Analysis of circulation and impact of extreme weather processes is the basis of refined disaster prevention and mitigation services. Based on meteorological observation data, reanalysis data and satellite data, the characteristics, atmospheric circulation background and its main impacts of the extreme heatwave event in the Sichuan Basin from 25 July to 9 August 2021 are analyzed. During this heatwave event, temperatures of 13 national meteorological stations broke the historical maximum temperature records, and high temperature days reached 14 days at six stations. The heatwave center was located in the central and southern parts of the Sichuan Basin, and the process intensity reached its peak in early August, and the daily maximum temperature (42.4 ℃) appeared at Xingwen station in Yibin. The analysis shows that the atmospheric circulation background of this heatwave is different from that of most previous heatwave processes, the direct role of the western Pacific subtropical high (subtropical high) during this process is not obvious, and the typhoon activity in the southeast coast prevents the westward extension of the subtropical high. The peripheral flow of the subtropical high is conducive to the maintenance of the anticyclonic system over the basin, and makes it difficult for water vapor from the south to reach the basin, which plays an important role in the development and maintenance of high temperature and heatwave. During this heatwave process, the average high temperature days in Chengdu reached 8.36 days and the heat island effect is significant. The impact of heatwave and urban heat islands effect on mega-cities like Chengdu worths attention.

Based on the conventional meteorological observations, the ERA5 (0.25°×0.25°) reanalysis data and FY-4A satellite cloud top brightness temperature data, the snow storm in the western Tibet from October 18 to 19, 2021 was analyzed, and the contribution of the low vortexes in northern India to the heavy snowfall was further studied. The results show that the heavy snowfall occurred under the background of the South Branch trough moving eastward and the abnormal activity of the Indian vortex, the high-level jet in front of the South Branch trough and the east-west double vortexes in northern India provided favorable circulation background for the strong snowfall in the west of the plateau. During this snowfall process, from northern India to the south of the Himalayas, the southeast low-level jet burst, establishing a water vapor transport channel from the bay of Bengal to the west, so that the water vapor in the bay of Bengal can be transported to the west. The low vortex system that generated in northwest India, on the one hand, made the water vapor from the bay of Bengal gather in the east of the low vortex and blocked its continuous transport westward. On the other hand, it enhanced the forcing effect between the southerly wind in the east of the low vortex and the plateau topography, so a large amount of water vapor can be continuously transported from the lower troposphere along the steep terrain on the southern slope of the plateau to the plateau, which provided sufficient water for the strong snowfall. The invasion of high-level potential vorticity is the main reason for the formation and development of the low vortex system in northwest India. In general, the low-level vortex system in the lower troposphere in northern India played a key role in the heavy snowfall process. In snow forecast in the plateau areas, it is necessary to strengthen tracking and monitoring of low vortex system in the lower troposphere in the low latitude.

Based on topography and forecasted 3-hour wind fields, relative humidity fields initialed from 20:00 BST and 08:00 BST by using the SWCWARMS (southwest center WRF ADAS real-time modeling system), the precipitation correction equation was constructed by calculating the terrain precipitation estimates combined with precipitation fields forecasted by SWCWARMS. The daily precipitation, precipitation processes in Sichuan Basin and in western Sichuan Basin during flood season from June to August during 2018-2020 are corrected, and the precipitation in the steep terrain transition zone from the eastern slope of western Sichuan Plateau to the western Sichuan Basin was tested and evalcated only. The results are as follows: (1) The TS of the precipitation correction value with each magnitude was improved compared with TS of forecasted precipitation by the SWCWARMS. The correction effect of precipitation forecasted initialed from 20:00 BST was better than that initialed from 08:00, and the correction effect of the precipitation processes in western Sichuan Province was the best for heavy rain and above. Compared with the SWCWARMS, the relative improvement rates of TS of corrected value of precipitation with heavy rain, torrential rain and heavy downpour were 19%, 25% and 37%, respectively, the hit ratio was higher, the false alarm rate and miss rate were decreased significantly. (2) The correction equations of precipitation had a good correction effect on both torrential rain and general precipitation cases of precipitation processes in western Sichuan Province occurring in the steep terrain transition zone, even for cases of precipitation area predicted by the SWCWARMS was far from the actual situation.

Based on daily temperature, precipitation, wind speed and relative humidity data in Fanjing Mountain from 2011 to 2019, the main climatic factors affecting tourism activities were analyzed. The forecast models of tourism climate comfort index and tourism meteorological index were established according to the reality of tourism. The results are as follows: (1) Fanjing Mountain had the climate characteristics of “suitable temperature, abundant precipitation, rain and heat in the same period, high air humidity, average wind of breeze”. The average temperature, precipitation, wind speed and relative humidity in recent 9 years were 8.4 ℃, 2073.7 mm, 3.8 m·s-1 and 91%, respectively. (2) In recent 9 years, the average days with comfortable and above feeling were 206.9 d in Fanjing Mountain, in which the more comfortable period was from June to August, the comfortable period was from April to May and September to October, the less comfortable period was in March and November, the uncomfortable period was from December to February of the following year. (3) The forecast model of tourism meteorological index in Fanjing Mountain included four factors such as comfort, precipitation, cloud cover and severe weather, which took the comprehensive consideration of tourism safety, comfort and appreciation, and it was easy to calculate and reasonable. It was more suitable for the weather forecast and service of mountain tourism.

Based on conventional observation data, intensive observation data at automatic weather stations in Guizhou, Doppler radar data and the T-ln P profiles and 6-hour reanalysis data from NCEP with 1°×1° spatial resolution, the environmental condition, formation mechanism and structure feature of a squall line in northern and central Guizhou from 17 to 18 May 2018 were analyzed. The results are as follows: (1) There was no cold air on the ground of Guizhou during the influence of the squall line, and the transport of  strong warm and wet advection in lower layer played an important role in increasing of thermal instability. The squall line was forced by warm advection, the characteristics of guiding streamline and trailing stratiform cloud area were obvious, and the moving of the squall line was mainly dispersive transmission. (2) The uplift condensation height was higher, so the dynamic uplift of shear line on 850 hPa played a key factor in strong convection weather in central and northern Guizhou. The instability and energy conditions in the east and north were better than that in the central of Guizhou, which was beneficial to stronger convection and higher rainfall density. (3) The convergence line of ground wind disturbance provided triggering and uplift mechanism for the organizational development of convection, which was a good indicator for the early warming of severe convective weather. (4) The interaction of ground cold pool and environment wind in lower layer played an important role in developing and maintaining of the squall line. The squall line moved and developed forward 1-hour allobaric center and ground mesoscale convergence line.

 The SAL quantitative verification method was used to evaluate precipitation forecast  results of the Grapes-Meso model on the rainfall event occurring on 5 September 2018 in Sichuan Province firstly, in this process, the optimal precipitation threshold scheme was selected through combination of three threshold schemes to identify the precipitation body. On this basis, SAL verification was applied to precipitation forecast of three heavy rainfall events in 2018 and rainfall processes from July to September in 2018 in Sichuan Province, and the verification results were compared with test score (TS) to understand prediction effect of the Grapes-Meso model in flood season in Sichuan Province. The conclusions are as follows: (1) The selected threshold determining method could identify the precipitation body well and the “connected neighborhood method” provided a good support for automatic identification of precipitation individuals. (2) The value of L reflected prediction effect of the model on precipitation forecast to a certain extent. If the value of L and the absolute value of A were both small, the probability of better prediction effect was bigger. (3)The overall prediction effect of the Grapes-Meso model on rainfall processes in Sichuan Province showed that the forecasted rainfall intensity was stronger and rainfall range was larger than actual rainfall, or forecasted precipitation center was smaller than the actual, or both cases were existed at the same time.

Analysis of the climatological features of Jiangnan rainy season precipitation anomalies shows that the month with the most
frequent anomalies of precipitation in Jiangnan is J une and the region is located in the middle of Jiangnan. The SVD analysis points
out that the related sea surface temperature anomaly (SSTA) distributes in the pattern as a“seesaw”. The key region is just located
in the most evident correlation area. The key period of SSTA impact is from May to J uly in the previous year in which SSTA has the
best negative correlation with April to May precipitation in Jiangnan. Under the influence of SSTA“seesaw”pattern the precipitation
anomalies in the following April to J une in East China exhibit opposite phase on both sides of the Yangtze River , the most evident
correlative region is Jiangnan and the middle of which is located in the most obvious area of SSTA impacts.