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Journal of Meteorological Research
Number of Followers: 3  
 
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ISSN (Print) 2095-6037 - ISSN (Online) 2198-0934
Published by Springer-Verlag Homepage  [2468 journals]
  • Unprecedented Hot Extremes Observed in City Clusters in China during
           Summer 2022

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      Abstract: We report here extreme daytime and nighttime temperatures, severe heatwaves, and compound hot events recorded in China’ five most densely populated city clusters in summer 2022. New records were set, with daytime maximum temperatures > 42°C in cities along the Yangtze River valley and extreme nighttime temperatures > 30°C. Widespread prolonged heatwaves lasting for > 40 days and compound hot days occurring for > 32 consecutive days were experienced in these city clusters. To explore the possible causes of these extreme events, we analyzed the linkages between the changes in the mean temperatures and hot extremes for different-sized cities in the city clusters. We found that megacities (e.g., Beijing, Shanghai, Guangzhou, etc.) and large cities (e.g., Baoding, Wuxi, Foshan, etc.), especially those located in central and eastern China, experienced unprecedented extreme high temperatures, not only in the daytime but also at night. We observed large increases in the mean temperatures and more frequent and more intense hot extremes in cities affected by both the background global warming and intensified urbanization. Megacities and large cities experienced higher and more frequent extreme temperatures and greater warming trends than medium- and small-sized cities (e.g., Zhangjiakou, Zhenjiang, Yaan, etc.). The evidence of the dependence of temperature trends on a city’ size shows that intensified heat island effects may increase the threat of hot extremes in cities undergoing rapid urbanization.
      PubDate: 2023-04-01
       
  • On the Nature of Caspian Clouds

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      Abstract: Caspian clouds (CCs) are formed between the southern coast of the Caspian Sea and the Alborz Mountains. The purpose of this study is to identify characteristics of CCs using aerosol, cloud, and meteorological data from Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA-2), Moderate Resolution Imaging Spectroradiometer (MODIS), and ECMWF Reanalysis version 5 (ERA5) during 2000–2020. During this period, we identified and investigated 636 days with CCs. The results indicated that the frequency (%) of these clouds was higher in the summer than in other seasons because synoptic system activity varies between hot and cold periods. The hot season with the beginning of high-pressure subtropical Azores activity and the formation of a stable atmosphere in northern Iran leads to more frequent occurrence of CCs. These clouds are mainly the low- and middle-level clouds in the region, e.g., stratus and altocumulus. CCs resulted in 13.9% of annual rainfall, and 55.9% and 18.7% of the summer and autumn rainfall, respectively, relative to total rainfall from all cloud types in the study region. In the multivariate regression analysis, CC precipitation exhibited a strong positive relationship with the cloud water path (CWP), cloud optical thickness (COT), and cloud effective radius (CER). A comparison of the mean and standard deviation of aerosol optical thickness (AOT) and aerosol index (AI) for CC and non-CC days did not show a significant difference. Examination of the synoptic patterns showed that the main factors in the formation of CCs are the specific environmental conditions of the region and the orographic lift of stable air masses. The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model indicated that the source of moisture for the formation of CCs was largely the Caspian Sea.
      PubDate: 2023-04-01
       
  • Effect of 2-m Temperature Data Assimilation in the CMA-MESO 3DVAR System

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      Abstract: Assimilation of surface observations including 2-m temperature (T2m) in numerical weather prediction (NWP) models remains a challenging problem owing to differences between the elevation of model terrain and that of actual observation stations. NWP results can be improved only if surface observations are assimilated appropriately. In this study, a T2m data assimilation scheme that carefully considers misrepresentation of model and station terrain was established by using the three-dimensional variational data assimilation (3DVAR) system of the China Meteorological Administration mesoscale model (CMA-MESO). The corresponding forward observation operator, tangent linear operator, and adjoint operator for the T2m observations under three terrain mismatch treatments were developed. The T2m data were assimilated in the same method as that adopted for temperature sounding data with additional representative errors, when station terrain was 100 m higher than model terrain; otherwise, the T2m data were assimilated by using the surface similarity theory assimilation operator. Furthermore, if station terrain was lower than model terrain, additional representative errors were stipulated and corrected. Test of a rainfall case showed that the observation innovation and analysis residuals both exhibited Gaussian distribution and that the analysis increment was reasonable. Moreover, it was found that on completion of the data assimilation cycle, T2m data assimilation obviously influenced the temperature, wind, and relative humidity fields throughout the troposphere, with the greatest impact evident in the lower layers, and that both the area and the intensity of rainfall were better forecasted, especially for the first 12 hours. Long-term continuous experiments for 2–28 February and 5–20 July 2020, further verified that T2m data assimilation reduced deviations not only in T2m but also in 10-m wind forecasts. More importantly, the precipitation equitable threat scores were improved over the two experimental periods. In summary, this study confirmed that the T2m data assimilation scheme that we implemented in the kilometer-scale CMA-MESO 3DVAR system is effective.
      PubDate: 2023-04-01
       
  • Impact of Antecedent Soil Moisture Anomalies over the Indo-China Peninsula
           on the Super Meiyu Event in 2020

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      Abstract: In the summer of 2020, a super Meiyu event occurred in the Yangtze River basin (YRB), causing enormous economic losses and human casualties. Recent studies have investigated the possible causes of this super Meiyu event from the perspective of anomalous atmospheric circulation activities and sea surface temperature (SST) anomalies; however, the influence of land surface processes has not garnered considerable attention. This study investigates the possible contributions of land surface processes to this extreme event based on observational analysis and numerical simulations, and shows that antecedent soil moisture (SM) anomalies over the Indo-China Peninsula (ICP) may have had a vital influence on the super Meiyu in 2020. Negative SM anomalies in May over the ICP increased the surface temperature and sensible heat flux. The “memory” of soil allowed the anomalies to persist into the Meiyu period. The heating of the lower atmosphere by the surface strengthened the western Pacific subtropical high, which caused an anomalous anticyclone from the ICP to Northwest Pacific and thus enhanced the southwesterly winds and vertical motion over the YRB. Consequently, the water vapor flux and convergence were strengthened. Sensitivity experiments based on the Weather Research and Forecasting (WRF) model further confirmed the results of observational analysis and indicated that the warm air heated by the ICP surface significantly warmed the lower troposphere from the ICP to Northwest Pacific under the influence of the background wind, thus increasing the geopotential height and inducing an anticyclone. The results of the sensitivity experiments showed that the SM anomalies in May over the ICP increased the precipitation by 10.6% from June to July over the YRB. These findings can improve our understanding of the mechanism of the super Meiyu event in 2020 and facilitate the prediction of extreme Meiyu events.
      PubDate: 2023-04-01
       
  • Impacts of the SSTs over Equatorial Central–Eastern Pacific and
           Southeastern Indian Ocean on the Cold and Rainy/Snowy/Icy Weather in
           Southern China

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      Abstract: Low temperature together with snow/freezing rain is disastrous in winter over southern China. Previous studies suggest that this is related to the sea surface temperature (SST) anomalies, especially La Niña conditions, over the equatorial central-eastern Pacific Ocean (EP). In reality, however, La Niña episodes are not always accompanied by rainy/snowy/icy (CRSI) days in southern China, such as the case in winter 2020/2021. Is there any other factor that works jointly with the EP SST to affect the winter CRSI weather in southern China' To address this question, CRSI days are defined and calculated based on station observation data, and the related SST anomalies and atmospheric circulations are examined based on the Hadley Centre SST data and the NCEP/NCAR reanalysis data for winters of 1978/1979-2017/2018. The results indicate that the CRSI weather with more CRSI days is featured with both decreased temperature and increased winter precipitation over southern China. The SSTs over both the EP and the southeastern Indian Ocean (SIO) are closely related to the CRSI days in southern China with correlation coefficients of −0.29 and 0.39, significant at the 90% and 95% confidence levels, respectively. The SST over EP affects significantly air temperature, as revealed by previous studies, with cooler EP closely related to the deepened East Asian trough, which benefits stronger East Asian winter monsoon (EAWM) and lower air temperature in southern China. Nevertheless, this paper discovers that the SST over SIO affects precipitation of southern China, with a correlation coefficient of 0.42, significant at the 99% confidence level, with warmer SIO correlated with deepened southern branch trough (SBT) and strengthened western North Pacific anomalous anticyclone (WNPAC), favoring more water vapor convergence and enhanced precipitation in southern China. Given presence of La Niña in both winters, compared to the winter of 2020/2021, the winter of 2021/2022 witnessed more CRSI days, perhaps due to the warmer SIO.
      PubDate: 2023-04-01
       
  • Convection Initiation of a Heavy Rainfall Event in the Coastal
           Metropolitan Region of Shanghai on the South Side of the Meiyu Front

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      Abstract: Accurate prediction of the convection initiation (CI) in urban areas is still a challenge. A heavy rainfall event, missed by the 9-km regional operational modeling system, occurred in the coastal urban area of the Shanghai metropolitan region (SMR) in the late morning on 28 July 2020 on the warm side to the south of the Meiyu front. In this study, observational analyses and convection-permitting simulations with a resolution of 3 km were conducted to investigate the CI mechanism of this rainfall event. The results showed that the CI was due to the interaction of urban heat island (UHI), northwesterly outflows from the Meiyu front precipitation system (MFPS), and northeasterly sea winds. First, the UHI created a lifting condition producing adiabatic cooling and the vertical moisture transport in the urban region. Then, the mesolow generated by the UHI induced and enhanced local low-level convergence near the CI region and accelerated the northwesterly outflows and the northeasterly sea winds as they converged to the UHI. The convection was triggered as a result of the strengthened low-level convergence when the enhanced northwesterly outflows and northeasterly sea winds approached the updraft zone caused by the UHI center. Sensitivity experiments with either the urban area of the SMR removed or the MFPS suppressed further revealed that the enhancement of the low-level convergence was mainly contributed by the UHI. The outflows and sea winds transported cold and moist air to the CI region and partly offset the negative contribution of the urban drying effect to the low-level relative humidity to facilitate the development of the deep moist absolute unstable layer during the CI. In addition, the MFPS also contributed to the enhancement of the northeasterly sea winds by influencing the land–sea pressure contrast on the north of the SMR.
      PubDate: 2023-04-01
       
  • Impact of Tropical Cyclones over the North Indian Ocean on Weather in
           China and Related Forecasting Techniques: A Review of Progress

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      Abstract: Tropical cyclones (TCs) over the North Indian Ocean (NIO) are closely related to Asian summer monsoon activities and have a great impact on the precipitation in the Tibetan Plateau, southwestern China, and even the middle and lower reaches of the Yangtze River. In this paper, the research progress on the impacting mechanisms of NIO TCs on the weather in China and associated forecasting techniques is synthesized and reviewed, including characteristics of the NIO TC activity, its variability under climate change, related precipitation mechanism, and associated forecasting techniques. On this basis, the limitations and deficiencies in previous research on the physical mechanisms and forecasting techniques of NIO TCs affecting the weather in China are elucidated and the directions for future investigations are discussed.
      PubDate: 2023-04-01
       
  • Synoptic-Scale Analysis on Development and Maintenance of the 19–21 July
           2021 Extreme Heavy Rainfall in Henan, Central China

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      Abstract: In this paper, synoptic-scale analyses of frontogenesis, moisture budget, and tropospheric diabatic heating are performed to reveal the development and maintenance mechanisms for the extreme heavy rainfall in Henan Province of central China from 19 to 21 July 2021, based on station observations and the ECMWF Reanalysis version 5 (ERA5) data. The results demonstrate that owing to the blocking effect of local topography, low-level wind convergence in Henan appeared underneath high-level divergence, conducive to development and maintenance of a midtropospheric low-pressure system saddled by the Asian continental high and the western Pacific subtropical high (WPSH), during the extreme heavy rainfall. In the lower troposphere, frontogenesis occurred in the θse intensive region, as a result of the divergence and horizontal deformation (which play equally important roles), generating frontal secondary circulation with strong vertical motion favorable to heavy rainfall. Moisture budget analysis reveals that 1) with the continuous strengthening of the easterly wind from the north side of Typhoon In-Fa (2106), strong wind shear and orographic uplift led to abnormally strong convergence of water vapor flux in the boundary layer in Henan; 2) there occurred extremely strong net inflow of moisture in the boundary layer from the east. Horizontally, both the apparent heat source <Q1> and the moisture sink <Q2> coincided with the area of heavy rainfall; vertically, however, Q1 exhibited a single peak with the heating center in the middle and upper troposphere, while large Q2 values evenly resided over 850–400 hPa; and Q1 (Q2) was dominated by vertical (horizontal) transport of potential temperature (moisture). These indicate that the latent heat release from condensation of initial heavy rainfall provided a positive feedback, leading to increasingly heavy precipitation. All these synoptic settings sustained the extreme rainfall process.
      PubDate: 2023-04-01
       
  • Recent Unusual Consecutive Spring Tropical Cyclones in North Atlantic and
           Winter Oceanic Precursor Signals

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      Abstract: Although the North Atlantic (NA) hurricane season has been reported to start increasingly early, historical long-term changes in NA spring tropical cyclone (TC) activity have not been examined in previous studies. We find that spring TCs have become more frequent and intense, and they have more closely approached the US coastline over the past four decades, thus increasing the probability of landfall. A long-term increase in the NA sea surface temperature (SST) and northward incursion of warm water may be responsible for the increasing number, intensity, and northward shift of spring TCs. In addition, the interdecadal winter NA SST tripole mode may induce a North Atlantic Oscillation (NAO)-like atmospheric response in the following spring, which controls spring TC tracks via modulating the strength and position of the NA subtropical high (or the Bermuda high). The superposition of the interdecadal positive winter SST tripole on the NA warming trend has contributed to the anomalous enhancement and northward shift of spring TC activity, increasing the spring TC risks in the US in the past six years.
      PubDate: 2023-04-01
       
  • Observed Vertical Structure of Precipitation over the Southeastern Tibetan
           Plateau in Summer 2021

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      Abstract: Mêdog County, with its mountains and valleys, is located in the southeastern Tibetan Plateau (TP) and at the lower reaches of the Yarlung Zangbo River. This area has the highest annual rainfall amount over the TP, and in situ measurements are very scarce due to frequent debris flows and transportation difficulties. A monitoring campaign focused on cloud and precipitation observations was established in Mêdog in 2019 as a part of the Second Tibetan Plateau Scientific Expedition and Research Program. This paper evaluates the accuracy of micro rain radar (MRR) measurements and investigates the variations in precipitation vertical structure in Mêdog using observations collected from the MRR, disdrometer, and rain gauges in summer 2021. The measurements from the three instruments show a strong consistency, with correlation coefficients exceeding 0.93. Although the profiles of integral rain parameters for different rain rate categories in Mêdog are similar to those in other regions, the vertical evolution of raindrop size distributions shows significant differences. For lightest rain, the evaporation of small raindrops and breakup of large raindrops are clear during their descent. For the rainfall rate category of 0.2–2.0 mm h−1 (2.0–20.0 mm h−1), concentrations of small and medium (large) drops show almost uniform vertical structures, while the large (medium) drop number displays a positive (negative) gradient. A disturbance at height of 1.5–2.0 km above ground level (AGL) is observed in the heavy rainfall due to strong updrafts. In general, the MRR measurements in Mêdog are robust. The raindrop breakup process is more apparent in Mêdog than in other regions, resulting in high concentration of size-limited raindrops. In addition, it is found that the interaction between steep terrain and Mêdog convective rain causes the strong updrafts between 1.5 and 2.0 km AGL.
      PubDate: 2023-02-01
       
  • Meteorological Tower Observed CO2 Flux and Footprint in the Forest of
           Xiaoxing’an Mountains, Northeast China

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      Abstract: The Xiaoxing’an Mountains, located in the temperate monsoon climate zone in Northeast China, have the largest and most complete virgin Korean pine forest in Asia, which has great potential for carbon sequestration. Based on the observational data of the eddy-covariance system at Wuying National Climate Observatory in January 2015—November 2017, the CO2 flux in the forest ecosystem around the observatory was quantitatively studied and the distribution characteristics of the flux source area were analyzed by the Kljun model and the Agroscope Reckenholz—Tänikon footprint tool, providing references for assessing the carbon source/sink potential of the unique forest area. The results showed that the annual total carbon flux around the observatory in 2015, 2016, and 2017 was −756.84, −834.73, and −629.37 gC m−2, respectively, higher than that of other forest ecosystems. The forest of the study area in the Xiaoxing’an Mountains was a strong carbon sink, with the strongest carbon fixation capacity in June and weakest in October, and the carbon flux of each month was less than zero. The flux source area under stable atmospheric conditions was larger than that under unstable conditions, and the source area was larger in the nongrowing season than in the growing season. The size of the source area was largest in winter, followed by spring, autumn, and summer. The maximum length of the source area was 1614.12 m (5639.33 m) under unstable (stable) conditions when the flux contribution rate was 90%. The peak flux contribution was located near the sensor (i.e., within 200 m) in all seasons. The contribution of the source area from the coniferous and broadleaved mixed forest on the west side of the observatory was greater than (3.4 times) that from the Korean pine forest on the east side.
      PubDate: 2023-02-01
       
  • Thermal Wind Imbalance along the Curved Streamline of the Secondary
           Circulation in Tropical Cyclones

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      Abstract: The thermal wind balance in tropical cyclone (TC) eyewalls has been controversial for decades. This study reveals the relationship between the acceleration and curvature on the TC secondary circulation streamline, providing a way to judge thermal wind balance or imbalance in TCs from a simple but clear perspective. According to the relationship between the curvature and acceleration on the streamline, the vertical and radial components of the acceleration cannot be zero simultaneously on the streamline curve, implying that the thermal wind imbalance corresponds to the curvature of the streamline. On the regular scales of TCs, we discuss the conditions of the thermal wind balance approximation and find that the conditions become more stringent with increasing altitudes. In the TC secondary circulation, as an indication of thermal wind imbalance, gradient wind imbalance can be found in the low-level eyewall since there is usually a large curvature when the inflow in the low-level eyewall turns into updrafts sharply. Additionally, gradient wind imbalance also appears at the top level of TC eyewalls because the stringent conditions are too easily broken there.
      PubDate: 2023-02-01
       
  • A Diagnostic Study of the Influence of Early Spring Soil Moisture in
           Southeastern China on Interannual Variability of the East Asian
           Subtropical Summer Monsoon Onset

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      Abstract: The effect of soil moisture (SM) on the onset of East Asian subtropical summer monsoon (EASSM) is investigated based on multiple sets of reanalysis data in the period of 1981–2010. It is found that the EASSM is characterized by persistent 2-m s−1 southerly winds for about 3 months in spring at 850 hPa over the subtropical region of East Asia. Considering this feature of the meridional winds, we define the EASSM onset date, and obtain that the climatological onset date is pentad 17.7, around 26 March. On the interannual timescale, the onset date of EASSM exhibits statistically significant correlation with the SM in southeastern China in the month preceding the onset, with wetter (drier) conditions being associated with later (earlier) onset. The physical process by which the preceding SM affects the EASSM onset is further explored by examining the surface energy balance as well as its impacts. Positive (negative) SM anomalies in southeastern China in the month before onset may induce negative (positive) surface temperature anomalies. The decreased (increased) surface temperature in southeastern China before the EASSM onset weakens (strengthens) the zonal sea-land thermal contrast in the surface and low-level atmosphere in the subtropical East Asia. The zonal sea-land thermal contrast in wetter (drier) years induces anomalous northerly (southerly) winds over southeastern China, which tends to delay (advance) the zonal thermal seasonal transition in spring and is conducive to a later (earlier) onset of EASSM. These results are helpful for understanding and prediction of the variability of EASSM and the EASSM onset.
      PubDate: 2023-02-01
       
  • Synoptic Climate Settings and Moisture Supply for the Extreme Heavy
           Snowfall in Northern China during 6–8 November 2021

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      Abstract: A record-breaking extreme heavy snowfall (EHS) event hit northern China during 6–8 November 2021, with two maximum snowfall centers in North China (NC) and Northeast China (NEC), which inflicted severe socioeconomic impacts. This paper compares the differences in the synoptic processes and moisture supply associated with the EHS event in NC and NEC, as well as the atmospheric circulation anomalies before the event, to provide a reference for better prediction and forecasting of EHS in northern China. Synoptic analyses show that a positively tilted, inverted 500-hPa trough channeled cold-air outbreaks into NC, while dynamic updrafts along the front below the trough promoted moisture convergence over this region. In NEC, the dynamic updraft south of the frontogenesis region firstly triggered a low-level Yellow—Bohai Sea cyclone, which then converged with the 500-hPa trough to ultimately form an NEC cold vortex. Calculation of the vorticity tendency indicates that absolute vorticity advection was a better indicator than absolute vorticity divergence for the movement of the trough/ridge at the synoptic scale. Moreover, NOAA’s HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory) model results reveal that the moisture for the EHS over NC mainly originated from the mid-to-low levels over the Asian—African region and the Eurasian mid-to-high latitudes, accounting for 32% and 31%, respectively. In contrast, the source of water vapor for the EHS over NEC was mainly the Eurasian mid-to-high latitudes and East Asia, with contributions of 38% and 28%, respectively. The findings of this study shed some fresh light on the distinctive contributions of different moisture sources to local precipitation. Further analyses of the atmospheric circulation anomalies in October reveal that a phase shift in the Arctic Oscillation related to the weakening of the polar vortex could have served as a useful indicator for the cold-air outbreaks in this EHS event.
      PubDate: 2023-02-01
       
  • The Monsoon Low-Level Jet: Climatology and Impact on Monsoon Rainfall over
           the West Coast and Central Peninsular India

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      Abstract: The monsoon low-level jet (MLLJ) originates at Mascarene high and after traveling thousands of kilometers enters India from the western boundary causing deep convection, cloudiness, and rainfall. Although its core lies at 850 hPa, it has a large vertical extent; therefore, different meteorological parameters at different levels have a large influence on the Indian summer monsoon rainfall. This study aims to examine the upper-air climatology of 9 stations on the west coast and central Peninsular India and to find out the effects of various parameters at different standard pressure levels on the Indian summer monsoon rainfall variability. We used the 34-yr (1971–2004) actual upper-air radiosonde/radio wind and standard synoptic surface observations data from these 9 stations and reported some new aspects of the MLLJ. The NCEP/NCAR and ECMWF reanalysis wind data have also been used to holistically study the features of MLLJ over sea and land areas. This study, as opposed to some recent studies, confirms the splitting of MLLJ into two branches, which can be seen on a few days during the monsoon season. Further analyses show that the change in geopotential height between 800 and 900 hPa has a strong bearing on the strength of MLLJ. The change in the upper-air pressure gradient force over the Indian landmass can cause a change in the wind speed of MLLJ during the monsoon season.
      PubDate: 2023-02-01
       
  • CRA-40/Atmosphere—The First-Generation Chinese Atmospheric Reanalysis
           (1979–2018): System Description and Performance Evaluation

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      Abstract: Atmospheric reanalysis reproduces the past atmospheric conditions through assimilation of historical meteorological observations with fixed version of a numerical weather prediction (NWP) model and data assimilation (DA) system. It is widely used in weather, climate, and even business-related research and applications. This paper reports the development of CMA’s first-generation global atmospheric reanalysis (RA) covering 1979–2018 (CRA-40; CRA refers to CMA-RA). CRA-40 is produced by using the Global Spectral Model (GSM)/Gridpoint Statistical Interpolation (GSI) at a 6-h time interval and a TL574 spectral (34-km) resolution with the model top at 0.27 hPa. A large number of reprocessed satellite data and widely collected conventional observations were assimilated during the reanalyzing process, including the reprocessed atmospheric motion vector (AMV) products from FY-2C/D/E/G satellites, dense conventional observations (at about 120 radiosonde and 2400 synoptic stations) over China, as well as MWHS-2 and GNSS-RO observations from FY-3C. The reanalysis fitting to observations is improved over time, especially for surface pressure with root-mean-square error reduced from 1.05 hPa in 1979 to 0.8 hPa, and for upper air temperature from 1.65 K in 1979 to 1.35 K, in 2018. The patterns of global analysis increments for temperature, specific humidity, and zonal wind are consistent with the changes in the observing system. Near surface temperature from the model’s 6-h forecast reflects the global warming trend reasonably. The CRA-40 precipitation pattern matches well with those of GPCP and other reanalyses. CRA-40 also successfully captures the QBO and its vertical and temporal development, hemispherical atmospheric circulation change, and moisture transport by the East Asian summer monsoon. CRA is now operationally running in near real time as a climate data assimilation system in CMA.
      PubDate: 2023-02-01
       
  • Effects of Soil Hydraulic Properties on Soil Moisture Estimation

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      Abstract: Accurate quantification of soil moisture is essential to understand the land surface processes. Soil hydraulic properties influence water transport in soil and thus affect the estimation of soil moisture. However, some soil hydraulic properties are only observable at a few field sites. In this study, the effects of soil hydraulic properties on soil moisture estimation are investigated by using the one-dimensional (1-D) Richards equation at ELBARA, which is part of the Maqu monitoring network over the Tibetan Plateau (TP), China. Soil moisture assimilation experiments are then conducted with the unscented weighted ensemble Kalman filter (UWEnKF). The results show that the soil hydraulic properties significantly affect soil moisture simulation. Saturated soil hydraulic conductivity (Ksat) is optimized based on its observations in each soil layer with a genetic algorithm (GA, a widely used optimization method in hydrology), and the 1-D Richards equation performs well using the optimized values. If the range of Ksat for a complete soil profile is known for a particular soil texture (rather than for arbitrary layers within the horizon), optimized Ksat for each soil layer can be obtained by increasing the number of generations in GA, although this increases the computational cost of optimization. UWEnKF performs well with optimized Ksat, and improves the accuracy of soil moisture simulation more than that with calculated Ksat. Sometimes, better soil moisture estimation can be obtained by using optimized saturated volumetric soil moisture content Ksat. In summary, an accurate soil profile can be obtained by using soil moisture assimilation with optimized soil hydraulic properties.
      PubDate: 2023-02-01
       
  • Performance of a Global Spectral Model with Dry Air-Mass and Total
           Air-Mass Conserving Dynamical Cores: A Case Study of the July 2021 Henan
           Extreme Rainfall Event

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      Abstract: This study investigates the effects of the assumption on the types of air-mass conservation prescribed in numerical models. First, predictions of the July 2021 (“21.7”) Henan extreme rainfall event from the Integrated Forecast System (IFS) at ECMWF were compared with those from the Yin-He Global Spectral Model (YHGSM), which is a global spectral model with total air-mass conservation (TMC) and dry air-mass conservation (DMC) options. Then, a sensitivity test between simulations from the YHGSM adopting TMC and DMC was conducted. The results show that both the IFS and YHGSM predicted relatively well the 24-h rainfall amount less than 100 mm day−1 on 20 and 21 July 2021 at lead times of 84, 60, and 36 h. For heavy precipitation exceeding 100 mm day−1, however, both models obviously underestimated the daily rainfall amount on 20 July 2021, but the YHGSM produced more precise and stable precipitation forecasts on these two days than the IFS, especially the maximum 24-h precipitation amount, with better consistency at lead times of 84, 60, and 36 h. These differences are further examined in the sensitivity test. Predictions from the YHGSM with DMC show rainfall distributions and daily rainfall amounts closer to the observations at longer lead times. It is inferred that considering sources or sinks of total water in dynamical cores with DMC may have positive feedback for the precise prediction of condensates. For extreme rainfall events, the high local loss of total water may have caused a loss of the atmospheric mass, leading to an additional decrease in surface pressure. Subsequently, the unbalanced pressure gradient force enhances the cyclonic rotation of surface wind and strengthens convergence in the lower troposphere, which in turn further strengthens the vertical velocity, circularly contributing to the enhanced precipitation if the water vapor condition is favorable.
      PubDate: 2023-02-01
       
  • Interdecadal Variability of Summer Precipitation in Northwest China and
           Associated Atmospheric Circulation Changes

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      Abstract: Daily precipitation data from 149 rain gauge stations in China and NCEP—NCAR reanalysis data during 1961–2018 are used to investigate the interdecadal variability of summer precipitation in Northwest China and related causes. The results suggest that, on the interdecadal timescale, Northwest China shifts into a rainy period from the year 1987, with an increase in the precipitation amount and intensity; an increase in the probability of moderate rain, heavy rain, torrential rain, and extremely heavy rain; and a decrease in the probability of light rain. More than 60% of the increase in precipitation can be attributed to rainfall with intensity above the grade of heavy rain. The associated inter-decadal variability of atmospheric circulations over midlatitude Eurasia in summer is examined and it is found that the interdecadal variability is mainly characterized by the Silk Road pattern (SRP), with a cyclonic circulation anomaly and an anticyclonic circulation anomaly over central Asia and Mongolia, respectively; enhanced ascending motion and atmospheric instability in Northwest China; and strengthened easterly winds caused by the Mongolian anti-cyclonic anomaly along the northern boundary of the Tibetan Plateau. On the south side of the Mongolian anticyclone, the water vapor transported from the Pacific and Indian Oceans as well as the South China Sea to Northwest China by easterly winds increases significantly, providing the main water vapor source for the increase in precipitation in Northwest China on the interdecadal timescale. The transition of the Atlantic multidecadal oscillation to a positive phase may be the main cause of the interdecadal transition of the SRP to a positive phase, resulting in the inter-decadal increase in summer precipitation in Northwest China.
      PubDate: 2022-12-01
      DOI: 10.1007/s13351-022-2021-6
       
  • Impacts of the Urban Spatial Landscape in Beijing on Surface and Canopy
           Urban Heat Islands

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      Abstract: How does the urban spatial landscape (USL) pattern affect the land surface urban heat islands (SUHIs) and canopy urban heat islands (CUHIs)' Based on satellite and meteorological observations, this case study compares the impacts of the USL pattern on SUHI and CUHI in the central urban area (CUA) of Beijing using the satellite land-surface-temperature product and hourly temperature data from automatic meteorological stations from 2009 to 2018. Eleven USL metrics—building height (BH), building density (BD), standard deviation of building height (BSD), floor area ratio (FAR), frontal area index (FAI), roughness length (RL), sky view factor (SVF), urban fractal dimension (FD), vegetation coverage (VC), impervious coverage (IC), and albedo (AB)—with a 500-m spatial resolution in the CUA are extracted for comparative analysis. The results show that SUHI is higher than CUHI at night, and SUHI is only consistent with CUHI at spatial—temporal scales at night, particularly in winter. Spatially, all 11 metrics are strongly correlated with both the SUHI and CUHI at night, with stronger correlation between most metrics and SUHI. VC, AB, and SVF have the greatest impact on both the SUHI and CUHI. High SUHI and CUHI values tend to appear in areas with BD ⩾ 0.26, VC ⩽ 0.09, AB ⩽ 0.09, and SVF ⩽ 0.67. In summer, most metrics have a greater impact on the SUHI than CUHI; the opposite is observed in winter. SUHI variation is affected primarily by VC in summer and by VC and AB in winter, which is different for the CUHI variation. The collective contribution of all 11 metrics to SUHI spatial variation in summer (61.8%) is higher than that to CUHI; however, the opposite holds in winter and for the entire year, where the cumulative contribution of the factors accounts for 66.6% and 49.6%, respectively, of the SUHI variation.
      PubDate: 2022-12-01
      DOI: 10.1007/s13351-022-2045-y
       
 
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