Objective assessment and analysis of abnormal causes of regional rainstorm events in South China

Based on the daily precipitation data at 192 surface meteorological stations in South China from 1961 to 2019, the standard for regional rainstorm events in South China is defined, and all regional rainstorm events in South China are objectively and quantitatively assessed. The intensity series of rainstorm events in South China during a whole year, the first and the second flood seasons are determined. The climatic characteristics and changes of its frequency and comprehensive intensity index were analyzed. Based on NCEP/NCAR reanalysis data, the characteristics of atmospheric circulation in strong and weak years of rainstorm events in the first and second flood season in South China are studied by using correlation and composite analysis methods. The results show that there are 1196 regional rainstorms in South China in recent 59 years, with an average of 20.3 events per year. 82.4% of them occurred in the flood season from April to September. The number of annual regional rainstorm events in South China increased by 0.45/(10a)^-1. The annual intensity index of regional rainstorm in South China increased significantly at a rate of 1.148·(10 a)^-1, especially in the second flood season. In the first flood season in South China, the westerly jet in upper levels intensifies, East Asian Trough in the southern part of Japan strengthened, and the subtropical high strengthened westward. Additionally, the low level South China was located at the intersection of the North-South airflow, so the rainstorm intensity in South China increased in the first flood season. In the second flood season of South China, the westerly jet in upper levels intensifies, the geopotential height of East Asia in the middle and middle latitudes increases, and the cyclonic circulation in the South China Sea and the Western Pacific increases. The sea level pressure in the North Pacific decreases in the middle and low latitudes, which is conducive to the formation of typhoons in the South China Sea and the Western Pacific. The rainstorm intensity in South China increases in the second flood season.