The summer monsoon transition zone in China is one of the regions with strong land-atmosphere interaction in the world, and it is also an area where extreme weather disasters are frequent and easy to cause serious economic losses. Further understanding of land-atmosphere interaction in the transition area will help to improve the disaster prevention and mitigation ability of this region. Based on the research results of the summer monsoon transition area related projects carried out by the Key Laboratory of Drought Climate Change and Disaster Reduction of China Meteorological Administration in recent years, this paper systematically summarizes the new progresses of land-atmosphere interaction in the summer monsoon transition zone, including the spatio-temporal distribution law of land-atmosphere interaction in the transition region, the new characteristics of the response of land surface water budget to summer monsoon, the spatio-temporal variation characteristics and development mechanism of the boundary layer, the influence of monsoon and land-atmosphere interaction on regional climate in the transition zone, the new progress of land-atmosphere interaction on crop yield in the transition zone and new schemes for parameterization of multi-factor and multi-scale kinetic roughness. According to the development trend of land-atmosphere interaction research in the summer monsoon transition zone, it is proposed that the multi-scale dynamic response of land-atmosphere interaction to summer monsoon should be explored in the future, and the climatic dynamic relationship between surface processes and key physical quantities in the atmospheric boundary layer should be established on the basis of the research on the response rule of land-atmosphere exchange multi-cycle process to the annual cycle of summer monsoon in order to improve and enhance the simulation of regional climate models in the future. This work is of great significance to promote the research of land-atmosphere coupling process in China, which can provide scientific and technological support for disaster prevention and mitigation in the summer monsoon transition zone in China.

In the summer of 2022, there was a climate anomaly with the highest average temperature and less precipitation since 1961 in China, accompanied by the strongest nationwide high temperature processes (except northeastern China) and a wide range and strong summer drought in the middle and lower reaches of the Yangtze River and Sichuan and Chongqing. For the persistence and extremity of the high temperature and drought event in this summer in China, this paper analyzes the temporal and spatial distribution characteristics and cirulation situation based on the daily maximum temperature and precipitation of 2162 meteorological stations in China from June to August 2022 and the daily reanalysis data of NCEP (National Centers for Environmental Prediction) /NCAR (National Center for Atmospheric Research), which will be of some references for the forecast of summer high temperature and drought with different time scales in southern China. The results show that in the summer of 2022, a total of 48 198 high temperature events occurred in 76.0% of the stations in China, among which 36.6% of the stations experienced 3001 extreme high temperature events. The stations with more than 20 extreme high temperature events all distributed in the Sichuan Basin, and the high temperature situation was more severe this year than the typical high temperature years since the 21st century. The nationwide high temperature process lasted from June 13 to August 30, a total of 79 days, and the strongest high temperature period was from August 11 to 24. According to the comprehensive ranking of high temperature station numbers, duration, intensity and impact range from strong to weak, the order is East China, Central China, southwestern China, northwestern China, North China and South China, among which, the extreme was the most in southwestern China, while there was no high temperature in northeastern China. The spatial and temporal distribution of drought is basically similar to that of high temperature, and the strongest drought period in China was in mid-August. In the summer of 2022, the 500 hPa over the middle and high latitudes of Eurasia showed a “two ridges and one trough” pattern, especially in July and August, the high pressures ridge near the Ural Mountains and the Sea of Okhotsk formed periodic blocking high. The strong subtropical system blocked the active cold air between the two high pressures to the north of 50°N most of the time, resulting in the pattern of “flood in the north and drought in the south” in China. The Iran high at low latitude extended abnormally to the east, and the western Pacific subtropical high was slightly northward and abnormally extended to the west. The air flow in the control area of the high pressure zone formed by the long-term connection of the two high pressures diverged and sank, which continuously blocked the transport of water vapor to the middle latitude, and was not conducive to the precipitation in the Yangtze River basin. At the same time, the south Asia high at high-level was abnormally eastward and moved to the opposite direction of western Pacific subtropical high at mid-level, which superimposed over the range of 80°E-120°E in the middle and late August, resulting in a stable barotropic structure of the high pressure system controlling a wide range of China, and the center was located over Sichuan and Chongqing region, which made Sichuan-Chongqing region become the large value center of high temperature days and the extreme high temperature event times.

Based on the hail data from eleven ground meteorological observation stations in the eastern agricultural region of Qinghai Province from 1961 to 2020, the temporal and spatial distribution of hail days, hail diameter, duration and disaster risk characteristics were analyzed by using statistical methods. The main conclusions are as follows: (1) The hail days in the agricultural area of eastern Qinghai Province decreased with climate tendency rate of 11.6 d·(10 a)^-1 in the past 60 years, which passed the significance test of \alpha=0.05, and after 1995, the anomaly of the total number of hail days changed from the positive to the negative. Hualong was the area with the largest number of hail days, and the number of hail days in Jianzha was the least. (2) Hail occurred mainly from May to September each year with seasonal differences. The diurnal variation of hail was obvious, and the peak occurred at 16:00 BST in the afternoon. (3) The number of hail days was positively correlated with altitude of stations and the correlation coefficient between them was as high as 0.97.(4) In the past 60 years, the hail processes with hail diameter less than 6 mm and the duration less than 9 min accounted for 58.33% and 73.55% of the total number of hail processes, respectively. (5) Xunhua was a low-risk area for hail, Hualong, Huangzhong and Huangyuan were medium-risk areas for hail, and Ledu was a high-risk or extremely high-risk area for hail, which was basically consistent with historical hail disasters.

Based on the daily minimum temperature and precipitation data from 126 meteorological observation stations in Beijing-Tianjin-Hebei (BTH) region from 1961 to 2017, the spatial and temporal characteristics of occurrence frequency of cold wave, and dry and wet characteristics of regional cold wave were analyzed by using defined discriminant index of dry and wet. The results are as follows: (1) The averageannual frequency of cold wave in BTH was more in the northwest than in the southeast, and 86% of the stations showed a decreasing trend in annual frequency of cold wave. (2) The accumulative occurrence station times of cold wave from 1961 to 2017 showed a significant decreasing trend (P<0.001) with a climate tendency rate of -5.7 station times per year and it mutated in 1983. The accumulative occurrence station times of cold wave from 1961 to 1971 reached a peak, it decreased sharply since 1972. The average accumulative occurrence station times of cold wave from 2007 to 2017 was the lowest in history. (3) The inter-annual variation of occurrence frequency of regional cold wave from 1961 to 2017 showed a decreasing trend with a climate tendency rate of -0.282 per decade. The frequency of regional cold wave was most in winter of 1960s, it was most in autumn and spring of 1970s, it reached the second peak in winter and spring of 2000s, and it was least in three seasons during 2011-2017. The frequency of regional cold wave was most in autumn, followed by winter and it was least in spring; the cold wave was most active in October and November. (4) The dry process was most in regional cold wave processes in BTH. From 2011 to 2017, the dry and wet characteristics of regional cold wave process showed the polarization distribution of dry process and wet process.

Spring frost is one of the serious agricultural natural disasters in northern China, which has new characteristics under the background of climate change. Based on the daily minimum air temperature data at 19 weather stations of Ningxia from 1961 to 2016, the temporal and spatial change characteristics and periodicity of the minimum air temperature and frost days during the spring frost stage were analyzed by using Mann-Kendall test, Sen’s slope, linear regression and wavelet analysis methods. The results show that the average value of the minimum air temperature and frost days during the spring frost stage was 0.59 ℃ and 3.9 d in Ningxia, respectively, and the minimum air temperature increased significantly, while the frost days decreased distinctly from 1961 to 2016, their Sen’s slopes were 0.005 ℃·a-1 and -0.05 d·a-1, respectively. The frost days were remarkably negative correlated with the minimum temperature during the spring frost stage, and their spatial distribution was opposite. The minimum temperature was higher in central Ningxia and lower in southern Ningxia, while the frost days was more in the south and less in the central. The minimum temperature showed an significant increasing trend at 74% weather stations of Ningxia, while the frost days appeared an significant decreasing trend at 53% stations during the spring frost stage from 1961 to 2016, and the change trends of them at same station weren’t exactly opposite, but Sen’s slopes of them were smaller. The periodic characteristics of the minimum temperature were similar to frost days during the spring frost stage, and the periods with 20-25 a and around 10 a were obvious and stable from 1961 to 2016, but the oscillation was sharp before 1967, then that tended to be flat after 1967, which indicated that the frost days during the spring frost stage were possible to decrease slowly in Ningxia in the future.

Based on the hourly precipitation of 327 stations from May to September in the upper reaches of the Yellow River Basin during 2015-2018, some characteristic quantities such as precipitation frequency, etc. were defined, and the variation characteristics of precipitation in 7 basins in flood season in study area were analyzed. The results are as follows: (1) There was a good consistency between precipitation and precipitation days in flood season in the upper reaches of the Yellow River Basin, with the highest in 2018 and the least in 2015. As for the spatial distribution of precipitation, it was highest in the river basin above Longyang Gorge which located in the southeast of Qinghai Province, while it was least in the Liujia Gorge-Lanzhou Basin in central Gansu Province. (2) The precipitation days in various tributaries of the same basin was quite different, due to the different geographical location and drainage area, especially in the river basin above Longyang Gorge and the Taohe River Basin. (3) There were three forms of the daily variation characteristics of precipitation frequency in the seven basins: double peak single valley type, single peak single valley type, and gentle type. On the whole, the number of maximum hourly precipitation more than 20 mm, the average hourly precipitation more than 2 mm, and the total number of precipitation processes increased after nightfall every day, and the heavy rainfall usually occurred around 19:00. (4) In the past four years, the events of maximum precipitation and the maximum hourly precipitation all occurred in Daxia River Basin, and the events of longest lasting precipitation occurred in Huangshui River Basin.

The effects of drought stress at different levels on photosynthetic characteristics, dry matter production and yield of winter wheat at flowering stage were examined in a common garden with rainout shelter during 2016－2017 growing season taking the winter wheat variety ‘Qi Mai 2’ as the test material. The experiment was consisted of five moisture treatments including T1 (an appropriate moisture level) and T2, T3, T4, T5 (one-time replenishment in accordance with the annual precipitation decrease of 20%, 50%, 75% and 100% in the critical period of winter wheat) and one rain-fed contrast test. The results indicate that under drought stress, chlorophyll-a content, photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), plant height, leaf area and total dry weight of aerial parts of winter wheat showed a decreasing trend at flowering stage, and the more serious the drought stress, the greater the decrease. The content of chlorophyll-a, Pn, Gs and Tr, plant height, leaf area and the total dry weight above ground under T5 treatment were reduced by 33.6%, 67.4%, 90.9%, 84.6%, 43.9%, 19.1% and 33.3%, compared with those of T1 test. With intensification of drought stress, the leaf water use efficiency (WUE) of winter wheat at flowering stage first increased and then decreased, and reached the highest under light drought stress. Drought stress could promote the transfer of plant nutrients to leaves and sheaths, and reduce nutrient supply to stems and ears, which was detrimental to the increase of winter wheat yield. In addition, drought stress at flowering stage also caused decrease of filling rate and increase of sterile ear rate of winter wheat, which made the theoretical yield of winter wheat reduce.

The characteristics of precipitation and lightning activity of a strong hailstorm occurring in Gansu Province on 25 August 2012 were studied by using the data from precipitation radar (PR), microwave imager (TMI) and lightning imager sensor (LIS) boarded on the TRMM (tropical rainfall measuring mission) satellite. The results show that there were three dispersed meso-β scale convective systems by analyzing the horizontal structure of the hailstorm. Although the pixel points of convection precipitation were less than that of stratiform precipitation, the averaged rain rate of convective precipitation was 8.2 times larger than that of stratiform precipitation. The top of the hail cloud could reach up to 13 km above ground with strong echoes (>55 dBZ) below about 7.5 km. The cloud thickness with the precipitation rate greater than 45 mm·h-1 was 7 km. The vertical distribution of precipitation profiles under different rain rates showed inhomogeneous characteristic. The precipitation rates of 50 mm·h-1 and 10 mm·h-1  increased first at lower levels and then decreased with the increase of height，their initial reduced height was about 9 km. Analysis of lightning activity of the hail precipitation process showed that most lightning flashes occurred near the strong convective region with radar echoes higher than 40 dBZ and polarization corrected brightness temperature of 85 GHz channel was lower than 210 K.

When applying wavelet transformation method in the retrieval of the boundary layer height by using lidar backscatter signals， the different selection of wavelet generating function may get different results．Therefore，the idealized lidar signal profiles during the daytime and nighttime have been built to explore which wavelet generating function will get the best performance．In this study，Haar wavelet covariance transformation has been used for the profiles of lidar backscatter signal，and Morlet and Mexican Hat wavelet transformation have been utilized for the gradient profiles of lidar backscatter signal，respectively．The results showed that the Haar function and the Mexican Hat function should be used as wavelet generating functions:the Haar function is more accurate and the Mexican Hat function is more stable． Furthermore，the changed wavelet dilation of the wavelet generating function also has been researched in order to test the sensitivity of the three different wavelet transformation methods to the wavelet dilation． The result show that whatever the idealized profile or the profile in which disturbance was added，the larger wavelet dilation can get a more stable and accurate diurnal boundary layer height and nocturnal mixing layer height．