In Press

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Original Paper
The concurrent record-breaking rainfall over Northwest India and North China in September 2021
Ying Na, Ri-Yu LU
, Available online   , Manuscript accepted  27 September 2022, doi: 10.1007/s00376-022-2187-y
Extremely heavy rainfall occurred over both Northwest India and North China in September 2021. The precipitation anomalies were 4.1 and 6.2 times interannual standard deviation and broke the record since the observational data were available, i.e., 1901 and 1951, respectively. In this month, the Asian upper-tropospheric westerly jet extremely displaced poleward over West Asia, and correspondingly, an anomalous cyclone appeared over India. The anomalous cyclone transported abundant water vapor into Northwest India, leading to the heavy rainfall over there. In addition, the Silk Road pattern, a teleconnection pattern over the Eurasian continent and fueled by the heavy rainfall in Northwest India, contributed to the heavy rainfall in North China. Our study emphasizes the roles of atmospheric teleconnection patterns in concurrent rainfall extremes in the remote regions, and the occurrence of rainfall extremes during the post- or pre-monsoon in the northern margins of monsoon regions.
Contrasting Regional Responses of Indian Summer Monsoon Rainfall to Exhausted Spring and Concurrently Emerging Summer El Niño Events
Krishnakumar E K, Abhilash S, Syam Sankar, Vijaykumar P, Santosh K R, Sreenath A V
, Available online   , Manuscript accepted  22 September 2022, doi: 10.1007/s00376-022-2114-2
The inverse relationship between the warm phase of the El Niño Southern Oscillation (ENSO) and the Indian Summer Monsoon Rainfall (ISMR) is well established. Yet, some El Niño events that occur in the early months of the year (boreal spring) transform into a neutral phase before the start of summer, whereas others begin in the boreal summer and persist in a positive phase throughout the summer monsoon season. This study investigates distinct influences of exhausted spring El Niño (springtime) and emerging summer El Niño (summertime) on the regional variability of ISMR. The two ENSO categories were formulated based on the time of occurrence of positive SST anomalies over the Niño-3.4 region in the Pacific. The ISMR's dynamical and thermodynamical responses to such events were investigated using standard metrics such as the Walker and Hadley circulations, vertically integrated moisture flux convergence (VIMFC), wind shear and upper atmospheric circulation. The monsoon circulation features are remarkably different in response to the exhausted spring El Niño and emerging summer El Niño phases, which distinctly dictate regional rainfall variability. The dynamical and thermodynamical responses reveal that exhausted spring El Niño events favour excess monsoon rainfall over eastern peninsular India and deficit rainfall over the core monsoon regions of central India. In contrast, emerging summer El Niño events negatively impact the seasonal rainfall over the country, except for a few regions along the west coast and northeast India.
Assessment of ERA5 and ERA-Interim in reproducing mean and extreme climates over West Africa
Imoleayo Gbode, Toju Babalola, Gulilat Diro, Joseph Daniel Intsiful
, Available online   , Manuscript accepted  22 September 2022, doi: 10.1007/s00376-022-2161-8
In situ data in West Africa are scarce, and reanalysis datasets could be an alternative source to alleviate the problem of data availability. Nevertheless, because of uncertainties in numerical prediction models and assimilation methods, amidst other things, existing reanalysis datasets can perform with various degrees of quality and accuracy. Therefore, proper assessment of their shortcomings and strengths should be performed prior to their usage. In this study, we examined the performance of ERA5 and ERA-interim (ERAI) products in representing the mean and extreme climates over West Africa for the period 1981-2018 using observations from CRU and CHIRPS. The major conclusion is that ERA5 showed a considerable decrease in precipitation and temperature biases and an improved representation of inter-annual variability in much of western Africa. Also, the annual cycle is better captured by ERA5 in three of the region’s climatic zones, specifically precipitation is well reproduced in Savannah and Guinea Coast and temperature in Sahel. In terms of extremes, ERA5 performance is superior but both reanalyses underestimate the intensity and frequency of heavy precipitations and overestimate the number of wet days as the numerical models used in reanalyses tend to drizzle more often. Despite the somewhat higher ERA5 performance in capturing observable long-term trends than the ERAI, both datasets are less successful in catching observed long-term trends. Although ERA5 has achieved considerable progress compared to its predecessor, improved datasets with better resolution and accuracy continue to be needed in sectors like agriculture and water resources to enable climate impact assessment.
A statistical algorithm for retrieving background value of absorbing aerosol index based on TROPOMI measurements
Fuying Tang, Weihe Wang, Fuqi Si, Haijin ZHOU, Yuhan Luo, Dongshang Yang, Yuanyuan Qian
, Available online   , Manuscript accepted  22 September 2022, doi: 10.1007/s00376-022-2093-3
The ultraviolet (UV) aerosol index (UVAI) is essential for monitoring the absorption during aerosol events. UVAI depends on the absorbing aerosol concentration, the viewing geometry, and the temporal drift of radiometric sensitivity. To efficiently detect absorbing aerosols with the highest precision and to improve the accuracy of long-term UVAI estimates, the background UVAI must be examined through the UVAI retrieval. This study presents a statistical method that calculates the background value of UVAI using TROPOspheric Monitoring Instrument (TROPOMI) observation data over the Pacific Ocean under clear-sky scenes. Radiative transfer calculations were performed to simulate the dependence of UVAI on aerosol type and viewing geometry. We firstly applied the background UVAI to reducing the effects of viewing geometry and the degradation of the TROPOMI irradiance measurements on the UVAI. The temporal variability of the background UVAI under the same viewing geometry and aerosol concentration was identified. Radiative transfer calculations were performed to study the changes in background UVAI using Aerosol Optical Depth from the Moderate Resolution Imaging Spectroradiometer (MODIS) and reflectance measurements from TROPOMI as input. The trends of the temporal variations in the background UVAI agreed with the simulations. Alterations in the background UVAI expressed the reflectance variations driven by the changes in satellite state. Decreasing trends in solar irradiance at 340 and 380 nm due to instrument degradation were identified. Our findings are valuable because they can be applied to future retrievals of UVAI from the Environmental Trace Gases Monitoring Instrument (EMI) onboard the GaoFen-5 satellite.
The Arctic sea ice thickness change in CMIP6’s historical simulations
Lanying Chen, Renhao Wu, Qi Shu, Chao Min, Qinghua Yang, Bo Han
, Available online   , Manuscript accepted  14 September 2022, doi: 10.1007/s00376-022-1460-4
This study assesses sea ice thickness (SIT) from the historical run of the Coupled Model Inter-comparison Project Phase 6 (CMIP6). The SIT reanalysis from the Pan-Arctic Ice Ocean Modeling and Assimilation System (PIOMAS) product is chosen as the validation reference data. Results show that most models can adequately reproduce the climatological mean, seasonal cycle, and long-term trend of Arctic Ocean SIT during 1979 - 2014, but significant inter-model spread exists. Differences in simulated SIT patterns among the CMIP6 models may be related to model resolution and sea ice model components. By comparing the climatological mean and trend for SIT among all models, we evaluate the Arctic SIT change in different seas during 1979 - 2014. Under the scenario of historical radiative forcing, the Arctic SIT will probably exponentially decay at -18 %/decade and plausibly reach its minimum (equilibrium) of 0.48 m since the 2070s.
Dataset of comparative observations for land surface processes over the semi-arid alpine grassland against alpine lake in the source region of the Yellow River
Xianhong Meng, Shihua LYU, Zhaoguo Li, Yinhuan Ao, Lijuan Wen, Lunyu Shang, Shaoying Wang, Mingshan Deng, Shaobo Zhang, Lin Zhao, Hao Chen, Di Ma, Suosuo LI, Lele Shu, Yingying An, Hanlin Niu
, Available online   , Manuscript accepted  14 September 2022, doi: 10.1007/s00376-022-2118-y
Thousands of lakes are located on the Tibetan Plateau (TP), playing a critical role in regional water cycle, weather and climate. In recent years, the areas of TP lakes are undergoing drastic changes and have become a research hotspot. However, the characteristics of the lake-atmosphere interaction over the high-altitude lakes are still unclear, which inhibits the model development and accurate simulation of lake climate effects. The source region of the Yellow River (SRYR) has the largest outflow lake and freshwater lake on the TP and is one of the most densely distributed lakes on the TP. Since 2011, three observation sites have been set up in the Ngoring Lake basin in the SRYR, to monitor the lake-atmosphere interaction and the difference of water- heat exchange over the land and lake surfaces. In this study, we present an eight-year (2012–2019) half-hourly observation dataset of lake–atmosphere interaction, composed of three sites. These sites represent the lake surface, the lakeside and the land, respectively. The observation items contain the basic meteorological elements, surface radiation, eddy covariance system, soil temperature and moisture (for land). The information of sites and instruments, the continuity and completeness of data and the differences of observation results among different sites are described in this study. These data have been used in the previous study to reveal a few energy and water exchange characteristics of TP lakes, to validate and improve the lake and land surface model. The dataset is available at National Cryosphere Desert Data Center.
Improvement and evaluation of the latest version of WRF-lake at a deep riverine reservoir
Shibo Guo, Dejun ZHU, Yongcan CHEN
, Available online   , Manuscript accepted  13 September 2022, doi: 10.1007/s00376-022-2180-5
The WRF-lake vertically one-dimensional (1D) water temperature model, as a sub-module of the Weather Research and Forecasting (WRF) system, is being widely used to investigate water-atmosphere interactions. But previous applications revealed that it cannot accurately simulate the water temperature in a deep riverine reservoir during large flow rate period, and whether it can produce sufficiently accurate heat flux through the water surface of deep riverine reservoirs remains uncertain. In this study, the WRF-lake model was improved for applications in large, deep riverine reservoirs by parametric scheme optimization, and the accuracy of heat flux calculation was evaluated compared with the results of a better physically-based model, the Delft3D-Flow, which was previously applied to different kinds of reservoirs successfully. The results show: (1) The latest version of WRF-lake can describe the surface water temperature to some extent, but performs poorly in the large flow period. We revised WRF-lake by modifying the vertical thermal diffusivity, then the water temperature simulation in the large flow period was improved significantly. (2) The latest version of WRF-lake overestimates the reservoir-atmosphere heat exchange around the year, mainly because of underestimating the energy downward transfer in the reservoir and resultant more heat remaining at the surface and returning to the atmosphere. The modification of vertical thermal diffusivity can improve the surface heat flux calculation significantly. (3) The longitudinal temperature variation and the temperature difference between inflow and outflow, which can not be considered in the 1D WRF-lake, can also affect the water surface heat flux.
Convective Storm VIL and Lightning Nowcasting Using Satellite and Weather Radar Measurements based on Multi-Task Learning Models
Yang Li, Yubao Liu, Rongfu SUN, Fengxia Guo, Xiaofeng Xu, Haixiang XU
, Available online   , Manuscript accepted  31 August 2022, doi: 10.1007/s00376-022-2082-6
Convective storms and lightning are among the most important weather phenomena that are challenging to forecast. In this study, a novel multi-task learning (MTL) encoder-decoder U-net neural network was developed to forecast convective storms and lightning with lead times for up to 90 min, using GOES-16 geostationary satellite infrared brightness temperatures (IRBTs), lightning flashes from Geostationary Lightning Mapper (GLM), and vertically integrated liquid (VIL) from Next Generation Weather Radar (NEXRAD). To cope with the heavily skewed distribution of lightning data, a spatiotemporal exponent-weighted loss function and log-transformed lightning normalization approach were developed. The effects of MTL, single-task learning (STL), and IRBTs as auxiliary input features on convection and lightning nowcasting were investigated. The results showed that normalizing the heavily skew-distributed lightning data along with a log-transformation dramatically outperforms the min-max normalization method for nowcasting an intense lightning event. The MTL model significantly outperformed the STL model for both lightning nowcasting and VIL nowcasting, particularly for intense lightning events. The MTL also helped delay the lightning forecast performance decay with the lead times. Furthermore, incorporating satellite IRBTs as auxiliary input features substantially improved lightning nowcasting, but produced little difference in VIL forecasting. Finally, the MTL model performed better for forecasting both lightning and the VIL of organized convective storms than for isolated cells.
CMIP6 Evaluation and Projection of Precipitation over Northern China: Further Investigation
Xiaoling Yang, Botao Zhou, Ying Xu, Zhen-Yu HAN
, Available online   , Manuscript accepted  31 August 2022, doi: 10.1007/s00376-022-2092-4
Based on 20 Coupled Model Intercomparison Project phase 6 (CMIP6) models, this article explored possible reasons for differences in simulation biases and projected changes of precipitation in northern China among all-model ensemble (AMME), "highest-ranked" model ensemble (BMME) and "lowest-ranked" model ensemble (WMME), from the perspective of atmospheric circulations and moisture budget. The results show that the BMME and AMME reproduce the East Asian winter circulations better than the WMME. Compared with the AMME and WMME, the BMME reduces the overestimation of evaporation, thereby improving the simulation of winter precipitation. The three ensemble simulated biases for the East Asian summer circulations are generally similar, featured with a stronger zonal pressure gradient between the mid-latitudes of the North Pacific and East Asia and a northward displacement of the East Asian westerly jet. However, the simulation of vertical moisture advection is improved in the BMME, contributing to slightly higher performance of the BMME than the AMME and WMME on summer precipitation in North and Northeast China. Concerning future changes, the BMME projects larger increases in precipitation in northern China during both seasons by the end of the 21st century under the Shared Socioeconomic Pathway 5-8.5 (SSP5-8.5) as compared to the AMME and WMME projections. One of the reasons is that the increase in evaporation projected by the BMME is larger. The BMME projected greater dynamic contribution also plays a role. In addition, larger changes of the nonlinear component in the BMME projection contribute to the larger increase in winter precipitation in northern China.
Summer extreme precipitation in the key region of the Sichuan-Tibet Railway
Jiali Ma, Xiuping Yao
, Available online   , Manuscript accepted  29 August 2022, doi: 10.1007/s00376-022-2133-z
The Sichuan-Tibet Railway, mainly located in the southeastern Qinghai-Tibet Plateau, is affected by summer extreme precipitation (SEP). Using rain-gauge daily precipitation and ERA5 reanalysis data for the summers of 1979–2020, the spatial-temporal distribution characteristics of SEP in the key region of Sichuan-Tibet Railway (28°N–33°N, 90°E–105°E, hereafter KR) are revealed and the cause for SEP amount (SEPA) variation in KR is investigated. The results show that SEPA in KR contributes to nearly 30% of summer precipitation and has a nocturnal percentage of nearly 75%. SEP exhibits significant regional differences. SEP thresholds are higher than 25mm d-1 in the plateau-dominated central-western KR (CWKR) and lower than 25mm d-1 in the basin-dominated eastern KR (EKR). Also, SEP in CWKR is less intense but more frequent than SEP in EKR. During the recent 42 years, SEPA in KR increased slightly while SEPA in CWKR increased significantly and peaked in the last decade. When anticyclonic circulation (AC) anomalies dominate the 500hPa level over the Bay of Bengal and Mongolia, the southerly and cyclonic shear over the southeastern plateau will be strengthened, favoring more SEPA in CWKR. When AC anomaly dominates the 500hPa over the Bohai Sea, the low-level easterly wind over the basin will be strengthened, favoring more SEPA in EKR. The strengthening of updraft, water vapor convergence and convective instability are conducive to more SEPA in KR. These results deepened the understanding of the characteristics and the cause for extreme precipitation in KR.
Extended Impact of Cold Air Invasions in East Asia in Response to a Warm South China Sea and Philippine Sea
Marco Y.-T. LEUNG, Dongxiao WANG, Wen ZHOU, Yuntao JIAN
, Available online   , Manuscript accepted  25 August 2022, doi: 10.1007/s00376-022-2096-0
During boreal winter, the invasion of cold air can lead to remarkable temperature drops in East Asia which can result in serious socioeconomic impacts. Here, we find that the intensity of strong synoptic cold days in the East China Sea and Indochina Peninsula are increasing. The enhanced synoptic cold days in these two regions are attributed to surface warming over the South China Sea and Philippine Sea (SCSPS). The oceanic forcing of the SCSPS on the synoptic cold days in the two regions is verified by numerical simulation. The warming of the SCSPS enhances the baroclinicity, which intensifies meridional wind and cold advection on synoptic timescales. This leads to a more extended region that is subject to the influence of cold invasion.
The combined effects of the tropical and extratropical quasi-biweekly oscillations in modulating the record-setting Meiyu rainfall in the summer of 2020
Zhen Huang, Shuanglin Li, Jianying Li, Chao Zhang
, Available online   , Manuscript accepted  25 August 2022, doi: 10.1007/s00376-022-2050-1
During June–July 2020, record strongest Meiyu occurred in the middle and lower reaches of the Yangtze River. The rainfall processes exhibited an obvious quasi-biweekly (biweekly in brief) variability, and there are altogether five cycles. It is found that the biweekly rainfall mainly arises from the collaborative effects of biweekly variablities from both the tropics and extratropics. As for the tropics, the biweekly meridional march and retreat of the western Pacific subtropical high (WPSH) is particularly evident. As for the extratropics, geopotential height anomalies near Lake Baikal are active. The former is attributed to the intensified biweekly activity of the southwest-northeast oriented East-Asian Pacific wavetrain (EAP) originating from the tropical western Pacific, while the latter is associated with the biweekly activities of the eastward propagating Eurasia mid-high latitudinal wavetrain and the westward propagating North Pacific wavetrain. In addition, it is found that the record-strongest convection anchoring over the tropical western Indian Ocean (IO) triggers anomalous descent over the tropical western Pacific, which modulates the biweekly activity of EAP. Meanwhile, the anomalous diabatic heating over IO causes changes of the meridional thermodynamic contrast across IO to the high-latitudes, which modulates the extratropical wavetrains. A further diagnosis of barotropic kinetic energy conversion suggests that the active occurrence of two extratropical biweekly wavetrains is attributed to the increased efficiency of energy conversion from basic flow. The westward propagation of the extratropical North Pacific wavetrain is attributed to the weakened and north-shifted upper-level westerly, which is caused by the SST warmth near the Kuroshio extension.
Response of the North Pacific Storm Track Activity in Cold Season to Multi-scale Oceanic Variations of Kuroshio Extension System: A Statistical Assessment
Peilong Yu, Minghao Yang, Chao Zhang, Yi Li, Lifeng Zhang, Shiyao Chen
, Available online   , Manuscript accepted  25 August 2022, doi: 10.1007/s00376-022-2044-z
In this paper, a statistical method called generalized equilibrium feedback analysis (GEFA) is used to investigate the responses of the North Pacific storm track (NPST) in cold season to the multi-scale oceanic variations of Kuroshio Extension (KE) system including its large-scale variation, oceanic front meridional shift and mesoscale eddy activity. Results show that in cold season from the lower to the upper troposphere, the KE large-scale variation significantly weakens the storm track activity over the central North Pacific south of 30°N. The northward shift of KE front significantly strengthens the storm track activity over the western and central North Pacific south of 40°N, resulting in a southward shift of NPST. In contrast, the NPST response to KE mesoscale eddy activity is not so significant and relatively shallow, which only shows some significant positive signals near the dateline in the lower and middle troposphere. Furthermore, it is found that baroclinicity and baroclinic energy conversion play an important role in the formation of NPST responses to the KE multi-scale oceanic variations.
Changes in Water Use Efficiency Caused by Climate Change, CO2 Fertilization, and Land Use Changes on the Tibetan Plateau
Binghao JIA, Xin LUO, Longhuan WANG, Xin LAI
, Available online   , Manuscript accepted  24 August 2022, doi: 10.1007/s00376-022-2172-5
Terrestrial ecosystem water use efficiency (WUE) is an important indicator for coupling plant photosynthesis and transpiration, and is also a key factor linking the carbon and water cycles between the land and atmosphere. However, under the combination of climate change and human intervention, the change in WUE is still unclear, especially on the Tibetan Plateau (TP). Therefore, satellite remote sensing data and process-based terrestrial biosphere models (TBMs) are used in this study to investigate the spatiotemporal variations of WUE over the TP from 2001 to 2010. Then, the effects of land use and land cover change (LULCC) and CO2 fertilization on WUE from 1981–2010 are assessed using TBMs. Results show that climate change is the leading contributor to the change in WUE on the TP, and temperature is the most important factor. LULCC makes a negative contribution to WUE (–20.63%), which is greater than the positive contribution of CO2 fertilization (11.65%). In addition, CO2 fertilization can effectively improve ecosystem resilience on the TP. On the northwest plateau, the effects of LULCC and CO2 fertilization on WUE are more pronounced during the driest years than the annual average. These findings can help researchers understand the response of WUE to climate change and human activity and the coupling of the carbon and water cycles over the TP.
Effects of a Dry-Mass Conserving Dynamical Core on the Simulation of Tropical Cyclones
Shaoying Li, Jun Peng, Weimin Zhang, Jianping Wu, Qiang Yao, Xiangrong Yang, Tengling LUO
, Available online   , Manuscript accepted  23 August 2022, doi: 10.1007/s00376-022-2085-3
The purpose of this paper was to investigate the effects of a dry-mass conserving (DMC) hydrostatic global spectral dynamical core on tropical cyclone (TC) simulation. Experiments were conducted with the DMC and total (moist) mass conserving (TMC) dynamical cores. The TC forecast performance was first evaluated considering 20 TCs in the West Pacific region observed during the 2020 typhoon season. The impacts of the DMC dynamical core on forecasts of individual TCs were then estimated. The DMC dynamical core improved both the track and intensity forecasts, and the TC intensity forecast improvement was much greater than the TC track forecast improvement. Sensitivity simulations indicated that the DMC dynamical core-simulated TC intensity was stronger regardless of the forecast lead time. In the DMC dynamical core, three-dimensional winds and warm and moist cores were consistently enhanced with the TC intensity. Drier air in the boundary inflow layer was found in the DMC dynamical core experiments at the early simulation times. Water vapor mixing ratio budget analysis indicated that this mainly depended on the simulated vertical velocity. Higher updraft above the boundary layer yielded a drier boundary layer, resulting in surface latent heat flux (SLHF) enhancement, the major energy source of TC intensification. The higher DMC dynamical core-simulated updraft in the inner core caused a higher net surface rain rate, producing higher net internal atmospheric diabatic heating and increasing the TC intensity. These results indicate that the stronger DMC dynamical core-simulated TCs are mainly related to the higher DMC vertical velocity.
The Regularized WSM6 Microphysical Scheme and its Validation in WRF 4D-Var
Sen YANG, Deqin Li, Liqiang CHEN, Zhiquan Liu, Xiang-Yu HUANG, Xiao PAN
, Available online   , Manuscript accepted  22 August 2022, doi: 10.1007/s00376-022-2058-6
A cold cloud assimilation scheme was developed that fully considered the water substances, i.e., water vapor, cloud water, rain, ice, snow, and graupel based on the single-moment WSM6 microphysical scheme and four-dimensional variational (4D-Var) data assimilation in the Weather Research and Forecasting data assimilation (WRFDA) system. The verification of the regularized WSM6 and its tangent linearity model (TLM) and adjoint mode model (ADM) were correctly proven. Two groups of single observation and real sounding data assimilation experiments were set up to further verify the correctness of the assimilation scheme. The results showed that with the consideration of ice, snow, and graupel in the assimilation system of the 4D-Var rather than the warm rain Kessler scheme, the water substances could be reasonably updated, further improving the forecast. Before it can be further applied in the assimilation of observational data, radar reflectivity and satellite radiance, the cold cloud assimilation scheme needs more verification, including the use of conventional ground and sounding observations in the 4D-Var assimilation system.
The Initial Mesoscale Vortexes Leading to the Formation of Tropical Cyclones in the Western North Pacific
Shenglan WU, Juan FANG
, Available online   , Manuscript accepted  22 August 2022, doi: 10.1007/s00376-022-2029-y
A statistical analysis of the initial vortexes leading to tropical cyclone (TC) formation in the western North Pacific (WNP) is conducted with the ECMWF ERA5 reanalysis data from 1999 to 2018. It is found that TCs in the WNP basically originate from three kinds of vortexes, i.e., a mid-level vortex (MV), a low-level vortex (LV), and a relatively deep vortex with notable vorticity in both the lower and middle troposphere (DV). Among them, LV and DV account for 47.9% and 24.2% of tropical cyclogenesis events, respectively, while only 27.9% of TCs develop from the MV, which is much lower than that which occurs in the North Atlantic and eastern Pacific. Such a difference might be ascribed to the active monsoon systems in the WNP all year round. Due to the nearly upright structure of mid-level convergence in the early pre-genesis stage, TC genesis efficiency is the highest in DV. Compared with MV, LV generally takes a shorter time to intensify to a TC because of the higher humidity and the stronger low-level cyclonic circulation, which is related to air-sea interaction and boundary-layer convergence. Further examination of the relationship between tropical cyclogenesis and large-scale flow patterns indicate that the TC genesis events associated with LV are primarily related to the monsoon shear line, monsoon confluence region, and monsoon gyre, while those associated with MV are frequently connected with easterly waves and wave energy dispersion of preexisting TC. Compared with other flow patterns, tropical cyclones usually form and intensify faster in the monsoon confluence region.
Elucidating Dominant Factors Affecting Land Surface Hydrological Simulations of the Community Land Model over China
Jianguo Liu, Zong-Liang Yang, Binghao Jia, Longhuan WANG, Ping WANG, Zhenghui Xie, Chunxiang Shi
, Available online   , Manuscript accepted  17 August 2022, doi: 10.1007/s00376-022-2091-5
In order to compare the impacts of the choice of land surface model (LSM) parameterization schemes, meteorological forcing, and land surface parameters on the land surface hydrological simulations, and explore to what extent the quality can be improved, a series of experiments with different LSMs, forcing datasets, and parameter datasets concerning soil texture and land cover were conducted. Six simulations are run for mainland China in 0.1o×0.1o grids from 1979 to 2008, and the simulated monthly soil moisture (SM), evapotranspiration (ET) and snow depth (SD) are then compared and assessed against observations. The results show that the meteorological forcing is the most important factor governing output. Beyond that, SM seems to be also very sensitive to soil texture information; SD is also very sensitive to snow parameterization scheme in LSM. The Community Land Model version 4.5 (CLM4.5) driven by newly developed observation-based regional meteorological forcing and land surface parameters (referred to as CMFD_CLM4.5_NEW) significantly improved the simulations in most cases over mainland China and its eight basins. It increased the correlation coefficient values from 0.46 to 0.54 for the SM modeling, from 0.54 to 0.67 for the SD simulations, and decreased the root-mean-square-error (RMSE) from 0.093 to 0.085 for the SM simulation, reduced the normalized RMSE from 1.277 to 0.201 for the SD simulations. This study indicates that the offline LSM simulation using refined LSM driven by newly developed observation-based regional meteorological forcing and land surface parameters can better model reginal land surface hydrological processes.
The Influence of Meridional Shift of the North Pacific Sea Surface Temperature Anomalies on the Arctic Stratospheric Polar Vortex
Tao Wang, Qiang Fu, Wenshou Tian, Hongwen Liu, Yifeng Peng, Fei Xie, Hongying Tian, Jiali Luo
, Available online   , Manuscript accepted  16 August 2022, doi: 10.1007/s00376-022-2033-2
This study examines the dependence of the Arctic stratospheric polar vortex (SPV) variations on the meridional positions of the sea surface temperature (SST) anomalies associated with the first leading mode of the North Pacific SST. The principal component 1 (PC1) of the first leading mode is obtained by empirical orthogonal function decomposition. The reanalysis data, numerical experiments and CMIP5 model outputs all suggest that the PC1 events (positive-minus-negative PC1 events) located relatively north (i.e., North PC1 events) are more easily to weaken the Arctic SPV compared to the PC1 events located south (i.e., South PC1 events). The analysis indicates that the North PC1-related Aleutian low anomaly is located over the northern North Pacific and thus enhances the climatological trough, which strengthens the planetary-scale wave 1 at mid-to-high latitudes and thereby weakens the SPV. The weakened stratospheric circulation further extends into troposphere and favors negative surface temperature anomalies over Eurasia. By contrast, the South PC1-related Aleutian low anomaly is located relatively south, and its constructive interference with the climatological trough is less efficient at high latitudes. Thus, the South PC1 events could not induce an evident enhancement of the planetary-scale waves at high latitudes and thereby a weakening of the SPV on average. The Eurasian cooling associated with South PC1 events (positive-minus-negative PC1 events) is also not prominent. Our results suggest that the meridional positions of the PC1 events may be useful for predicting the Arctic SPV and Eurasian surface temperature variations.
Response of Freezing/Thawing Indexes to the Wetting Trend under Warming Climate Conditions over the Qinghai –Tibetan Plateau during 1961–2010: A Numerical Simulation
Xuewei FANG, Zhi LI, Chen CHENG, Klaus FRAEDRICH, Anqi WANG, Yihui CHEN, Yige XU, Shihua LYU
, Available online   , Manuscript accepted  15 August 2022, doi: 10.1007/s00376-022-2109-z
Since the 1990s, the Qinghai–Tibetan Plateau (QTP) has experienced a strikingly warming and wetter climate that alters the thermal and hydrological properties of frozen ground. A positive correlation between the warming and thermal degradation in permafrost or seasonally frozen ground (SFG) has long been recognized. Still, a predictive relationship between historical wetting under warming climate conditions and frozen ground has not yet been well demonstrated, despite the expectation that it will become even more important because precipitation over the QTP has been projected to increase continuously in the near future. This study investigates the response of the thermal regime to historical wetting in both permafrost and SFG areas and examines their relationships separately using the Community Land Surface Model version 4.5. Results show that wetting before the 1990s across the QTP mainly cooled the permafrost body in the arid and semiarid zones, with significant correlation coefficients of 0.60 and 0.48, respectively. Precipitation increased continually at the rate of 6.16 mm decade–1 in the arid zone after the 1990s but had a contrasting warming effect on permafrost through a significant shortening of the thawing duration within the active layer. However, diminished rainfall in the humid zone after the 1990s also significantly extended the thawing duration of SFG. The relationship between the ground thawing index and precipitation was significantly negatively correlated (−0.75). The dual effects of wetting on the thermal dynamics of the QTP are becoming critical because of the projected increases in future precipitation.
Effects of Dropsonde Data in Field Campaigns on Forecasts of Tropical Cyclones over the Western North Pacific in 2020 and the Role of CNOP Sensitivity
Xiaohao QIN, Wansuo DUAN, Pak-Wai CHAN, Boyu CHEN, Kang-Ning HUANG
, Available online   , Manuscript accepted  12 August 2022, doi: 10.1007/s00376-022-2136-9
Valuable dropsonde data were obtained from multiple field campaigns targeting tropical cyclones, namely Higos, Nangka, Saudel, and Atsani, over the western North Pacific by the Hong Kong Observatory and Taiwan Central Weather Bureau in 2020. The conditional nonlinear optimal perturbation (CNOP) method has been utilized in real-time to identify the sensitive regions for targeting observations adhering to the procedure of real-time field campaigns for the first time. The observing system experiments were conducted to evaluate the effect of dropsonde data and CNOP sensitivity on TC forecasts in terms of track and intensity, using the Weather Research and Forecasting model. It is shown that the impact of assimilating all dropsonde data on both track and intensity forecasts is case-dependent. However, assimilation using only the dropsonde data inside the sensitive regions displays unanimously positive effects on both the track and intensity forecast, either of which obtains comparable benefits to or greatly reduces deterioration of the skill when assimilating all dropsonde data. Therefore, these results encourage us to further carry out targeting observations for the forecast of tropical cyclones according to CNOP sensitivity.
Influences of MJO-induced Tropical Cyclones on the Circulation-Convection Inconsistency for the 2021 South China Sea Summer Monsoon Onset
Yanying CHEN, Ning JIANG, Yang AI, Kang XU, Longjiang MAO
, Available online   , Manuscript accepted  12 August 2022, doi: 10.1007/s00376-022-2103-5
The South China Sea Summer Monsoon (SCSSM) onset is characterized by an apparent seasonal conversion of circulation and convection. Accordingly, various indices have been introduced to identify the SCSSM onset date. However, the onset dates as determined by various indices can be very inconsistent. It not only limits the determination of onset dates but also misleads the assessment of prediction skills. In 2021, the onset time as identified by the circulation criteria was May 20, which is 12 days earlier than that deduced by also considering the convection criteria. The present study mainly ascribes such circulation-convection inconsistency to the activities of tropical cyclones (TCs) modulated by the Madden-Julian Oscillation (MJO). The convection of TC “Yaas” (2021) acted as an upper-level diabatic heat source to the north of the SCS, facilitating the circulation transition. Afterward, TC “Choi-wan” (2021) over the western Pacific aided the westerlies to persist at lower levels while simultaneously suppressing moist convection over the SCS. Accurate predictions using the ECMWF S2S forecast system were obtained only after the MJO formation. The skillful prediction of the MJO during late spring may provide an opportunity to accurately predict the establishment of the SCSSM several weeks in advance.
A New Sensitivity Analysis Approach Using Conditional Nonlinear Optimal Perturbations and Its Preliminary Application
Qiujie REN, Mu MU, Guodong SUN, Qiang WANG
, Available online   , Manuscript accepted  12 August 2022, doi: 10.1007/s00376-022-1445-3
Simulations and predictions using numerical models show considerable uncertainties, and parameter uncertainty is one of the most important sources. It is impractical to improve the simulation and prediction abilities by reducing the uncertainties of all parameters. Therefore, identifying the sensitive parameters or parameter combinations is crucial. This study proposes a novel approach: conditional nonlinear optimal perturbations sensitivity analysis (CNOPSA). The CNOPSA method fully considers the nonlinear synergistic effects of parameters in the whole parameter space and quantitatively estimates the maximum effects of parameter uncertainties, prone to extreme events. Results of the analytical g-function test indicate that the CNOPSA method can effectively identify the sensitivity of variables. Numerical results of the theoretical five-variable grassland ecosystem model show that the maximum influence of the simulated wilted biomass caused by parameter uncertainty can be estimated and computed by employing the CNOPSA method. The identified sensitive parameters can easily change the simulation or prediction of the wilted biomass, which affects the transformation of the grassland state in the grassland ecosystem. The variance-based approach may underestimate the parameter sensitivity because it only considers the possibility of limited parameter samples from a statistical view. This study verifies that the CNOPSA method is effective and feasible for exploring the important and sensitive physical parameters or parameter combinations in numerical models.
A Precursory Signal of June–July Precipitation over the Yangtze River Basin: December–January Tropospheric Temperature over the Tibetan Plateau
Xiaying ZHU, Mingzhu YANG, Ge LIU, Yanju LIU, Weijing LI, Sulan NAN, Linhai SUN
, Available online   , Manuscript accepted  04 August 2022, doi: 10.1007/s00376-022-2079-1
The prediction of summer precipitation over the Yangtze River basin (YRB) has long been challenging, especially during June–July (JJ), when the mei-yu generally occurs. This study explores the potential signal for the YRB precipitation in JJ and reveals that the Tibetan Plateau tropospheric temperature (TPTT) in the middle and upper levels during the preceding December–January (DJ) is significantly correlated with JJ YRB precipitation. The close connection between the DJ TPTT anomaly with JJ YRB precipitation may be due to the joint modulation of the DJ ENSO and spring TP soil temperatures. The lagged response to an anomalously cold TPTT during the preceding DJ is a TPTT that is still anomalously cold during the following JJ. The lower TPTT can lead to an anomalous anticyclone to the east of Lake Baikal, an anomalous cyclone at the middle latitudes of East Asia, and an anomalous anticyclone over the western North Pacific. Meanwhile, the East Asian westerly jet shifts southward in response to the meridional thermal gradient caused by the colder troposphere extending from the TP to the east of Lake Baikal. The above-mentioned circulation anomalies constitute the positive anomaly of the East Asia-Pacific pattern, known to be conducive to more precipitation over the YRB. Since the DJ TPTT contains both the land (TP soil temperature) and ocean (ENSO) signals, it has a closer relationship with the JJ precipitation over the YRB than the DJ ENSO alone. Therefore, the preceding DJ TPTT can be considered an alternative predictor of the JJ YRB precipitation.
Toward Quantifying the Increasing Accessibility of the Arctic Northeast Passage in the Past Four Decades
Chao MIN, Xiangying ZHOU, Hao LUO, Yijun YANG, Yiguo WANG, Jinlun ZHANG, Qinghua YANG
, Available online   , Manuscript accepted  04 August 2022, doi: 10.1007/s00376-022-2040-3
Sea ice, one of the most dominant barriers to Arctic shipping, has decreased dramatically over the past four decades. Arctic maritime transport is hereupon growing in recent years. To produce a long-term assessment of trans-Arctic accessibility, we systematically revisit the daily Arctic navigability with a view to the combined effects of sea ice thickness and concentration throughout the period 1979−2020. The general trends of Navigable Windows (NW) in the Northeast Passage show that the number of navigable days is steadily growing and reached 89±16 days for Open Water (OW) ships and 163±19 days for Polar Class 6 (PC6) ships in the 2010s, despite high interannual and interdecadal variability in the NWs. More consecutive NWs have emerged annually for both OW ships and PC6 ships since 2005 because of the faster sea ice retreat. Since the 1980s, the number of simulated Arctic routes has continuously increased, and optimal navigability exists in these years of record-low sea ice extent (e.g., 2012 and 2020). Summertime navigability in the East Siberian and Laptev Seas, on the other hand, varies dramatically due to changing sea ice conditions. This systematic assessment of Arctic navigability provides a reference for better projecting the future trans-Arctic shipping routes.
Possibility of Solid Hydrometeor Growth Zone Identification Using Radar Spectrum Width
Sung-Ho SUH, Eun-Ho CHOI, Hong-Il KIM, Woonseon JUNG
, Available online   , Manuscript accepted  02 August 2022, doi: 10.1007/s00376-022-1472-0
In this study, the correlation between simulated and measured radar velocity spectrum width (σv) is investigated. The results show that the dendrites growth zones (DGZs) and needles growth zones (NGZs) mostly contain dendrites (DN) and needles (NE), respectively. Clear σv zones (1.1 < σv (m s–1) < 1.3 and 0.3 < σv (m s–1) < 0.7 for the DGZ and NGZ, respectively) could be identified in the case studies (27 and 28 February 2016) near altitudes corresponding to temperatures of –15°C and –5°C, according to the Japan Meteorological Agency and mesoscale model reanalysis data. Oblate particles with diverse particle shapes were observed in the DGZ with σv > 1.2 m s–1, a differential reflectivity (ZDR) higher than 0 dB, and a cross-correlation coefficient (ρhv) less than 0.96. In contrast, prolate particles with relatively uniform shapes were observed in the NGZ with σv < 0.6 m s–1, a ZDR less than 0 dB, and ρhv higher than 0.97. The simulation results show that the DN exhibited a larger σv compared to the NE, and this observed σv was strongly dependent on the wind fluctuations (v’) due to turbulence or wind shear. In contrast, the NE exhibited a significantly small σv ~ 0.55 m s–1, which converges irrespective of v’. In addition, a strong correlation between the measured σv values at five radar elevation angles (θ = 6.2°, 9.1°, 13.1°, 19°, and 80°) and those simulated in this study confirmed the significance of the analysis results.
Decomposition of Fast and Slow Cloud Responses to Quadrupled CO2 Forcing in BCC–AGCM2.0 over East Asia
Xixun ZHOU, Bing XIE, Hua ZHANG, Jingyi HE, Qi CHEN
, Available online   , Manuscript accepted  01 August 2022, doi: 10.1007/s00376-022-1441-7
In this study, the decomposed fast and slow responses of clouds to an abruptly quadrupled CO2 concentration (approximately 1139 ppmv) in East Asia (EA) are obtained quantitatively by using a general circulation model, BCC–AGCM2.0. Our results show that in the total response, the total cloud cover (TCC), low cloud cover (LCC), and high cloud cover (HCC) all increased north of 40°N and decreased south of 40°N except in the Tibetan Plateau (TP). The mean changes of the TCC, LCC, and HCC in EA were –0.74%, 0.38%, and –0.38% in the total response, respectively; 1.05%, –0.03%, and 1.63% in the fast response, respectively; and –1.79%, 0.41%, and –2.01% in the slow response, respectively. By comparison, we found that changes in cloud cover were dominated by the slow response in most areas in EA due to the changes in atmospheric temperature, circulation, and water vapor supply together. Overall, the changes in the cloud forcing over EA related to the fast and slow responses were opposite to each other, and the final cloud forcing was dominated by the slow response. The mean net cloud forcing (NCF) in the total response over EA was –1.80 W m–2, indicating a cooling effect which partially offset the warming effect caused by the quadrupled CO2. The total responses of NCF in the TP, south China (SC), and northeast China (NE) were –6.74 W m–2, 6.11 W m–2, and –7.49 W m–2, respectively. Thus, the local effects of offsetting or amplifying warming were particularly obvious.
Ground-Based Atmospheric CO2, CH4, and CO Column Measurements at Golmud in the Qinghai-Tibetan Plateau and Comparisons with TROPOMI/S5P Satellite Observations
Minqiang ZHOU, Qichen NI, Zhaonan CAI, Bavo LANGEROCK, Jingyi JIANG, Ke CHE, Jiaxin WANG, Weidong NAN, Yi LIU, Pucai WANG
, Available online   , Manuscript accepted  18 July 2022, doi: 10.1007/s00376-022-2116-0
Measurements of carbon dioxide (CO2), methane (CH4), and carbon monoxide (CO) are of great importance in the Qinghai-Tibetan region, as it is the highest and largest plateau in the world affecting global weather and climate systems. In this study, for the first time, we present CO2, CH4, and CO column measurements carried out by a Bruker EM27/SUN Fourier-transform infrared spectrometer (FTIR) at Golmud (36.42ºE, 94.91ºN, 2808 m) in August 2021. The mean and standard deviation of the column-average dry-air mixing ratio of CO2, CH4, and CO (XCO2, XCH4, and XCO) are 409.3 ± 0.4 ppm, 1905.5 ± 19.4 ppb, and 103.1 ± 7.7 ppb, respectively. The differences between the FTIR co-located TROPOMI/S5P satellite measurements at Golmud are 0.68 ± 0.64% (13.1 ± 12.2 ppb) for XCH4 and 9.81 ± 3.48% (–10.7 ± 3.8 ppb) for XCO, which are within their retrieval uncertainties. High correlations for both XCH4 and XCO are observed between the FTIR and S5P satellite measurements. Using the FLEXPART model and satellite measurements, we find that enhanced CH4 and CO columns in Golmud are affected by anthropogenic emissions transported from North India. This study provides an insight into the variations of the CO2, CH4, and CO columns in the Qinghai-Tibetan Plateau.
Role of Ocean Dynamics in the Seasonal Hadley Cell: A Response to Idealized Arctic Amplification
Haijin DAI, Qiang YAO
, Available online   , Manuscript accepted  18 July 2022, doi: 10.1007/s00376-022-2057-7
How atmospheric and oceanic circulations respond to Arctic warming at different timescales are revealed with idealized numerical simulations. Induced by local forcing and feedback, Arctic warming appears and leads to sea-ice melting. Deep-water formation is inhibited, which weakens the Atlantic Meridional Overturning Circulation (AMOC). The flow and temperature in the upper layer does not respond to the AMOC decrease immediately, especially at mid-low latitudes. Thus, nearly uniform surface warming in mid-low latitudes enhances (decreases) the strength (width) of the Hadley cell (HC). With the smaller northward heat carried by the weaker AMOC, the Norwegian Sea cools significantly. With strong warming in Northern Hemisphere high latitudes, the long-term response triggers the “temperature-wind-gyre-temperature” cycle, leading to colder midlatitudes, resulting in strong subsidence and Ferrel cell enhancement, which drives the HC southward. With weaker warming in the tropics and stronger warming at high latitudes, there is a stronger HC with decreased width. A much warmer Southern Hemisphere appears due to a weaker AMOC that also pushes the HC southward. Our idealized model results suggest that the HC strengthens under both warming conditions, as tropical warming determines the strength of the HC convection. Second, extreme Arctic warming led by artificially reduced surface albedo decreases the meridional temperature gradient between high and low latitudes, which contracts the HC. Third, a warmer mid-high latitude in the Northern (Southern) Hemisphere due to surface albedo feedback (weakened AMOC) in our experiments pushes the HC northward (southward). In most seasons, the HC exhibits the same trend as that described above.
The Roles of Low-level Jets in “21·7” Henan Extremely Persistent Heavy Rainfall Event
Yuhan LUO, Yu DU
, Available online   , Manuscript accepted  14 July 2022, doi: 10.1007/s00376-022-2026-1
An extremely heavy rainfall event lasting from 17 to 22 July 2021 occurred in Henan Province of China, with accumulated precipitation of more than 1000 mm over a 6-day period that exceeded its mean annual precipitation. The present study examines the roles of persistent low-level jets (LLJs) in maintaining the precipitation using surface station observations and reanalysis datasets. The LLJs triggered strong ascending motions and carried moisture mainly from the outflow of Typhoon In-fa (2021). The varying directions of the LLJs well corresponded to the meridional shifts of the rainfall The precipitation rate reached a maximum during 20−21 July as the LLJs strengthened and expanded vertically into double LLJs, including synoptic-weather-system-related LLJs (SLLJs) at 850–700 hPa and boundary-layer jets (BLJs) at ~950 hPa. The coupling of the SLLJ and BLJ provided strong mid- and low-level convergence on 20 July, whereas the SLLJ produced mid-level divergence at its entrance that coupled with low-level convergence at the terminus of the BLJ on 21 July. The formation mechanisms of the two types of LLJs are further examined. The SLLJs and the low-pressure vortex (or inverted trough) varied synchronously as a whole and were affected by the southwestward movement of the WPSH in the rainiest period. The persistent large total pressure gradient force at low levels also maintained the strength of low-level geostrophic winds, thus sustaining the BLJs on the synoptic scale. The results based on a Du-Rotunno 1D model show that the Blackadar and Holton mechanisms jointly governed the BLJ dynamics on the diurnal scale.
Analysis on Precipitation Efficiency of the “21.7” Henan Extremely Heavy Rainfall Event
Lei YIN, Fan PING, Jiahua MAO, Shuanggen JIN
, Available online   , Manuscript accepted  14 July 2022, doi: 10.1007/s00376-022-2054-x
A record-breaking heavy rainfall event that occurred in Zhengzhou, Henan province during 19–21 July 2021 is simulated using the Weather Research and Forecasting Model, and the large-scale precipitation efficiency (LSPE) and cloud-microphysical precipitation efficiency (CMPE) of the rainfall are analyzed based on the model results. Then, the key physical factors that influenced LSPE and CMPE, and the possible mechanisms for the extreme rainfall over Zhengzhou are explored. Results show that water vapor flux convergence was the key factor that influenced LSPE. Water vapor was transported by the southeasterly winds between Typhoon In-Fa and the subtropical high, and the southerly flow of Typhoon Cempaka, and converged in Zhengzhou due to the blocking by the Taihang and Funiu Mountains in western Henan province. Strong moisture convergence centers were formed on the windward slope of the mountains, which led to high LSPE in Zhengzhou. From the perspective of CMPE, the net consumption of water vapor by microphysical processes was the key factor that influenced CMPE. Quantitative budget analysis suggests that water vapor was mainly converted to cloud water and ice-phase particles and then transformed to raindrops through melting of graupel and accretion of cloud water by rainwater during the heavy precipitation stage. The dry intrusion in the middle and upper levels over Zhengzhou made the high potential vorticity descend from the upper troposphere and enhanced the convective instability. Moreover, the intrusion of cold and dry air resulted in the supersaturation and condensation of water vapor, which contributed to the heavy rainfall in Zhengzhou.
A Time Neighborhood Method for the Verification of Landfalling Typhoon Track Forecast
Daosheng XU, Jeremy Cheuk-Hin LEUNG, Banglin ZHANG
, Available online   , Manuscript accepted  30 June 2022, doi: 10.1007/s00376-022-1398-6
Landfalling typhoons can cause disasters over large regions. The government and emergency responders need to take measures to mitigate disasters according to the forecast of landfall position, while slight timing error can be ignored. The reliability of operational model forecasts of typhoon landfall position needs to be evaluated beforehand, according to the forecasts and observation of historical cases. In the evaluation of landfalling typhoon track, the traditional method based on point-to-point matching methods could be influenced by the predicted typhoon translation speed. Consequently, the traditional track evaluation method may result in a large track error even if the predicted landfall position is close to observation. The purpose of this paper is to address the above issue using a simple evaluation method of landfalling typhoon track forecast based on the time neighborhood approach. In this new method, the timing error was lessened to highlight the importance of the position error during the landfall of typhoon. The properties of the time neighborhood method are compared with the traditional method based on numerical forecast results of 12 landfalling typhoon cases. Results demonstrated that the new method is not sensitive to the sampling frequency, and that the difference between the time neighborhood and traditional method will be more obvious when the moving speed of typhoon is moderate (between 15−30 km h−1). The time neighborhood concept can be easily extended to a broader context when one attempts to examine the position error more than the timing error.
Unprecedented Heatwave in Western North America during Late June of 2021: Roles of Atmospheric Circulation and Global Warming
Chunzai WANG, Jiayu ZHENG, Wei LIN, Yuqing WANG
, Available online   , Manuscript accepted  30 June 2022, doi: 10.1007/s00376-022-2078-2
An extraordinary and unprecedented heatwave swept across western North America (i.e., the Pacific Northwest) in late June of 2021, resulting in hundreds of deaths, a massive die-off of sea creatures off the coast, and horrific wildfires. Here, we use observational data to find the atmospheric circulation variabilities of the North Pacific and Arctic-Pacific-Canada patterns that co-occurred with the development and mature phases of the heatwave, as well as the North America pattern, which coincided with the decaying and eastward movement of the heatwave. Climate models from the Coupled Model Intercomparison Project (Phase 6) are not designed to simulate a particular heatwave event like this one. Still, models show that greenhouse gases are the main reason for the long-term increase of average daily maximum temperature in western North America in the past and future.
Aerosol-Cloud-Precipitation Interactions in a Closed-cell and Non-homogenous MBL Stratocumulus Cloud
Xiaojian ZHENG, Xiquan DONG, Dale M. WARD, Baike XI, Peng WU, Yuan WANG
, Available online   , Manuscript accepted  30 June 2022, doi: 10.1007/s00376-022-2013-6
A closed-cell marine stratocumulus case during the Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) aircraft field campaign is selected to examine the heterogeneities of cloud and drizzle microphysical properties and the aerosol-cloud-precipitation interactions. The spatial and vertical variabilities of cloud and drizzle microphysics are found in two different sets of flight legs: Leg-1 and Leg-2, which are parallel and perpendicular to the cloud propagation, respectively. The cloud along Leg-2 was close to adiabatic, where cloud-droplet effective radius and liquid water content linearly increase from cloud base to cloud top with less drizzle. The cloud along Leg-1 was sub-adiabatic with lower cloud-droplet number concentration and larger cloud-droplet effective, but higher drizzle droplet number concentration, larger drizzle droplet median diameter and drizzle liquid water content. The heavier drizzle frequency and intensity on Leg-1 were enhanced by the collision-coalescence processes within cloud due to strong turbulence. The sub-cloud precipitation rate on Leg-1 was significantly higher than that along Leg-2. As a result, the sub-cloud accumulation mode aerosols and CCN on Leg-1 were depleted, but the coarse model aerosols increased. This further leads to a counter-intuitive phenomenon that the CCN is less than cloud-droplet number concentration for Leg-1. The average CCN loss rates are −3.89 \begin{document}$\mathrm{c}{\mathrm{m}}^{-3}\;{\mathrm{h}}^{-1}$\end{document} and −0.77 \begin{document}$\mathrm{c}{\mathrm{m}}^{-3}\;{\mathrm{h}}^{-1}$\end{document} on Leg-1 and Leg-2, respectively. The cloud and drizzle heterogeneities inside the same stratocumulus can significantly alter the sub-cloud aerosols and CCN budget. Hence it should be treated with caution in the aircraft assessment of aerosol-cloud-precipitation interactions.
The Characteristics and Controlling Factors of Water and Heat Exchanges over the Alpine Wetland in the East of the Qinghai–Tibet Plateau
Jinlei CHEN, Yuan YUAN, Xianyu YANG, Zuoliang WANG, Shichang KANG, Jun WEN
, Available online   , Manuscript accepted  30 June 2022, doi: 10.1007/s00376-022-1443-5
Alpine wetland is one of the typical underlying surfaces on the Qinghai–Tibet Plateau. It plays a crucial role in runoff regulation. Investigations on the mechanisms of water and heat exchanges are necessary to understand the land surface processes over the alpine wetland. This study explores the characteristics of hydro-meteorological factors with in situ observations and uses the Community Land Model 5 to identify the main factors controlling water and heat exchanges. Latent heat flux and thermal roughness length were found to be greater in the warm season (June–August) than in the cold season (December–February), with a frozen depth of 20–40 cm over the alpine wetland. The transfers of heat fluxes were mainly controlled by longwave radiation and air temperature and affected by root distribution. Air pressure and stomatal conductance were also important to latent heat flux, and soil solid water content was important to sensible heat flux. Soil temperature was dominated by longwave radiation and air temperature, with crucial surface parameters of initial soil liquid water content and total water content. The atmospheric control factors transitioned to precipitation and air temperature for soil moisture, especially at the shallow layer (5 cm). Meanwhile, the more influential surface parameters were root distribution and stomatal conductance in the warm season and initial soil liquid water content and total water content in the cold season. This work contributes to the research on the land surface processes over the alpine wetland and is helpful to wetland protection.
Objective Identification and Climatic Characteristics of Heavy-Precipitation Northeastern China Cold Vortexes
Xu CHEN, Xiaoyong ZHUGE, Xidi ZHANG, Yuan WANG, Daokai XUE
, Available online   , Manuscript accepted  30 June 2022, doi: 10.1007/s00376-022-2037-y
The northeastern China cold vortex (NCCV) plays an important role in regional rainstorms over East Asia. Using the National Centers for Environmental Prediction Final reanalysis dataset and the Global Precipitation Measurement product, an objective algorithm for identifying heavy-precipitation NCCV (HPCV) events was designed, and the climatological features of 164 HPCV events from 2001 to 2019 were investigated. The number of HPCV events showed an upward linear trend, with the highest frequency of occurrence in summer. The most active region of HPCV samples was the Northeast China Plain between 40°–55°N. Most HPCV events lasted 3–5 days and had radii ranging from 250 to 1000 km. The duration of HPCV events with larger sizes was longer. About half of the HPCV events moved into (moved out of) the definition region (35°–60°N, 115–145°E), and half initiated (dissipated) within the region. The initial position was close to the western boundary of the definition region, and the final position was mainly near the eastern boundary. The locations associated with the precipitation were mostly concentrated within 2000 km southeast of the HPCV systems, and they were farther from the center in the cold season than in the warm season.
Impact of perturbation schemes on the ensemble prediction in a coupled Lorenz model
Qian Zou, Quanjia Zhong, Jiangyu Mao, Ruiqiang Ding, Deyu Lu, Jianping Li, Xuan LI
, Available online   , Manuscript accepted  01 June 2022, doi: 10.1007/s00376-022-1376-z
Based on a simple coupled Lorenz model, we investigate how to consider a suitable initial perturbation scheme for ensemble forecasting in a multiscale system involving slow dynamics and fast dynamics. Four initial perturbation approaches are used in the ensemble forecasting experiments: random perturbation (RP), the bred vector (BV), the ensemble transform Kalman filter (ETKF) and the nonlinear local Lyapunov vector (NLLV) methods. Results show that, regardless of the method used, the ensemble averages behave indistinguishably from the control forecasts during the first few time steps. Due to different error growth in different time-scale systems, the ensemble averages perform better than the control forecast after a very short period of lead time in a fast subsystem, but after a relatively long period of time in a slow subsystem. As a result of coupled dynamic processes, whether adding perturbations to fast variables or to slow variables can contribute to an improvement in the forecasting skill for fast variables and slow variables. When it comes to the initial perturbation approaches, the NLLVs show higher forecasting skill than BVs or RPs overall. NLLVs and ETKFs had nearly equivalent prediction skill, and NLLVs won by a narrow margin. In particular, when adding perturbations to slow variables, independent perturbations (NLLVs and ETKFs) perform much better in the ensemble prediction. These results are simply implied in a real coupled air–sea model. For the prediction of oceanic variables, independent perturbations (NLLVs) and adding perturbations to oceanic variables will be expected to perform better in the ensemble prediction.
Kinetic Energy Budgets during the Rapid Intensification of Typhoon Rammasun (2014)
Xin QUAN, Xiaofan LI
, Available online   , Manuscript accepted  30 May 2022, doi: 10.1007/s00376-022-2060-z
In this study, Typhoon Rammasun (2014) was simulated using the Weather Research and Forecasting model to examine the kinetic energy during rapid intensification (RI). Budget analyses revealed that in the inner area of the typhoon, the conversion from symmetric divergent kinetic energy associated with the collocation of strong cyclonic circulation and inward flow led to an increase in the symmetric rotational kinetic energy in the lower troposphere. The increase in the symmetric rotational kinetic energy in the mid and upper troposphere resulted from the upward transport of symmetric rotational kinetic energy from the lower troposphere. In the outer area, both typhoon and Earth’s rotation played equally important roles in the conversion from symmetric divergent kinetic energy to symmetric rotational kinetic energy in the lower troposphere. The decrease in the symmetric rotational kinetic energy in the upper troposphere was caused by the conversion to asymmetric rotational kinetic energy through the collocation of symmetric tangential rotational winds and the radial advection of asymmetric tangential rotational winds by radial environmental winds.
On the Key Dynamical Processes Supporting the 21.7 Zhengzhou Record-breaking Hourly Rainfall in China
Peng WEI, Xin XU, Ming XUE, Chenyue ZHANG, Yuan WANG, Kun ZHAO, Ang ZHOU, Shushi ZHANG, Kefeng ZHU
, Available online   , Manuscript accepted  24 May 2022, doi: 10.1007/s00376-022-2061-y
An extremely heavy rainfall event occurred in Zhengzhou, China, on 20 July 2021 and produced an hourly rainfall rate of 201.9 mm, which broke the station record for mainland China. Based on radar observations and a convection-permitting simulation using the WRF-ARW model, this paper investigates the multiscale processes, especially those at the mesoscale, that support the extreme observed hourly rainfall. Results show that the extreme rainfall occurred in an environment characteristic of warm-sector heavy rainfall, with abundant warm moist air transported from the ocean by an abnormally northward-displaced western Pacific subtropical high and Typhoon In-Fa (2021). However, rather than through back building and echo training of convective cells often found in warm-sector heavy rainfall events, this extreme hourly rainfall event was caused by a single, quasi-stationary storm in Zhengzhou. Scale separation analysis reveals that the extreme-rain-producing storm was supported and maintained by the dynamic lifting of low-level converging flows from the north, south, and east of the storm. The low-level northerly flow originated from a mesoscale barrier jet on the eastern slope of the Taihang Mountain due to terrain blocking of large-scale easterly flows, which reached an overall balance with the southerly winds in association with a low-level meso-β-scale vortex located to the west of Zhengzhou. The large-scale easterly inflows that fed the deep convection via transport of thermodynamically unstable air into the storm prevented the eastward propagation of the weak, shallow cold pool. As a result, the convective storm was nearly stationary over Zhengzhou, resulting in record-breaking hourly precipitation.
Estimation of Lightning-Generated NOx in the Continental Area of China Based on Cloud-to-Ground Lightning Location Data
Qi LI, Fengxia GUO, Xiaoyu JU, Ze LIU, Mingjun GAN, Kun ZHANG, Binbin CAI
, Available online   , Manuscript accepted  19 May 2022, doi: 10.1007/s00376-022-1329-6
Lightning-generated nitrogen oxides (LNOx) have a major influence on the atmosphere and global climate change. Therefore, it is of great importance to obtain a more accurate estimation of LNOX. The aim of this study is to provide a reference for the accurate estimation of the total LNOx in the continental area of China based on cloud-to-ground lightning (CG) location data from 2014 to 2018. The energy of each CG flash was based on the number of return strokes per CG flash, the peak current of each return stroke, and the assumed CG breakdown voltage. The energy of intracloud lightning (IC) was based on the estimated frequencies of IC and the assumed energy of each IC flash. Combining the energy of lightning and the number of nitric oxide (NO) molecules produced by unit energy (ρno), the total LNOX production in the continental area of China was determined. The LNOx in the continental area of China estimated in this study is in the range 0.157–0.321 × 109 kg per year [Tg(N) yr–1], which is on the high end of other scholars’ works. Negative cloud-to-ground lightning (NCG) flashes produce the most moles of NOx, while positive cloud-to-ground lightning (PCG) flashes produce the least total moles of NOx. The breakdown voltage of PCG is greater than that of IC or NCG, while the latter has a greater output of LNOx.
Circulation Patterns Linked to the Positive Sub-Tropical Indian Ocean Dipole
Chibuike Chiedozie IBEBUCHI
, Available online   , Manuscript accepted  18 May 2022, doi: 10.1007/s00376-022-2017-2
The positive phase of the subtropical Indian Ocean dipole (SIOD) is one of the climatic modes in the subtropical southern Indian Ocean that influences the austral summer inter-annual rainfall variability in parts of southern Africa. This paper examines austral summer rain-bearing circulation types (CTs) in Africa south of the equator that are related to the positive SIOD and the dynamics through which specific rainfall regions in southern Africa can be influenced by this relationship. Four austral summer rain-bearing CTs were obtained. Among the four CTs, the CT that featured (i) enhanced cyclonic activity in the southwest Indian Ocean; (ii) positive widespread rainfall anomaly in the southwest Indian Ocean; and (iii) low-level convergence of moisture fluxes from the tropical South Atlantic Ocean, tropical Indian Ocean, and the southwest Indian Ocean, over the south-central landmass of Africa, was found to be related to the positive SIOD climatic mode. The relationship also implies that positive SIOD can be expected to increase the amplitude and frequency of occurrence of the aforementioned CT. The linkage between the CT related to the positive SIOD and austral summer homogeneous regions of rainfall anomalies in Africa south of the equator showed that it is the principal CT that is related to the inter-annual rainfall variability of the south-central regions of Africa, where the SIOD is already known to significantly influence its rainfall variability. Hence, through the large-scale patterns of atmospheric circulation associated with the CT, the SIOD can influence the spatial distribution and intensity of rainfall over the preferred landmass through enhanced moisture convergence.
Understanding the Development of the 2018/19 Central Pacific El Niño
Chengyang GUAN, Xin WANG, Haijun YANG
, Available online   , Manuscript accepted  18 May 2022, doi: 10.1007/s00376-022-1410-1
A central Pacific (CP) El Niño event occurred in 2018/19. Previous studies have shown that different mechanisms are responsible for different subtypes of CP El Niño events (CP-I El Niño and CP-II El Niño). By comparing the evolutions of surface winds, ocean temperatures, and heat budgets of the CP-I El Niño, CP-II El Niño, and 2018/19 El Niño, it is illustrated that the subtropical westerly anomalies in the North Pacific, which led to anomalous convergence of Ekman flow and surface warming in the central equatorial Pacific, played an important role in the 2018/19 El Niño event as well as in the CP-II El Niño. Although the off-equatorial forcing played a vital role, it is found that the equatorial forcing acted as a driving (damping) term in boreal spring (summer) of the 2018/19 El Niño. The 2018/19 El Niño provides a timely and vivid example that helps illustrate the proposed mechanism of the CP El Niño, which could be leveraged to improve El Niño predictability.
Macro- and Micro-physical Characteristics of Different Parts of Mixed Convective-stratiform Clouds and Differences in Their Responses to Seeding
Dejun LI, Chuanfeng ZHAO, Peiren LI, Cao Liu, Dianli GONG, Siyao LIU, Zhengteng YUAN, Yingying CHEN
, Available online   , Manuscript accepted  11 May 2022, doi: 10.1007/s00376-022-2003-8
This study investigates the cloud macro- and micro-physical characteristics in the convective and stratiform regions and their different responses to the seeding for mixed convective-stratiform clouds that occurred in Shandong province on 21 May 2018, based on the observations from the aircraft, the Suomi National Polar-Orbiting Partnership (NPP) satellite, and the high-resolution Himawari-8 (H8) satellite. The aircraft observations show that convection was deeper and radar echoes were significantly enhanced with higher tops in response to seeding in the convective region. This is linked with the conversion of supercooled liquid droplets to ice crystals with released latent heat, resulting in strengthened updrafts, enhanced radar echoes, higher cloud tops, and more and larger precipitation particles. In contrast, in the stratiform cloud region, after the Silver Iodide (AgI) seeding, the radar echoes become significantly weaker at heights close to the seeding layer, with the echo tops lowered by 1.4–1.7 km. In addition, a hollow structure appears at the height of 6.2–7.8 km with a depth of about 1.6 km and a diameter of about 5.5 km, and features such as icing seeding tracks appear. These suggest that the transformation between droplets and ice particles was accelerated by the seeding in the stratiform part. The NPP and H8 satellites also show that convective activity was stronger in the convective region after seeding; while in the stratiform region, a cloud seeding track with a width of 1–3 km appears 10 km downstream of the seeding layer 15 minutes after the AgI seeding, which moves along the wind direction as width increases.
The Coordinated Influence of Indian Ocean Sea Surface Temperature and Arctic Sea Ice on Anomalous Northeast China Cold Vortex Activities with Different Paths during Late Summer
Yitong LIN, Yihe FANG, Chunyu ZHAO, Zhiqiang GONG, Siqi YANG, Yiqiu YU
, Available online   , Manuscript accepted  11 May 2022, doi: 10.1007/s00376-022-1415-9
The Northeast China cold vortex (NCCV) during late summer (from July to August) is identified and classified into three types in terms of its movement path using machine learning. The relationships of the three types of NCCV intensity with atmospheric circulations in late summer, the sea surface temperature (SST), and Arctic sea ice concentration (SIC) in the preceding months, are analyzed. The sensitivity tests by the Community Atmosphere Model version 5.3 (CAM5.3) are used to verify the statistical results. The results show that the coordination pattern of East Asia-Pacific (EAP) and Lake Baikal high pressure forced by SST anomalies in the North Indian Ocean dipole mode (NIOD) during the preceding April and SIC anomalies in the Nansen Basin during the preceding June results in an intensity anomaly for the first type of NCCV. While the pattern of high pressure over the Urals and Okhotsk Sea and low pressure over Lake Baikal during late summer—which is forced by SST anomalies in the South Indian Ocean dipole mode (SIOD) in the preceding June and SIC anomalies in the Barents Sea in the preceding April—causes the intensity anomaly of the second type. The third type is atypical and is not analyzed in detail. Sensitivity tests, jointly forced by the SST and SIC in the preceding period, can well reproduce the observations. In contrast, the results forced separately by the SST and SIC are poor, indicating that the NCCV during late summer is likely influenced by the coordinated effects of both SST and SIC in the preceding months.
On the Influences of Urbanization on the Extreme Rainfall over Zhengzhou on 20 July 2021: A Convection-Permitting Ensemble Modeling Study
Yali LUO, Jiahua ZHANG, Miao YU, Xudong LIANG, Rudi XIA, Yanyu GAO, Xiaoyu GAO, Jinfang YIN
, Available online   , Manuscript accepted  11 May 2022, doi: 10.1007/s00376-022-2048-8
This study investigates the influences of urban land cover on the extreme rainfall event over the Zhengzhou city in central China on 20 July 2021 using the Weather Research and Forecasting model at a convection-permitting scale [1-km resolution in the innermost domain (d3)]. Two ensembles of simulation (CTRL, NURB), each consisting of 11 members with a multi-layer urban canopy model and various combinations of physics schemes, were conducted using different land cover scenarios: (i) the real urban land cover, (ii) all cities in d3 being replaced with natural land cover. The results suggest that CTRL reasonably reproduces the spatiotemporal evolution of rainstorms and the 24-h rainfall accumulation over the key region, although the maximum hourly rainfall is underestimated and displaced to the west or southwest by most members. The ensemble mean 24-h rainfall accumulation over the key region of heavy rainfall is reduced by 13%, and the maximum hourly rainfall simulated by each member is reduced by 15–70 mm in CTRL relative to NURB. The reduction in the simulated rainfall by urbanization is closely associated with numerous cities/towns to the south, southeast, and east of Zhengzhou. Their heating effects jointly lead to formation of anomalous upward motions in and above the planetary boundary layer (PBL), which exaggerates the PBL drying effect due to reduced evapotranspiration and also enhances the wind stilling effect due to increased surface friction in urban areas. As a result, the lateral inflows of moisture and high-θe (equivalent potential temperature) air from south and east to Zhengzhou are reduced.
Multi-scale Incremental Analysis Update Scheme and Its Application to Typhoon Mangkhut Prediction
Yan GAO, Jiali FENG, Xin XIA, Jian SUN, Yulong MA, Dongmei CHEN, Qilin WAN
, Available online   , Manuscript accepted  11 May 2022, doi: 10.1007/s00376-022-1425-7
In the traditional incremental analysis update (IAU) process, all analysis increments are treated as constant forcing in a model’s prognostic equations over a certain time window. This approach effectively reduces high-frequency oscillations introduced by data assimilation. However, as different scales of increments have unique evolutionary speeds and life histories in a numerical model, the traditional IAU scheme cannot fully meet the requirements of short-term forecasting for the damping of high-frequency noise and may even cause systematic drifts. Therefore, a multi-scale IAU scheme is proposed in this paper. Analysis increments were divided into different scale parts using a spatial filtering technique. For each scale increment, the optimal relaxation time in the IAU scheme was determined by the skill of the forecasting results. Finally, different scales of analysis increments were added to the model integration during their optimal relaxation time. The multi-scale IAU scheme can effectively reduce the noise and further improve the balance between large-scale and small-scale increments in the model initialization stage. To evaluate its performance, several numerical experiments were conducted to simulate the path and intensity of Typhoon Mangkhut (2018) and showed that: (1) the multi-scale IAU scheme had an obvious effect on noise control at the initial stage of data assimilation; (2) the optimal relaxation time for large-scale and small-scale increments was estimated as 6 h and 3 h, respectively; (3) the forecast performance of the multi-scale IAU scheme in the prediction of Typhoon Mangkhut (2018) was better than that of the traditional IAU scheme. The results demonstrate the superiority of the multi-scale IAU scheme.
Assimilation of Ocean Surface Wind Data by the HY-2B Satellite in GRAPES: Impacts on Analyses and Forecasts
Jincheng WANG, Xingwei JIANG, Xueshun SHEN, Youguang ZHANG, Xiaomin WAN, Wei HAN, Dan WANG
, Available online   , Manuscript accepted  06 May 2022, doi: 10.1007/s00376-022-1349-2
The ocean surface wind (OSW) data retrieved from microwave scatterometers have high spatial accuracy and represent the only wind data assimilated by global numerical models on the ocean surface, thus playing an important role in improving the forecast skills of global medium-range weather prediction models. To improve the forecast skills of the Global/Regional Assimilation and Prediction System Global Forecast System (GRAPES_GFS), the HY-2B OSW data is assimilated into the GRAPES_GFS four-dimensional variational assimilation (4DVAR) system. Then, the impacts of the HY-2B OSW data assimilation on the analyses and forecasts of GRAPES_GFS are analyzed based on one-month assimilation cycle experiments. The results show that after assimilating the HY-2B OSW data, the analysis errors of the wind fields in the lower-middle troposphere (1000–600 hPa) of the tropics and the southern hemisphere (SH) are significantly reduced by an average rate of about 5%. The impacts of the HY-2B OSW data assimilation on the analysis fields of wind, geopotential height, and temperature are not solely limited to the boundary layer but also extend throughout the entire troposphere after about two days of cycling assimilation. Furthermore, assimilating the HY-2B OSW data can significantly improve the forecast skill of wind, geopotential height, and temperature in the troposphere of the tropics and SH.
Relationships between Cloud Droplet Spectral Relative Dispersion and Entrainment Rate and Their Impacting Factors
Shi LUO, Chunsong LU, Yangang LIU, Yaohui LI, Wenhua GAO, Yujun QIU, Xiaoqi XU, Junjun LI, Lei ZHU, Yuan WANG, Junjie WU, Xinlin YANG
, Available online   , Manuscript accepted  06 May 2022, doi: 10.1007/s00376-022-1419-5
Cloud microphysical properties are significantly affected by entrainment and mixing processes. However, it is unclear how the entrainment rate affects the relative dispersion of cloud droplet size distribution. Previously, the relationship between relative dispersion and entrainment rate was found to be positive or negative. To reconcile the contrasting relationships, the Explicit Mixing Parcel Model is used to determine the underlying mechanisms. When evaporation is dominated by small droplets, and the entrained environmental air is further saturated during mixing, the relationship is negative. However, when the evaporation of big droplets is dominant, the relationship is positive. Whether or not the cloud condensation nuclei are considered in the entrained environmental air is a key factor as condensation on the entrained condensation nuclei is the main source of small droplets. However, if cloud condensation nuclei are not entrained, the relationship is positive. If cloud condensation nuclei are entrained, the relationship is dependent on many other factors. High values of vertical velocity, relative humidity of environmental air, and liquid water content, and low values of droplet number concentration, are more likely to cause the negative relationship since new saturation is easier to achieve by evaporation of small droplets. Further, the signs of the relationship are not strongly affected by the turbulence dissipation rate, but the higher dissipation rate causes the positive relationship to be more significant for a larger entrainment rate. A conceptual model is proposed to reconcile the contrasting relationships. This work enhances the understanding of relative dispersion and lays a foundation for the quantification of entrainment-mixing mechanisms.
Comparative Analysis of the Generalized Omega Equation and Generalized Vertical Motion Equation
Baofeng JIAO, Lingkun RAN, Na LI, Ren CAI, Tao QU, Yushu ZHOU
, Available online   , Manuscript accepted  04 May 2022, doi: 10.1007/s00376-022-1435-5
Research on vertical motion in mesoscale systems is an extraordinarily challenging effort. Allowing for fewer assumptions, a new form of generalized vertical motion equation and a generalized Omega equation are derived in the Cartesian coordinate system (nonhydrostatic equilibrium) and the isobaric coordinate system (hydrostatic equilibrium), respectively. The terms on the right-hand side of the equations, which comprise the Q vector, are composed of three factors: dynamic, thermodynamic, and mass. A heavy rain event that occurred from 18 to 19 July 2021 in southern Xinjiang was selected to analyze the characteristics of the diagnostic variable in the generalized vertical motion equation (\begin{document}$ {Q_z} $\end{document}) and the diagnostic variable in the generalized Omega equation (\begin{document}${Q_p}$\end{document}) using high-resolution model data. The results show that the horizontal distribution of the \begin{document}${Q_z}$\end{document}-vector divergence at 5.5 km is roughly similar to the distribution of the \begin{document}${Q_p}$\end{document}-vector divergence at 500 hPa, and that both relate well to the composite radar reflectivity, vertical motion, and hourly accumulated precipitation. The \begin{document}${Q_z}$\end{document}-vector divergence is more effective in indicating weak precipitation. In vertical cross sections, regions with alternating positive and negative large values that match the precipitation are mainly concentrated in the middle levels for both forms of Q vectors. The temporal evolutions of vertically integrated \begin{document}${Q_z}$\end{document}-vector divergence and \begin{document}${Q_p}$\end{document}-vector divergence are generally similar. Both perform better than the classical quasigeostrophic Q vector and nongeostrophic Q vector in indicating the development of the precipitation system.
Evolution of Meteorological Conditions during a Heavy Air Pollution Event under the Influence of Shallow Foehn in Urumqi, China
Xia LI, Keming ZHAO, Shiyuan ZHONG, Xiaojing YU, Zhimin FENG, Yuting ZHONG, Ayitken MAULEN, Shuting LI
, Available online   , Manuscript accepted  11 April 2022, doi: 10.1007/s00376-022-1422-x
Urumqi, located on the northern slope of the Tianshan Mountains in northwestern China, is one of the most polluted cities in the world. Of particular importance is the influence of terrain-induced shallow foehn, known locally as elevated southeasterly gale (ESEG). It usually modulates atmospheric boundary layer structure and wind field patterns and produces favorable meteorological conditions conducive to hazardous air pollution. During 2013–17, Urumqi had an average of 50 d yr–1 of heavy pollution (daily average PM2.5 concentration >150 μg m–3), of which 41 days were in winter. The majority (71.4%) of heavy pollution processes were associated with the shallow foehn. Based on microwave radiometer, wind profiler, and surface observations, the surface meteorological fields and boundary layer evolution during the worst pollution episode in Urumqi during 16–23 February 2013 are investigated. The results illustrate the significant role of shallow foehn in the building, strengthening, and collapsing of temperature inversions. There were four wind field patterns corresponding to four different phases during the whole pollution event. The most serious pollution phase featured shallow foehn activity in the south of Urumqi city and the appearance of an intense inversion layer below 600 m. Intense convergence caused by foehn and mountain–valley winds was sustained during most of the phase, resulting in pollutants sinking downward to the lower boundary layer and accumulating around urban area. The key indicators of such events identified in this study are highly correlated to particulate matter concentrations and could be used to predict heavy pollution episodes in the feature.
The Microphysical Characteristics of Wintertime Cold Clouds in North China
Xuexu WU, Minghuai WANG, Delong ZHAO, Daniel ROSENFELD, Yannian ZHU, Yuanmou DU, Wei ZHOU, Ping TIAN, Jiujiang SHENG, Fei WANG, Deping DING
, Available online   , Manuscript accepted  28 March 2022, doi: 10.1007/s00376-022-1274-4
The microphysical characteristics of wintertime cold clouds in North China were investigated from 22 aircraft observation flights from 2014 to 2017, 2020, and 2021. The clouds were generated by mesoscale weather systems with little orographic component. Over the mixed-phase temperature range (–40°C to 0°C), the average fraction of liquid, mixed-phase, and ice cloud was 4.9%, 23.3%, and 71.8%, respectively, and the probability distribution of ice mass fraction was a half-U-shape, suggesting that ice cloud was the primary cloud type. The wintertime mixed-phase clouds in North China were characterized by large cloud droplet number concentration, small liquid water content (LWC), and small effective diameter of cloud droplets. The main reason for larger cloud droplet number concentration and smaller effective diameter of cloud droplets was the heavy pollution in winter in North China, while for smaller LWC was the lower temperature during flights and the difference in air mass type. With the temperature increasing, cloud droplet number concentration, LWC, and the size of ice particles increased, but ice number concentration and effective diameter of cloud droplets decreased, similar to other mid-latitude regions, indicating the similarity in the temperature dependence of cloud properties of mixed-phase clouds. The variation of the cloud properties and ice habit at different temperatures indicated the operation of the aggregation and riming processes, which were commonly present in the wintertime mixed-phase clouds. This study fills a gap in the aircraft observation of wintertime cold clouds in North China.
Simulating Aerosol Optical Depth and Direct Radiative Effects over the Tibetan Plateau with a High-Resolution CAS FGOALS-f3 Model
Min ZHAO, Tie DAI, Hao WANG, Qing BAO, Yimin LIU, Hua ZHANG, Guangyu SHI
, Available online   , Manuscript accepted  20 March 2022, doi: 10.1007/s00376-022-1424-8
Current global climate models cannot resolve the complex topography over the Tibetan Plateau (TP) due to their coarse resolution. This study investigates the impacts of horizontal resolution on simulating aerosol and its direct radiative effect (DRE) over the TP by applying two horizontal resolutions of about 100 km and 25 km to the Chinese Academy of Sciences Flexible Global Ocean-Atmosphere Land System (CAS FGOALS-f3) over a 10-year period. Compared to the AErosol RObotic NETwork observations, a high-resolution model (HRM) can better reproduce the spatial distribution and seasonal cycles of aerosol optical depth (AOD) compared to a low-resolution model (LRM). The HRM bias and RMSE of AOD decreased by 0.08 and 0.12, and the correlation coefficient increased by 0.22 compared to the LRM. An LRM is not sufficient to reproduce the aerosol variations associated with fine-scale topographic forcing, such as in the eastern marginal region of the TP. The difference between hydrophilic aerosols in an HRM and LRM is caused by the divergence of the simulated relative humidity (RH). More reasonable distributions and variations of RH are conducive to simulating hydrophilic aerosols. An increase of the 10-m wind speed in winter by an HRM leads to increased dust emissions. The simulated aerosol DREs at the top of the atmosphere (TOA) and at the surface by the HRM are –0.76 W m–2 and –8.72 W m–2 over the TP, respectively. Both resolution models can capture the key feature that dust TOA DRE transitions from positive in spring to negative in the other seasons.
The asymmetric connection of SST in the Tasman Sea with respect to the opposite phases of ENSO in austral summer
Xueqian Sun, Shuanglin Li, Stefan Liess
, Available online   , Manuscript accepted  11 February 2022, doi: 10.1007/s00376-022-0421-y
Using linear regression and composite analyses, this study identifies a pronounced asymmetric connection of sea surface temperature (SST) in the Tasman Sea with the two opposite phases of El Niño-Southern Oscillation (ENSO) during austral summer. In El Niño years, the SST anomalies (SSTAs) in the Tasman Sea exhibit a dipolar pattern with weak warmth in the northwest and modest cooling in the southeast, while during La Niña years the SSTAs exhibit a basin-scale warmth with greater amplitude. Investigations on the underlying mechanism suggest that this asymmetry arises from the oceanic heat transport, especially the anomalous Ekman meridional heat fluxes induced by the zonal wind stress anomalies, rather than the surface heat fluxes on the air-sea interface. A further analysis demonstrates that the asymmetry of oceanic heat transport between El Niño and La Niña years is driven by the asymmetric atmospheric circulation over the Tasman Sea stimulated by the asymmetric diabatic heating in the tropical Pacific between the two opposite ENSO phases.
Microphysical Characteristics of Extreme-Rainfall Convection over the Pearl River Delta Region, South China from Polarimetric Radar Data during the Pre-summer Rainy Season
Hao HUANG, Kun ZHAO, Johnny C. L. CHAN, Dongming HU
, Available online   , Manuscript accepted  09 February 2022, doi: 10.1007/s00376-022-1319-8
During the pre-summer rainy season, heavy rainfall occurs frequently in South China. Based on polarimetric radar observations, the microphysical characteristics and processes of convective features associated with extreme rainfall rates (ERCFs) are examined. In the regions with high ERCF occurrence frequency, sub-regional differences are found in the lightning flash rate (LFR) distributions. In the region with higher LFRs, the ERCFs have larger volumes of high reflectivity factor above the freezing level, corresponding to more active riming processes. In addition, these ERCFs are more organized and display larger spatial coverage, which may be related to the stronger low-level wind shear and higher terrain in the region. In the region with lower LFRs, the ERCFs have lower echo tops and lower-echo centroids. However, no clear differences of the most unstable convective available potential energy (MUCAPE) exist in the ERCFs in the regions with different LFR characteristics. Regardless of the LFRs, raindrop collisional coalescence is the main process for the growth of raindrops in the ERCFs. In the ERCFs within the region with lower LFRs, the main mechanism for the rapid increase of liquid water content with decreasing altitude below 4 km is through the warm-rain processes converting cloud drops to raindrops. However, in those with higher LFRs, the liquid water content generally decreases with decreasing altitude.
Assimilation of the FY-4A AGRI Clear-Sky Radiance Data in a Regional Numerical Model and Its Impact on the Forecast of the “21·7” Henan Extremely Persistent Heavy Rainfall
Lan XU, Wei CHENG, Zhongren DENG, Juanjuan LIU, Bin WANG, Bin LU, Shudong WANG, Li DONG
, Available online   , Manuscript accepted  14 January 2022, doi: 10.1007/s00376-022-1380-3
Assimilation of the Advanced Geostationary Radiance Imager (AGRI) clear-sky radiance in a regional model is performed. The forecasting effectiveness of the assimilation of two water vapor (WV) channels with conventional observations for the “21·7” Henan extremely heavy rainfall is analyzed and compared with a baseline test that assimilates only conventional observations in this study. The results show that the 24-h cumulative precipitation forecast by the assimilation experiment with the addition of the AGRI exceeds 500 mm, compared to a maximum value of 532.6 mm measured by the national meteorological stations, and that the location of the maximum precipitation is consistent with the observations. The results for the short periods of intense precipitation processes are that the simulation of the location and intensity of the 3-h cumulative precipitation is also relatively accurate. The analysis increment shows that the main difference between the two sets of assimilation experiments is over the ocean due to the additional ocean observations provided by FY-4A, which compensates for the lack of ocean observations. The assimilation of satellite data adjusts the vertical and horizontal wind fields over the ocean by adjusting the atmospheric temperature and humidity, which ultimately results in a narrower and stronger WV transport path to the center of heavy precipitation in Zhengzhou in the lower troposphere. Conversely, the WV convergence and upward motion in the control experiment are more dispersed; therefore, the precipitation centers are also correspondingly more dispersed.
Assimilation of FY-3D MWTS-II Radiance with 3D Precipitation Detection and the Impacts on Typhoon Forecasts
Luyao QIN, Yaodeng CHEN, Gang MA, Fuzhong WENG, Deming MENG, Peng ZHANG
, Available online   , Manuscript accepted  07 January 2022, doi: 10.1007/s00376-022-1252-x
Precipitation detection is an essential step in radiance assimilation because the uncertainties in precipitation would affect the radiative transfer calculation and observation errors. The traditional precipitation detection method for microwave only detects clouds and precipitation horizontally, without considering the three-dimensional distribution of clouds. Extending precipitation detection from 2D to 3D is expected to bring more useful information to the data assimilation without using the all-sky approach. In this study, the 3D precipitation detection method is adopted to assimilate Microwave Temperature Sounder-2 (MWTS-II) onboard the Fengyun-3D, which can dynamically detect the channels above precipitating clouds by considering the near-real-time cloud parameters. Cycling data assimilation and forecasting experiments for Typhoons Lekima (2019) and Mitag (2019) are carried out. Compared with the control experiment, the quantity of assimilated data with the 3D precipitation detection increases by approximately 23%. The quality of the additional MWTS-II radiance data is close to the clear-sky data. The case studies show that the average root-mean-square errors (RMSE) of prognostic variables are reduced by 1.7% in the upper troposphere, leading to an average reduction of 4.53% in typhoon track forecasts. The detailed diagnoses of Typhoon Lekima (2019) further show that the additional MWTS-II radiances brought by the 3D precipitation detection facilitate portraying a more reasonable circulation situation, thus providing more precise structures. This paper preliminarily proves that 3D precipitation detection has potential added value for increasing satellite data utilization and improving typhoon forecasts.
Application of a Neural Network to Store and Compute the Optical Properties of Non-Spherical Particles
Jinhe YU, Lei BI, Wei HAN, Xiaoye ZHANG
, Available online   , Manuscript accepted  23 November 2021, doi: 10.1007/s00376-021-1375-5
Radiative transfer simulations and remote sensing studies fundamentally require accurate and efficient computation of the optical properties of non-spherical particles. This paper proposes a deep learning (DL) scheme in conjunction with an optical property database to achieve this goal. Deep neural network (DNN) architectures were obtained from a dataset of the optical properties of super-spheroids with extensive shape parameters, size parameters, and refractive indices. The dataset was computed through the invariant imbedding T-matrix method. Four separate DNN architectures were created to compute the extinction efficiency factor, single-scattering albedo, asymmetry factor, and phase matrix. The criterion for designing these neural networks was the achievement of the highest prediction accuracy with minimal DNN parameters. The numerical results demonstrate that the determination coefficients are greater than 0.999 between the prediction values from the neural networks and the truth values from the database, which indicates that the DNN can reproduce the optical properties in the dataset with high accuracy. In addition, the DNN model can robustly predict the optical properties of particles with high accuracy for shape parameters or refractive indices that are unavailable in the database. Importantly, the ratio of the database size (~127 GB) to that of the DNN parameters (~20 MB) is approximately 6810, implying that the DNN model can be treated as a highly compressed database that can be used as an alternative to the original database for real-time computing of the optical properties of non-spherical particles in radiative transfer and atmospheric models.
Ice Nucleation of Cirrus Clouds Related to the Transported Dust Layer Observed by Ground-Based Lidars over Wuhan, China
Yun HE, Fan YI, Fuchao LIU, Zhenping YIN, Jun ZHOU
, Available online   , Manuscript accepted  14 October 2021, doi: 10.1007/s00376-021-1192-x
Cirrus clouds related to transported dust layers were identified on 22 occasions with ground-based polarization lidar from December 2012 to February 2018 over Wuhan (30.5°N, 114.4°E), China. All the events occurred in spring and winter. Cirrus clouds were mostly located above 7.6 km on top of the aloft dust layers. In-cloud relative humidity with respect to ice (RHi) derived from water vapor Raman lidar as well as from ERA5 reanalysis data were used as criteria to determine the possible ice nucleation regimes. Corresponding to the two typical cases shown, the observed events can be classified into two categories: (1) category A (3 cases), in-cloud peak RHi ≥ 150%, indicating competition between heterogeneous nucleation and homogeneous nucleation; and (2) category B (19 cases), in-cloud peak RHi < 150%, revealing that only heterogeneous nucleation was involved. Heterogeneous nucleation generally took place during instances of cirrus cloud formation in the upper troposphere when advected dust particles were present. Although accompanying cloud-top temperatures ranged from –51.9°C to –30.4°C, dust-related heterogeneous nucleation contributed to primary ice nucleation in cirrus clouds by providing ice nucleating particle concentrations on the order of 10-3 L-1 to 102 L-1. Heterogeneous nucleation and subsequent crystal growth reduced the ambient RHi to be less than 150% by consuming water vapor and thus completely inhibited homogeneous nucleation.
Calculating the Climatology and Anomalies of Surface Cloud Radiative Effect Using Cloud Property Histograms and Cloud Radiative Kernels
Chen ZHOU, Yincheng LIU, Quan WANG
, Available online   , Manuscript accepted  08 September 2021, doi: 10.1007/s00376-021-1166-z
Cloud radiative kernels (CRK) built with radiative transfer models have been widely used to analyze the cloud radiative effect on top of atmosphere (TOA) fluxes, and it is expected that the CRKs would also be useful in the analyses of surface radiative fluxes, which determines the regional surface temperature change and variability. In this study, CRKs at the surface and TOA were built using the Rapid Radiative Transfer Model (RRTM). Longwave cloud radiative effect (CRE) at the surface is primarily driven by cloud base properties, while TOA CRE is primarily decided by cloud top properties. For this reason, the standard version of surface CRK is a function of latitude, longitude, month, cloud optical thickness (τ) and cloud base pressure (CBP), and the TOA CRK is a function of latitude, longitude, month, τ and cloud top pressure (CTP). Considering that the cloud property histograms provided by climate models are functions of CTP instead of CBP at present, the surface CRKs on CBP-τ histograms were converted to CTP-τ fields using the statistical relationship between CTP, CBP and τ obtained from collocated CloudSat and MODIS observations. For both climate model outputs and satellites observations, the climatology of surface CRE and cloud-induced surface radiative anomalies calculated with the surface CRKs and cloud property histograms are well correlated with those calculated from surface radiative fluxes. The cloud-induced surface radiative anomalies reproduced by surface CRKs and MODIS cloud property histograms are not affected by spurious trends that appear in Clouds and the Earth's Radiant Energy System (CERES) surface irradiances products.
Multiscale Combined Action and Disturbance Characteristics of Pre-summer Extreme Precipitation Events over South China
Hongbo LIU, Ruojing YAN, Bin WANG, Guanghua CHEN, Jian LING, Shenming FU
, Available online   , Manuscript accepted  08 September 2021, doi: 10.1007/s00376-021-1172-1
The dominant frequency modes of pre-summer extreme precipitation events (EPEs) over South China (SC) between 1998 and 2018 were investigated. The 67 identified EPEs were all characterized by the 3–8-d (synoptic) frequency band. However, multiscale combined modes of the synoptic and three low-frequency bands (10 20-d (quasi-biweekly, QBW); 15–40-d (quasi-monthly, QM); and 20–60-d (intraseasonal)) accounted for the majority (63%) of the EPEs, and the precipitation intensity on the peak wet day was larger than that of the single synoptic mode. It was found that EPEs form within strong southwesterly anomalous flows characterized by either lower-level cyclonic circulation over SC or a deep trough over eastern China. Bandpass-filtered disturbances revealed the direct precipitating systems and their life cycles. Synoptic-scale disturbances are dominated by mid–high latitude troughs, and the cyclonic anomalies originate from downstream of the Tibetan Plateau (TP). Given the warm and moist climate state, synoptic-scale northeasterly flows can even induce EPEs. At the QBW and QM scales, the disturbances originate from the tropical Pacific, downstream of the TP, or mid–high latitudes (QBW only). Each is characterized by cyclonic–anticyclonic wave trains and intense southwesterly flows between them within a region of large horizontal pressure gradient. The intraseasonal disturbances are confined to tropical regions and influence SC by marginal southwesterly flows. It is concluded that low-frequency disturbances provide favorable background conditions for EPEs over SC and synoptic-scale disturbances ultimately induce EPEs on the peak wet days. Both should be simultaneously considered for EPE predictions over SC.
Characteristics of Pre-summer Daytime Cloud Regimes over Coastal South China from the Himawari-8 Satellite
Mingxin LI, Yali LUO, Min MIN
, Available online   , Manuscript accepted  30 August 2021, doi: 10.1007/s00376-021-1148-1
Using the high spatiotemporal resolution (2 km-and-10 min) data from the Advanced Himawari Imager onboard the Himawari-8 satellite, this study documents the fine-scale characteristics of daytime cloud regimes (CRs) over coastal South China during the pre-summer rainy season (April–June). Six CRs (CR1–CR6) are identified based on the joint frequency distribution of cloud top brightness temperature and cloud optical thickness, namely, the optically thin-to-moderate cloud mixture, optically thin warm clouds with cirrus, optically thick warm clouds, weak convective cloud mixture, strong convective clouds, and extreme, deep convective clouds. The optically thick warm clouds are the major CR during April and May, with higher frequencies over land, especially along the urban agglomeration, rather than the offshore which may be an indicator of the higher aerosol concentrations being a contributing factor over the cities. The CRs with weak convective cloud mixtures and strong convective clouds appear more frequently over the land, while the two CRs with optically thinner clouds occur mainly offshore. Synoptic flow patterns (SPs) are objectively identified and examined focusing on those favoring the two major rain-producing CRs (CR5 and CR6) and the highly reflective CR with optically thick warm clouds (CR3). The two SPs favoring CR5 and CR6 are characterized by abundant moisture with low-level jets after monsoon onset, and a northwest high-southeast low pattern with strong dynamic convergence along the coastline, respectively. The non-convective CR3 with high reflectance is related to a SP that features the western North Pacific subtropical high extending more westward, leading to a moderate moisture supply and a wide range of convective available potential energy, but also, large convective inhibition.
A Machine Learning-based Cloud Detection Algorithm for the Himawari-8 Spectral Image
Chao LIU, Shu YANG, Di DI, Yuanjian YANG, Chen ZHOU, Xiuqing HU, Byung-Ju SOHN
, Available online   , Manuscript accepted  20 April 2021, doi: 10.1007/s00376-021-0366-x
Cloud Masking is one of the most essential products for satellite remote sensing and downstream applications. This study develops machine learning-based (ML-based) cloud detection algorithms using spectral observations for the Advanced Himawari Imager (AHI) onboard the Himawari-8 geostationary satellite. Collocated active observations from Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) are used to provide reference labels for model development and validation. We introduce both daytime and nighttime algorithms that differ according to whether solar band observations are included, and the artificial neural network (ANN) and random forest (RF) techniques are adopted for comparison. To eliminate the influences of surface conditions on cloud detection, we introduce three models with different treatments of the surface. Instead of developing independent ML-based algorithms, we add surface variables in a binary way that enhances the ML-based algorithm accuracy by ~5%. Validated against CALIOP observations, we find that our daytime RF-based algorithm outperforms the AHI operational algorithm by improving the accuracy of cloudy pixel detection by ~5%, while at the same time, reducing misjudgment by ~3%. The nighttime model with only infrared observations is also slightly better than the AHI operational product but may tend to overestimate cloudy pixels. Overall, our ML-based algorithms can serve as a reliable method to provide cloud mask results for both daytime and nighttime AHI observations. We furthermore suggest treating the surface with a set of independent variables for future ML-based algorithm development.
Evaluating the Impacts of Cloud Microphysical and Overlap Parameters on Simulated Clouds in Global Climate Models
Haibo WANG, Hua ZHANG, Bing XIE, Xianwen JING, Jingyi HE, Yi LIU
, Available online   , Manuscript accepted  18 March 2021, doi: 10.1007/s00376-021-0369-7
The improvement of the accuracy of simulated cloud-related variables, such as the cloud fraction, in global climate models (GCMs) is still a challenging problem in climate modeling. In this study, the influence of cloud microphysics schemes (one-moment versus two-moment schemes) and cloud overlap methods (observation-based versus a fixed vertical decorrelation length) on the simulated cloud fraction was assessed in the BCC_AGCM2.0_CUACE/Aero. Compared with the fixed decorrelation length method, the observation-based approach produced a significantly improved cloud fraction both globally and for four representative regions. The utilization of a two-moment cloud microphysics scheme, on the other hand, notably improved the simulated cloud fraction compared with the one-moment scheme; specifically, the relative bias in the global mean total cloud fraction decreased by 42.9%–84.8%. Furthermore, the total cloud fraction bias decreased by 6.6% in the boreal winter (DJF) and 1.64% in the boreal summer (JJA). Cloud radiative forcing globally and in the four regions improved by 0.3%−1.2% and 0.2%−2.0%, respectively. Thus, our results showed that the interaction between clouds and climate through microphysical and radiation processes is a key contributor to simulation uncertainty.
Estimations of Land Surface Characteristic Parameters and Turbulent Heat Fluxes over the Tibetan Plateau Based on FY-4A/AGRI Data
Nan GE, Lei ZHONG, Yaoming MA, Yunfei FU, Mijun ZOU, Meilin CHENG, Xian WANG, Ziyu HUANG
, Available online   , Manuscript accepted  10 November 2020, doi: 10.1007/s00376-020-0169-5
Accurate estimates of land surface characteristic parameters and turbulent heat fluxes play an important role in the understanding of land–atmosphere interaction. In this study, Fengyun-4A (FY-4A) Advanced Geostationary Radiation Imager (AGRI) satellite data and the China Land Data Assimilation System (CLDAS) meteorological forcing dataset CLDAS-V2.0 were applied for the retrieval of broadband albedo, land surface temperature (LST), radiation flux components, and turbulent heat fluxes over the Tibetan Plateau (TP). The FY-4A/AGRI and CLDAS-V2.0 data from 12 March 2018 to 30 April 2018 were first used to estimate the hourly turbulent heat fluxes over the TP. The time series data of in-situ measurements from the Tibetan Observation and Research Platform were divided into two halves—one for developing retrieval algorithms for broadband albedo and LST based on FY-4A, and the other for the cross validation. Results show the root-mean-square errors (RMSEs) of the FY-4A retrieved broadband albedo and LST were 0.0309 and 3.85 K, respectively, which verifies the applicability of the retrieval method. The RMSEs of the downwelling/upwelling shortwave radiation flux and downwelling/upwelling longwave radiation flux were 138.87/32.78 W m−2 and 51.55/17.92 W m−2, respectively, and the RMSEs of net radiation flux, sensible heat flux, and latent heat flux were 58.88 W m−2, 82.56 W m−2 and 72.46 W m−2, respectively. The spatial distributions and diurnal variations of LST and turbulent heat fluxes were further analyzed in detail.
Evaluation of a Regional Ensemble Data Assimilation System for Typhoon Prediction
Lili Lei, Yangjinxi Ge, Zhe-Min Tan, Yi Zhang, Kekuan Chu, Xin Qiu, Qifeng Qian
, Available online  
An ensemble Kalman filter (EnKF) combined with the Advanced Research Weather Research and Forecasting model (WRF) is cycled and evaluated for western North Pacific (WNP) typhoons of year 2016. Conventional in-situ data, radiance observations, and tropical cyclone (TC) minimum sea level pressure (SLP) are assimilated every 6 h using an 80-member ensemble. For all TC categories, the 6-h ensemble priors from the WRF/EnKF system have appropriate amount of variance for TC tracks, but have insufficient variance for TC intensity. The 6-h ensemble priors from the WRF/EnKF system tend to overestimate the intensity for weak storms, but underestimate the intensity for strong storms. The 5-d deterministic forecasts launched from the ensemble mean analyses of WRF/EnKF are compared to the NCEP and ECMWF operational control forecasts. Results show that the WRF/EnKF forecasts generally have larger track errors than the NCEP and ECMWF forecasts for all TC categories, because the regional simulation cannot better represent the large-scale environment than the global simulation. The WRF/EnKF forecasts produce smaller intensity errors and biases than the NCEP and ECMWF forecasts for typhoons, but the opposite is true for tropical storms and severe tropical storms. The 5-d ensemble forecasts from the WRF/EnKF system for seven typhoon cases show appropriate variance for TC track and intensity with short forecast lead times, but have insufficient spread with long forecast lead times. The WRF/EnKF system provides better ensemble forecasts and higher predictability for TC intensity than the NCEP and ECMWF ensemble forecasts.
News & Views
Earth Summit Mission 2022: Scientific Expedition and Research on Mt. Qomolangma Helps Reveal the Synergy between Westerly Winds and Monsoon and the Resulting Climatic and Environmental Effects
Yaoming MA, Weiqiang MA, Huaguang DAI, Lei ZHANG, Fanglin SUN, Jinqiang ZHANG, Nan YAO, Jianan HE, Zhixuan BAI, Yuejian XUAN, Yunshuai ZHANG, Yuan YUAN, Chenyi YANG, Weijun SUN, Ping ZHAO, Minghu DING, Kongju ZHU, Jie HU, Bian Bazhuga, Bai Juepingcuo, Zhuo Ma, Ren Qingnima, Suo Langwangdui, Yang Zong, Haikun WEN
, Available online   , Manuscript accepted  12 August 2022, doi: 10.1007/s00376-022-2166-3
“Earth summit mission 2022” is one of the landmark scientific research activities of the Second Tibetan Plateau Scientific Expedition and Research (STEP). This scientific expedition firstly used advanced technology and methods to detect vertical meteorological elements and produce forecasts for mountain climbing. The “Earth summit mission 2022” Qomolangma scientific expedition exceeded an altitude of over 8000 meters for the first time and carried out a comprehensive scientific investigation mission on the summit of Mt. Qomolangma. Among the participants, the westerly–monsoon synergy and influence team stationed in the Mt. Qomolangma region had two tasks: 1) detecting the vertical structure of the atmosphere for parameters such as wind, temperature, humidity, and pressure with advanced instruments for high-altitude detection at the Mt. Qomolangma base camp; and 2) observing extreme weather processes to ensure that members of the mountaineering team could successfully reach the top. Through this scientific expedition, a better understanding of the vertical structure and weather characteristics of the complex area of Mt. Qomolangma is gained.
Will the Historic Southeasterly Wind over the Equatorial Pacific in March 2022 Trigger a Third-year La Niña Event?
Xianghui FANG, Fei ZHENG, Kexin LI, Zeng-Zhen HU, Hongli REN, Jie WU, Xingrong CHEN, Weiren LAN, Yuan YUAN, Licheng FENG, Qifa CAI, Jiang ZHU
, Available online   , Manuscript accepted  26 June 2022, doi: 10.1007/s00376-022-2147-6
Based on the updates of the Climate Prediction Center and International Research Institute for Climate and Society (CPC/IRI) and the China Multi-Model Ensemble (CMME) El Niño-Southern Oscillation (ENSO) Outlook issued in April 2022, La Niña is favored to continue through the boreal summer and fall, indicating a high possibility of a three-year La Niña (2020–23). It would be the first three-year La Niña since the 1998–2001 event, which is the only observed three-year La Niña event since 1980. By examining the status of air–sea fields over the tropical Pacific in March 2022, it can be seen that while the thermocline depths were near average, the southeasterly wind stress was at its strongest since 1980. Here, based on a quaternary linear regression model that includes various relevant air–sea variables over the equatorial Pacific in March, we argue that the historic southeasterly winds over the equatorial Pacific are favorable for the emergence of the third-year La Niña, and both the anomalous easterly and southerly wind stress components are important and contribute ~50% of the third-year La Niña growth, respectively. Additionally, the possible global climate impacts of this event are discussed.
Volcanoes and Climate: Sizing up the Impact of the Recent Hunga Tonga-Hunga Ha'apai Volcanic Eruption from a Historical Perspective
Meng ZUO, Tianjun ZHOU, Wenmin MAN, Xiaolong CHEN, Jian LIU, Fei LIU, Chaochao GAO
, Available online   , Manuscript accepted  23 February 2022, doi: 10.1007/s00376-022-2034-1
An undersea volcano at Hunga Tonga-Hunga Ha'apai (HTHH) near the South Pacific island nation of Tonga, erupted violently on 15 January 2022. Potential climate impact of the HTHH volcanic eruption is of great concern to the public; here, we intend to size up the impact of the HTHH eruption from a historical perspective. The influence of historical volcanic eruptions on the global climate are firstly reviewed, which are thought to have contributed to decreased surface temperature, increased stratospheric temperature, suppressed global water cycle, weakened monsoon circulation and El Niño-like sea surface temperature. Our understanding of the impacts of past volcanic eruptions on global-scale climate provides potential implication to evaluate the impact of the HTHH eruption. Based on historical simulations, we estimate that the current HTHH eruption with an intensity of 0.4 Tg SO2 injection will decrease the global mean surface temperature by only 0.004°C in the first year after eruption, which is within the amplitude of internal variability at the interannual time scale and thus not strong enough to have significant impacts on the global climate.
Letters & Notes
A Quantitative Method of Detecting Transient Rossby Wave Phase Speed: No Evidence of Slowing down with Global Warming
Yashu WU, Jianhua LU
, Available online   , Manuscript accepted  06 July 2022, doi: 10.1007/s00376-022-2164-5
Based on the Complex Empirical Orthogonal Functions (CEOFs) of bandpass-filtered daily streamfunction fields, a quantitative method of detecting transient (synoptic) Rossby wave phase speed (RWPhS) is presented. The transient RWPhS can be objectively calculated by the distance between a high (or low) center in the real part of a CEOF mode and its counterpart in the imaginary part of the same CEOF mode divided by the time span between two adjacent peaks (or bottoms) of two principal component curves for the real and imaginary parts of that CEOF mode. The new detection method may partly reveal the spatiotemporal heterogeneity of Rossby wave prorogation. Although the mean westerly jet at 200 hPa doubles the speed of its counterpart at 500 hPa, the estimated RWPhS at both levels are around 1000 km d–1 and quantitatively consistent with the quasigeostrophic-theory-based RWPhS, confirming that the meridional potential vorticity gradient induced by the barotropic and baroclinic shears of mean flow, together with the β effect, play an essential role in Rossby wave propagation. Both observations over the past four decades and a 150-year historical simulation suggest no evidence for slowing wintertime transient Rossby waves in the Northern Hemisphere, but possible regional changes are not excluded. We emphasize that not only the mean flow speed, but also the barotropic and baroclinic shears of the mean flow, and their associated contributions to the meridional potential vorticity (PV) gradient, should be considered in investigating the possible change of Rossby waves with global warming.
Notes & Letters
The Importance of the Shape Parameter in a Bulk Parameterization Scheme to the Evolution of the Cloud Droplet Spectrum during Condensation
Jun ZHANG, Jiming SUN, Wei DENG, Wenhao HU, Yongqing WANG
, Available online   , Manuscript accepted  22 June 2022, doi: 10.1007/s00376-022-2065-7
The shape parameter of the Gamma size distribution plays a key role in the evolution of the cloud droplet spectrum in the bulk parameterization schemes. However, due to the inaccurate specification of the shape parameter in the commonly used bulk double-moment schemes, the cloud droplet spectra cannot reasonably be described during the condensation process. Therefore, a newly-developed triple-parameter condensation scheme with the shape parameter diagnosed through the number concentration, cloud water content, and reflectivity factor of cloud droplets can be applied to improve the evolution of the cloud droplet spectrum. The simulation with the new parameterization scheme was compared to those with a high-resolution Lagrangian bin scheme, the double-moment schemes in a parcel model, and the observation in a 1.5D Eulerian model that consists of two cylinders. The new scheme with the shape parameter varying with time and space can accurately simulate the evolution of the cloud droplet spectrum. Furthermore, the volume-mean radius and cloud water content simulated with the new scheme match the Lagrangian analytical solutions well, and the errors are steady, within approximately 0.2%.
Data-driven Estimation of Cloud Effects on Surface Irradiance at Xianghe, a Suburban Site on the North China Plain
Mengqi LIU, Jinqiang ZHANG, Hongrong SHI, Disong FU, Xiangao XIA
, Available online   , Manuscript accepted  10 March 2022, doi: 10.1007/s00376-022-1414-x
Clouds are a dominant modulator of the energy budget. The cloud shortwave radiative effect at the surface (CRE) is closely related to the cloud macro- and micro-physical properties. Systematic observation of surface irradiance and cloud properties are needed to narrow uncertainties in CRE. In this study, 1-min irradiance and Total Sky Imager measurements from 2005 to 2009 at Xianghe in North China Plain are used to estimate cloud types, evaluate cloud fraction (CF), and quantify the sensitivities of surface irradiance with respect to changes in CF whether clouds obscure the sun or not. The annual mean CF is 0.50, further noting that CF exhibits a distinct seasonal variation, with a minimum in winter (0.37) and maximum in summer (0.68). Cumulus occurs more frequently in summer (32%), which is close to the sum of the occurrence of stratus and cirrus. The annual CRE is –54.4 W m–2, with seasonal values ranging from –29.5 W m–2 in winter and –78.2 W m–2 in summer. When clouds do not obscure the sun, CF is a dominant factor affecting diffuse irradiance, which in turn affects global irradiance. There is a positive linear relationship between CF and CRE under sun-unobscured conditions, the mean sensitivity of CRE for each CF 0.1 increase is about 1.2 W m–2 [79.5° < SZA (Solar Zenith Angle) < 80.5°] to 7.0 W m–2 (29.5° < SZA < 30.5°). When clouds obscure the sun, CF affects both direct and diffuse irradiance, resulting in a non-linear relationship between CF and CRE, and the slope decreases with increasing CF. It should be noted that, although only data at Xianghe is used in this study, our results are representative of neighboring areas, including most parts of the North China Plain.
Identification of Convective and Stratiform Clouds Based on the Improved DBSCAN Clustering Algorithm
Yuanyuan ZUO, Zhiqun HU, Shujie YUAN, Jiafeng ZHENG, Xiaoyan YIN, Boyong LI
, Available online   , Manuscript accepted  09 November 2021, doi: 10.1007/s00376-021-1223-7
A convective and stratiform cloud classification method for weather radar is proposed based on the density-based spatial clustering of applications with noise (DBSCAN) algorithm. To identify convective and stratiform clouds in different developmental phases, two-dimensional (2D) and three-dimensional (3D) models are proposed by applying reflectivity factors at 0.5° and at 0.5°, 1.5°, and 2.4° elevation angles, respectively. According to the thresholds of the algorithm, which include echo intensity, the echo top height of 35 dBZ (ET), density threshold, and ε neighborhood, cloud clusters can be marked into four types: deep-convective cloud (DCC), shallow-convective cloud (SCC), hybrid convective-stratiform cloud (HCS), and stratiform cloud (SFC) types. Each cloud cluster type is further identified as a core area and boundary area, which can provide more abundant cloud structure information. The algorithm is verified using the volume scan data observed with new-generation S-band weather radars in Nanjing, Xuzhou, and Qingdao. The results show that cloud clusters can be intuitively identified as core and boundary points, which change in area continuously during the process of convective evolution, by the improved DBSCAN algorithm. Therefore, the occurrence and disappearance of convective weather can be estimated in advance by observing the changes of the classification. Because density thresholds are different and multiple elevations are utilized in the 3D model, the identified echo types and areas are dissimilar between the 2D and 3D models. The 3D model identifies larger convective and stratiform clouds than the 2D model. However, the developing convective clouds of small areas at lower heights cannot be identified with the 3D model because they are covered by thick stratiform clouds. In addition, the 3D model can avoid the influence of the melting layer and better suggest convective clouds in the developmental stage.
Letters and Notes
Alternation of the Atmospheric Teleconnections Associated with the Northeast China Spring Rainfall during a Recent 60-Year Period
Zhiwei ZHU, Rui LU, Shanshan FU, Hua CHEN
, Available online   , Manuscript accepted  06 May 2022, doi: 10.1007/s00376-022-2024-3
Northeast China (NEC) is China’s national grain production base, and the local precipitation is vital for agriculture during the springtime. Therefore, understanding the dynamic origins of the NEC spring rainfall (NECSR) variability is of socioeconomic importance. This study reveals an interdecadal change in the atmospheric teleconnections associated with the NECSR during a recent 60-year period (1961–2020). Before the mid-1980s, NECSR had been related to a Rossby wave train that is coupled with extratropical North Atlantic sea surface temperature (SST), whereas, since the mid-1980s, NECSR has been linked to a quite different Rossby wave train that is coupled with tropical North Atlantic SST. Both Rossby wave trains could lead to enhanced NECSR through anomalous cyclones over East Asia. The weakening of the westerly jet over North America is found to be mainly responsible for the alternation of the atmospheric teleconnections associated with NECSR during two epochs.