forcing, in part due to higher solar irradiance in the melting season. These calculations used ambient snowpack properties; if all snowpacks were instead assumed to be optically thick, the albedo reduction would be 20-50% larger for new snow in the Arctic and North America and 120-300% larger for old snow. Accounting for non-BC LAPs reduces the albedo reduction by BC in the Arctic, North America, and China by 32%, 29% and 70% respectively for new snow and 11%, 7% and 51% for old snow. BC-in-snow albedo reduction computed using two-layer model agrees reasonably with that computed using multi-layer model. Biases in BC concentration or snow depth often lead to nonlinear biases in BC-induced albedo reduction.
      PubDate: 2017-08-05T13:05:39.111161-05:
      DOI: 10.1002/2017JD027070
  • Multivariate quadrature for representing cloud condensation nuclei
           activity of aerosol populations
    • Authors: Laura Fierce; Robert L. McGraw
      Abstract: Atmospheric aerosol is comprised of distinct multicomponent particles that are continuously modified as they are transported in the atmosphere. Resolving variability in particle physical and chemical properties requires tracking high-dimensional probability density functions, which is not practical in large-scale atmospheric simulations. Reduced representations of atmospheric aerosols are needed for efficient regional- and global-scale chemical transport models. Although the aerosol size-composition distribution is described by a high-dimensional probability density function, here we show that cloud condensation nuclei (CCN) activity of aerosol populations can be represented with high accuracy using an optimized set of representative particles. The sparse representation of the aerosol mixing state, designed for use in quadrature-based moment models, is constructed from a linear program that is combined with an entropy-inspired cost function. Unlike reduced representations common to large-scale atmospheric models, such as modal and sectional schemes, the maximum-entropy approach described here is not confined to pre-determined size bins or assumed distribution shapes. This study is a first step toward a quadrature-based aerosol scheme that will track multivariate aerosol distributions with sufficient computational efficiency for large-scale simulations.
      PubDate: 2017-07-26T23:50:25.15736-05:0
      DOI: 10.1002/2016JD026335
  • An influence of extreme southern hemisphere cold surges on the North
           Atlantic Subtropical High through a shallow atmospheric circulation
    • Authors: A. R. Bowerman; R. Fu, L. Yin, D. N. Fernando, P. A. Arias, R. E. Dickinson
      Abstract: Previous studies have attributed inter-hemisphere influences of the atmosphere to the latitudinal propagation of planetary waves crossing the equator, to the triggering of equatorial Kelvin waves, or to monsoonal circulation. Over the American-Atlantic sector, such cross-equatorial influences rarely occur during boreal summer due to unfavorable atmospheric conditions. We have observed that an alternative mechanism provides an inter-hemisphere influence. When episodes of extreme cold surges and upper tropospheric westerly winds occur concurrently over southern hemisphere Amazonia, cold surges from extratropical South America can penetrate deep into southern Amazonia. Although they do not appear to influence upper tropospheric circulation of the northern hemisphere, extremely strong southerly cross-equatorial advection (> 2σ) of cold and dense air in the lower troposphere can reach as least 10°N. Such cold advection increases the northward cross-equatorial pressure gradient in the lower to middle troposphere, thus shallow northerly return flow below 500 hPa. This return flow and the strong lower tropospheric southerly cross-equatorial flow form an anomalous shallow meridional circulation spanning from southern Amazonia to the subtropical North Atlantic, with increased geopotential height anomalies exceeding +1 standard deviation to at least 18°N. It projects onto the southern edge of the NASH, increasing its pressure and leading to equatorward expansion of NASH’s southern boundary. These anomalies enhance the North Atlantic Subtropical High (NASH), leading to its equatorward expansion. These extreme cold surges can potentially improving the predictability of weather patterns of the tropical and subtropical Atlantic, including the variability of the NASH’s southern edge.
      PubDate: 2017-07-21T10:20:23.575848-05:
      DOI: 10.1002/2017JD026697
  • Evaluation of NASA Deep Blue/SOAR aerosol retrieval algorithms applied to
           AVHRR measurements
    • Authors: A. M. Sayer; N. C. Hsu, J. Lee, N. Carletta, S.-H. Chen, A. Smirnov
      Abstract: The Deep Blue (DB) and Satellite Ocean Aerosol Retrieval (SOAR) algorithms have previously been applied to observations from sensors like the Moderate Resolution Imaging Spectroradiometers (MODIS) and Sea-viewing Wide Field-of-view Sensor (SeaWiFS) to provide records of midvisible aerosol optical depth (AOD) and related quantities over land and ocean surfaces respectively. Recently, DB and SOAR have also been applied to Advanced Very High Resolution Radiometer (AVHRR) observations from several platforms (NOAA11, NOAA14, and NOAA18), to demonstrate the potential for extending the DB and SOAR AOD records. This study provides an evaluation of the initial version (V001) of the resulting AVHRR-based AOD data set, including validation against Aerosol Robotic Network (AERONET) and ship-borne observations, and comparison against both other AVHRR AOD records and MODIS/SeaWiFS products at select long-term AERONET sites. Although it is difficult to distil error characteristics into a simple expression, the results suggest that one standard deviation confidence intervals on retrieved AOD of ±(0.03+15%) over water and ±(0.05+25%) over land represent the typical level of uncertainty, with a tendency towards negative biases in high-AOD conditions, caused by a combination of algorithmic assumptions and sensor calibration issues. Most of the available validation data are for NOAA18 AVHRR, although performance appears to be similar for the NOAA11 and NOAA14 sensors as well.
      PubDate: 2017-07-20T22:55:28.900108-05:
      DOI: 10.1002/2017JD026934
  • Retrieving Near-Global Aerosol Loading over Land and Ocean from AVHRR
    • Authors: N. C. Hsu; J. Lee, A. M. Sayer, N. Carletta, S.-H. Chen, C. J. Tucker, B. N. Holben, S.-C. Tsay
      Abstract: The spaceborne Advanced Very High Resolution Radiometer (AVHRR) sensor data record is approaching 40 years, providing a crucial asset for studying long-term trends of aerosol properties regionally and globally. However, due to limitations of its channels’ information content, aerosol optical depth (AOD) data from AVHRR over land are still largely lacking. In this paper, we describe a new physics-based algorithm to retrieve aerosol loading over both land and ocean from AVHRR for the first time. The over-land algorithm is an extension of our Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and Moderate Resolution Imaging Spectroradiometer (MODIS) Deep Blue algorithm, while a simplified version of our Satellite Ocean Aerosol Retrieval (SOAR) algorithm is used over ocean. We compare retrieved AVHRR AOD with that from MODIS on a daily and seasonal basis, and find in general good agreement between the two. For the satellites with equatorial crossing times within two hours of solar noon, the spatial coverage of the AVHRR aerosol product is comparable to that of MODIS, except over very bright arid regions (such as the Sahara), where the underlying surface reflectance at 630 nm reaches the critical surface reflectance. Based upon comparisons of the AVHRR AOD against Aerosol Robotic Network (AERONET) data, preliminary results indicate that the expected error confidence interval envelope is around ±(0.03+15%) over ocean and ±(0.05+25%) over land for this first version of the AVHRR aerosol products. Consequently, these new AVHRR aerosol products can contribute important building blocks for constructing a consistent long-term data record for climate studies.
      PubDate: 2017-07-20T06:27:13.096366-05:
      DOI: 10.1002/2017JD026932
  • Issue Information
    • Pages: 9555 - 9556
      Abstract: No abstract is available for this article.
      PubDate: 2017-10-13T11:11:53.02879-05:0
      DOI: 10.1002/jgrd.53229
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