Abstract:Extreme hot and dry climate anomalies occurred in central-eastern China during the summer of 2022, which had a significant impact on the economy, agriculture, and human community.This paper reviews the spatiotemporal characteristics and causes of this hot and dry event occurrence.During the summer of 2022, the areal mean extreme high temperature frequency, monthly mean daily maximum temperature, and high temperature days reached their highest values since 1979, while the areal mean monthly mean precipitation over central-eastern China reached its lowest since 1979.These climate anomalies are caused by the combined effects of several factors, including anomalous strong western Pacific subtropical highs, increased disturbances in mid-latitude westerlies, and tropical sea surface temperature anomalies.In addition, the paper explores the influence of global warming trend on this extreme event, As well as the future changes in extreme heat and drought in China.

Abstract:The Yellow River basin is an important ecological barrier in our country, as well as the main producing area for grain crops.In recent years, the distribution of local water resources has had a significant impact on the development of urbanization.As a result, it is critical to understand the impact of urbanization on the water cycle in the Yellow River basin.The characteristics of groundwater change in the Yellow River basin are analyzed using the Resource Environmental Science and Data Center's Yellow River basin underlying surface data, population density and Gravity Recovery and Climate Experiment (Grace) data from the world population data set, and grid data of total land water storage from the Center for Space Research (CSR).The results show that there is a significant spatial difference in the degree of urbanization in the Yellow River basin, with urbanization intensifying in the lower reaches of the Yellow River and, in particular, the groundwater showing a downward trend.Comparing the difference of groundwater change among new urbanization, constant urbanization and disappearing urbanization in the Yellow River basin reveals that the decrease of groundwater is the smallest in the disappearing cities and towns;groundwater levels in new towns have fallen less than in areas that have always been urban.Therefore, as a typical arid and semi-arid region, the urbanization of the Yellow River basin plays a significant role in the reduction of groundwater, and the recent urbanization of groundwater protection has achieved good results.

Abstract:Central Asia is located in the interior of Eurasia, in the key area of westerly wind control.With accelerating global warming in recent years, observations and simulations show a trend of warming and humidification in Central Asia.SST anomalies in the North Atlantic induce atmospheric vorticity anomalies, which propagate anomalies influencing large-scale circulation and vertical motion in Central Asia via Rossby wave activity, leading to an increase in precipitation anomalies.Simultaneously, the SST anomalies in the equatorial Pacific Ocean and the North Indian Ocean cause abnormal water vapor transport from the Arabian Peninsula to Central Asia, as well as the westerly-monsoon synergy, the remote correlation phase transition of the Silk Road, the changes of land surface type, local circulation and soon, which all contribute in varying degrees to the warming and humidification process in Central Asia.This paper aims to outline the main influencing factors and mechanisms of precipitation anomalies in Central Asia in the past 20 years, and to propose future research directions based on previous research.

Abstract:The second mode of 200 hPa meridional wind over the Eurasian continent, known as the North Atlantic-East and North Asia (NAENA) pattern, has a significant impact on the multi-scale climatic variability in Eurasia.This paper examined the NAENA pattern's influence on summer drought and flood anomalies in Xinjiang, China, as well as its possible mechanism, based on an analysis of the NAENA pattern's maintenance mechanism.The vorticity budget analysis of atmospheric wave train shows that the advection of perturbed vorticity by the climatological rotational zonal wind can be compensated by the advection of mean vorticity caused by the anomalous rotational meridional wind.The energy conversion analysis indicates that the NAENA pattern is maintained mainly through the baroclinic energy conversion from the basic flow with the North Atlantic SST tripolar pattern being the important external driving factor for maintaining the NAENA pattern.Moreover, regression analysis shows that the NAENA pattern can significantly affect drought and flood anomalies in Xinjiang by promoting Southern Xinjiang summer precipitation.When the NAENA pattern is in phase with a cyclone over Central Asia, the vertical ascending motion in Southern Xinjiang tends to be above normal;however, more water vapor is transported into Southern Xinjiang, leading to an anomalous increase in precipitation over Xinjiang.This study is of great significance because it reveals the mechanisms of summer drought and flood in arid areas of northwest China, which is conducive to climate prediction and disaster prevention and mitigation.

Abstract:Climate wetness and dryness are important indicators of regional climate characteristics, which are the comprehensive results of the water cycle and land surface evapotranspiration in the context of global warming.Based on the wbtness index and the spatiotemporal variation of precipitation and potential evapotranspiration (PET), this paper analyzes the characteristics of climate change and climatic dry-wet variation and its effects on soil moisture in arid and semi-arid areas in China.The analysis reveals that China's arid and semi-arid areas have become wet in the past 50 years.The monthly difference in PET and precipitation between arid and semi-arid areas is inconsistent over the year, and the arid area is obviously larger than the semi-arid area.From March to September, the PET is greater than the precipitation in the arid areas, and from March to June, the PET is greater than the precipitation in the semi-arid areas of China.Since July, the difference has decreased significantly.The wetness index is analyzed using the difference method, and the results show that precipitation has more influence on the wetness index than PET in the arid region, while precipitation and PET have similar influences in the semi-arid region.For a long time, the majority of soil moisture in arid and semi-arid areas in China has gradually become drier, especially the plough layer, which has shown a trend of becoming drier throughout the entire region, indicating that there are great potential drought risks in northern agricultural and animal husbandry production.

Abstract:Climate change over drylands in a warmer climate and its responses to Earth's orbit can be better understood by an investigation of climate change across arid Central Asia during the Last Interglacial (LIG).Based on the Paleoclimate Modelling Intercomparison Project Phase 4 (PIMP4), we analyze variations in wet-dry conditions and associated mechanisms over arid Central Asia during the LIG.The results show that during the LIG, the annual precipitation is reduced by 0.7% over arid Central Asia, with precipitation decreasing by 2.8% over Central Asia while increasing by 1.8% over Xinjiang compared to the preindustrial period.Over Central Asia, the precipitation decreases by 1.8%, 1.3%, 1.5%, and 10.3% in the spring, summer, autumn, and winter, respectively, indicating that the reduced annual precipitation is mainly due to changes in winter.According to the moisture budget equation, variations in precipitation over Central Asia during the rainy season (winter and spring) are mainly caused by the vertical dynamic term.Moreover, precipitation over Xinjiang is reduced by 5.9% and 3.8% in spring and winter, respectively, and increased by 14.1% in summer and 8.6% in autumn, indicating that the majority of the rise in annual precipitation is a result of precipitation in summer.Consequently, the moisture budget analysis suggests that the vertical dynamic and thermodynamic terms contribute most to the variations in precipitation over Xinjiang.In addition, the aridity index over arid Central Asia decreases by approximately 10.2% during the LIG based on the Penman-Montieth method, which indicates that arid Central Asia experience a drier condition.Furthermore, drylands over arid Central Asia had expanded during that period, including the transition from semi-arid to arid regions, from sub-humid and semi-arid to arid regions, and from humid to sub-humid, semi-arid and arid regions.Hence, the reduced aridity index over arid Central Asia is a result of increased potential evaporation at that time.This increased evaporation is further linked with negative contributions from decreased surface air temperature and increased relative humidity, as well as positive contributions from enhanced surface wind speed and available energy.Our study provides a possible map for changes in wet-dry conditions over arid Central Asia in a warmer climate induced by the Earth's orbit and sheds light on risk assessments for arid Central Asia in different warming scenarios.

Abstract:Using machine learning models (MLMs) to develop high-accuracy evapotranspiration (ET) products is important for investigating the terrestrial hydrological changes in arid and semi-arid regions in the context global warming.Based on the 12 flux stations in Northwest China and multi-source observation datasets, we present a 5-km gridded ET product based on 4 MLMs including the random forest, the extreme gradient boosting, the support vector regression, and the artificial neural network, and analyze the long-term ET trend over Northwest China.The cross-validation results show that all the four models can simulate the daily ET reasonably well, with the root-mean-square error (RMSE) smaller than 0.57 mm·d^-1and the R² up to 0.73~0.88.Moreover, the Sharply additive explanations (SHAP) method reveals that all the models treat the net radiation, vegetation indexes and soil moisture as the most important predictors and capture the limitation effect of soil water on ET reasonably well, indicating a good physical interpretability of the 4 MLMs.No model always has superiority, and the ensemble mean of the 4 models shows a 7%-20% and 45%-70% smaller RMSE than the individual member and other ET products.The ensemble ET shows an increasing trend over the Northwest China during 2001-2018, with a mean increase of 19 mm/(10 a).In addition, the rate of growth of ET is greater than the rate of increase of precipitation in the Hetao region and the middle and northeastern parts of Inner Mongolia, suggesting an intensified drying trend in these regions.

Abstract:South Xinjiang, located in the Eurasian hinterland, has a typical temperate continental arid climate, which is affected by complex topography, weather system paths, and special atmospheric circulation and water vapor conditions, resulting in sudden and significant regional characteristics of heavy rainfall.At present, global numerical prediction models and mesoscale numerical models have a very limited ability to forecast heavy rainfall in South Xinjiang.In recent years, many research teams have conducted large-scale outfield observation experiments in the Tarim Basin and gained more insight into the convective triggering mechanism, the configuration and evolution characteristics of high and low altitude systems, and the physical processes of rainfall clouds that cause heavy rainfall in South Xinjiang.They have also shed more light on the mechanisms and causes of heavy rainfall.This paper summarizes and reviews the climatic characteristics, large-scale circulation background, mesoscale system development, water vapor transport, and precipitation dynamics mechanisms of heavy rainfall in southern Xinjiang, and proposes scientific questions that need further study, with a goal of providing references for further research on heavy rainfall in southern Xinjiang and improving the accuracy of heavy rainfall forecasting, as well as disaster prevention and mitigation.

Abstract:Based on the daily precipitation of 24 global climate models from the sixth phase of the Coupled Model Intercomparison Project 6 (CMIP6) multimodel simulations, the generalized extreme value distribution (GEV) is introduced to study the risks of extreme precipitation that expected to occur every 20, 50, and 100 years over China under 1.5 and 2℃ global warming levels.In comparison to the historical period (1995-2014), the changes in the probability of the risk of extreme precipitation under 1.5 and 2℃ global warming present an overall increasing trend.Although their spatial distributions show similar characteristics, the additional half degree of global warming will lead to a higher risk.For example, extreme precipitation that occurs once every 50 years will become once every 14 or 17 years under the 1.5 and 2℃ global warming, respectively, and extreme precipitation will become more frequent.There are regional differences in how each region reacts to global warming, among which the middle and upper reaches of the Yangtze and Yellow Rivers and the Qinghai-Tibet Plateau region in Western China, and the middle and lower reaches of the Yangtze and Yellow Rivers and their tributaries in Eastern China, are regions that are highly sensitive to climate change, with probability ratios of 3 or even 5 or more.Furthermore, the influence and contribution measures of location and scale parameters on the probability ratios are explored theoretically using probability distributions, which are also used to explore the influences of climate means and variability changes on the risks of extreme precipitation.The results show that there are significant differences between Eastern China and Western China in the incremental changes of location and scale parameters, and in the rates of probability changes, which lead to differences in the factors that influence the risk of extreme precipitation.In Western China, although the changes in climate means and variabilities of extreme precipitation are small, the probability ratios increase significantly due to the high rates of changes in probability.In contrast, the change rates are small, but the climate means and variabilities are increasing significantly, which also lead to an increase in Eastern China.Moreover, compared to the location parameters, the increased risks in most regions of China are mainly due to the anticipated changes in scale parameters of extreme precipitation.

Abstract:Based on the meteorological observation data from more than 2 400 ground stations in China from 1961 to 2022, the basic climatic characteristics and major meteorological events over China during the the summer of 2022 are summarized.The climate in China during the summer of 2022 has been generally warm and dry, with the hottest summer on record occurring since 1961 by a margin of more than 1.1℃.It has also been the second-driest summer on record, and the total precipitation in summer over China has been 290.6 mm, 12.3% less than normal, with most of the southern half of China (apart from Guangdong Province) having seasonal rainfall 20% to 50% below average.China has had the most extensive and long-lasting high temperature event since 1961, extending from mid-June to the end of August.The heat is particularly severe in the Yangtze River Basin, which is also experiencing its driest summer on record.The compound of high temperature and drought has had a negative impact on agricultural, water resources, energy supply and the navigability of the Yangtze River.There were 19 regional rainstorm events across the country, and flooding occurs in the Pearl River Basin and the Songliao River Basin.There are abnormally fewer typhoons generating and making landfall in summer.The typhoon Chaba, which is the first typhoon to make landfall in China this year, has had a huge impact and strong intensity.

Abstract:Compound heat waves consider the effect of temperature, humidity, wind speed and radiation simultaneously, and have a major impact on human health and ecological diversity.Based on the wet-bulb globe temperature (WBGT), this paper identifies the compound heat waves in China, and quantitatively analyzes the synergy of urbanization and the Western Pacific subtropical high (WPSH) intensification on the compound heat waves in China.Results show that the number of days, intensity and impact range of compound heat waves in China are gradually increasing from 1979 to 2019, especially after 2010, which are about four times larger than those in the 1980s.The synergy of urbanization rapid development and WPSH intensification has intensified the compound heat waves in China.The early urbanization has contributed 9.2%, 12.5% and 7.5% to the increases in the number of days, intensity and impact range of the compound heat waves, respectively.During the same period, WPSH has a positive contribution of about 30% to the three heat wave indexes, and the contribution can be up to about 70% when the strengthening effect of global warming on WPSH is considered.It suggests that the synergy of the rapid urbanization and the enhanced WPSH under the background of global warming can explain more than 80% of the jump in the compound heat waves in China at the beginning of the 21st century.

Abstract:Using composition, EOF, correlation and other methods on the daily precipitation data from Guangxi surface meteorological observation stations from 1961 to 2020 and NCEP/NCAR reanalysis data, the impact of the onset of the South China Sea summer monsoon on the heavy rainfall in Guangxi in June is studied, revealing the causes of the climate anomalies on the heavy rainfall days.The results show that the East Asian trough is significantly stronger when the onset of the South China Sea summer monsoon is earlier, and the meridional circulation in the middle and high latitudes is strengthened.Additionally, the southwesterly wind along the South China coast is significantly stronger, which is combined with the strong meridional winds in the middle and high latitudes.While the Madden-Julian oscillation (MJO) from the Indian Ocean to the oceanic continent is in the phase of convection enhancement, moving significantly eastward with the low-frequency convection belt being transported to Guangxi under the guidance of the southwest monsoon, the northerly wind, guided by the circulation, moves southward and the north-south wind converges over Guangxi.Also, it indicates that the updraft is significantly stronger over Guangxi, as indicated by the increased frequency of rainstorm days and, on the other hand, the relatively low number of rainstorm days.

Abstract:The adaptive areas of the "Chan-hom" typhoon (1509) in 2015 are created by ETKF adaptive observing systems.Further assimilation experiments are carried out based on the sensitive area at the first target moment, using the OSSE method and WRF mode.According to the experiments, the sensitive areas of "CHAN-HOM" are mostly found in the northeast and southeast sides of the center.The results show that the assimilation of simulated observation data in the sensitive area better improves the prediction accuracy of the height field and track than the assimilation of conventional observation data, while having a negative impact on the prediction of precipitation.

Abstract:Based on the ERA-Interim reanalysis data four times a day provided by ECMWF from 1979 to 2019, the main period of the intraseasonal oscillation and its propagation characteristics of zonal wind at upper troposphere over the eastern Tibetan Plateau in summer are studied by the Morlet wavelet, filtering and composite analysis methods.Results show that the 10-30 d is the main period of intraseasonal oscillation of zonal wind at upper troposphere over the eastern Tibetan Plateau in summer and its intensity has significant interannual difference.In the strong intraseasonal oscillation years, the low-frequency oscillation process lasts for a long time and has a large amplitude.The center of variance of intraseasonal oscillation influences the mid-troposphere from the upper troposphere, which shows an equivalent barotropic structure.In zonal propagation, the intraseasonal oscillation centers mainly propagate eastward from the eastern Tibetan Plateau for three times and reach the Western Pacific region.In meridional propagation, 10-30 d intraseasonal oscillation centers propagating southward from the mid and higher latitudes converge with centers from the lower latitude on the south side of the Plateau for four times from June to August, and their intensities are strengthened.The strong oscillation centers can propagate southward to the lower latitude areas.The phase evolution of intraseasonal oscillation is mainly manifested in the alternation of the low-frequency anticyclone and the low-frequency cyclone center over the eastern Tibetan Plateau, which leads to the intensity change of low-frequency easterly wind and low-frequency westerly wind over the eastern Tibetan Plateau.In the extremely active phase of ISO, the low-frequency westerly wind over the eastern Tibetan Plateau is the strongest.