The study of the impacts of climate change and human activities on the vegetation of the Gansu section of the Yellow River main stream is of significance for the construction of ecological security in the Yellow River Basin. Based on the NDVI (Normalized Difference Vegetation Index) of MODIS (Moderate-resolution Imaging Spectroradiometer) and the precipitation and air temperature data of 18 ground meteorological observation stations in the Gansu section of the Yellow River main stream, the spatial and temporal characteristics of NDVI and the contribution of climate change and human activities to the changes of NDVI in the Gansu section of the Yellow River main stream from 2001 to 2020 were quantitatively analyzed by using the methods of linear trend analysis, bias correlation analysis and residual analysis. The results show that the growth rate of NDVI in the Gansu section of the Yellow River main stream was 0.05·(10 a)^-1 from 2001 to 2020, of which the 2001-2010 period was a slow increase stage with a growth rate of 0.04·(10 a)^-1, and the 2011-2020 period was a rapid increase stage with a growth rate of 0.08·(10 a)^-1. The vegetation ecology of the Gansu section of the Yellow River main stream showed a benign development in the past 20 years, and the vegetation improvement area was located in the north-central part of Linxia Hui Autonomous Prefecture, Lanzhou City and the southeastern part of Baiyin City. The climatic factors that dominate the changes of NDVI in the study area are different, the positive correlation between NDVI and air temperature is higher in most parts of Gannan Prefecture, while the correlation between NDVI and precipitation is more significant in the northern part of Linxia Prefecture, Lanzhou City and Baiyin City. The vegetation change in the Gansu section of the Yellow River Basin is the result of the joint action of climate factors and human activities, and the contribution of climate factors to the NDVI change from 2001 to 2020 is 75.27%, while the contribution of human activities is 24.73%. Climate factors are still the dominant factors for the vegetation change in the Gansu section of the Yellow River Basin, but the influence of human activities on the vegetation change is gradually deepening.

In order to study the variation trend of climatic characteristics of consecutive dry days (CDDS) in arid and semi-arid regions of China the variation characteristics of CDDS and their differences in the period of 1961-1990 and 1991-2020 in the study area were analyzed by using the daily precipitation data from 74 meteorological stations in arid and semi-arid regions of China from 1961 to 2022. The CDDS equal to 16 days and above were focused on, and the CDDS of 16 to 25 days, 26 to 40 days, 41 to 60 days and above 60 days were defined as partial drought, moderate drought, severe drought and extreme drought, respectively. The results show that there are significant differences in the occurrence times and days of CDDS for 16 days and above in arid and semi-arid climates in China. Especially, the occurrence times and days of CDDS corresponding to severe drought and extreme drought in arid regions are about twice times and six times of those in semi-arid regions, respectively. Compared with the period of 1961-1990, during 1991-2020, the frequency of meteorological drought with different grades in the western part of the study area decreased significantly, while it increased in the central and eastern parts of the study area. In the western part of the study area, the number of drought days with different grades also decreased obviously, the number of partial and moderate drought days decreased, and the number of severe and extreme drought days increased. The significance test of difference show that there was no mutation in the frequency and days of drought with different grades at most meteorological stations in the arid and semi-arid regions.

Sandstorm is a serious natural disaster in north China. It is of great significance to carry out relevant research to improve the forecast level of this kind of catastrophic weather. Based on the RegCM-dust model, an extended period numerical prediction analysis of a typical severe sandstorm process in north China is conducted, and the results are compared with NCEP reanalysis data and other analysis results. The results show that the regions with high sediment discharge simulated by the model are mainly located in southern Xinjiang, Mongolia and western Inner Mongolia. The model has a certain forecasting ability for 10 m wind speed, but the simulated wind speed is smaller than the reanalysis data. The changes of dust column content and total sedimentation simulated by the model can reflect the characteristics of the dust storm weather process. The simulated sand-dust mixing ratio has a certain correspondence with the urban pollution index, which indicates that the model has certain forecasting ability for the pollution weather caused by sand-dust.

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 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.