Based on the tropical cyclone data in the North Indian Ocean released by the Joint Typhoon Warning Center (JTWC) and the synchronous reanalysis data with a resolution of 1°×1° from ERA-interim during 1979-2018, a study was conducted to investigate the monthly characteristics of different environmental factors affecting the tropical cyclone genesis over the North Indian Ocean. The results show that the sea surface temperature of the North Indian Ocean increased significantly, with moderate vertical wind shear (5- 10 m·s^-1) between 200 hPa and 850 hPa and sufficient water vapor supply in May, which led to the first peak of tropical cyclones of the whole year. From July to September, the relative humidity in the middle and lower troposphere was very plenty. However, due to the large vertical wind shear between 200 hPa and 850 hPa, it was difficult for disturbed convection to form a warm center structure, which was not conducive to the generation of tropical cyclones. From October to November, the average sea surface temperature of the North Indian Ocean was 27-29 ℃, accompanying with higher relative humidity of the middle atmosphere and more cyclonic circulation at 850 hPa. The vertical wind shear between 200 hPa and 850 hPa was reduced to 5-15 m·s^-1. Under these favorable environmental conditions, the generation of tropical cyclones over the North Indian Ocean reached the second peak of the whole year. Through quantitative study of different environmental factors affecting the monthly variability of tropical cyclones in two sea areas of the North Indian Ocean, it was found that although the number of tropical cyclones generated over the Arabian Sea and the Bay of Bengal assumed a bimodal monthly variation, the environmental factors presented different characteristics in the same month.

Based on NCEP/NCAR FNL reanalysis data, merged hourly gridded precipitation product from CMORPH-AWS data and retrieved TBB data from FY-2G satellite, the impact of ROANU storm over the Bay of Bengal on the torrential rainstorm in South China from 19 to 20 May 2016 was diagnosed and analyzed. The results show that the development and maintenance of upper trough on 500 hPa over South China provided important large scale environment background for the occurrence of heavy rainfall, and the coupling of high and low level jet stimulated the development of MCS (mesoscale convective system), which promoted the heavy rainfall. Accompanied with the northern subtropical westerly jet, the ROANU storm over the Bay of Bengal supplied important water vapor channel for the occurrence of heavy rainfall in South China. In addition, during the moving northeastward of ROANU, the positive potential vorticity advection propagating eastward was significant on isentropic surface of 315 K, and the divergence and pumping of upper jet resulted in cyclonic vortex in lower level of South China, which provided dynamic condition for the occurrence and development of rainstorm. So the peripheral cloud cluster of storm ROANU over the Bay of Bengal not only transported water vapor to the heavy precipitation area of South China, but also the warm and moist airflow further strengthened stratification instability and accumulated unstable energy.

Based on lightning density grid data of lightning imaging sensors (LIS) on TRMM satellite and precipitation data at six weather stations of Yunnan Province during 1998-2013, the spatial and temporal distribution characteristics of lightning activities over Yunnan Province and its relationship with precipitation were analyzed. The results are as follows: (1) The mean lightning density over Yunnan Province was 4.7 fl·km-2·a-1. The distribution of lightning density was closely related with terrain, and it gradually increased from northwest to southeast with ‘V’ zonal shape. The lightning over Xishuangbanna, southeastern Puer and the border between China and Vietnam of southern Yunnan was active, and the maximum was 30.6 fl·km-2·a-1. (2) The lightning activity had obvious seasonal and diurnal variation over Yunnan. The lightning activity in spring and summer was obviously more than that in autumn and winter, and it  mainly occurred over southern Yunnan in spring and eastern Yunnan in summer. The monthly distribution of lightning activity appeared typical single peak pattern over northern Yunnan, the peak occurred in July or August, and the monthly lightning density was well related with monthly precipitation. However, the monthly distribution of lightning activity showed double peaks pattern over western, southern and southeastern Yunnan, and the peaks were in April and July or August, respectively, but the correlation between lightning density and precipitation was poor. The lightning over most regions of Yunnan occurred from 16:00 LT to 20:00 LT, and it usually occurred in the nighttime over high density areas. (3) The influence of terrain and altitude on lightning activity was significant over Yunnan, and lightning activity was closely related with monsoon.