The precipitation anomaly in the first half of July in southern China has an important impact on the double cropping rice production and social economy in China. Based on daily precipitation data from 2400 national meteorological stations in China, the daily reanalysis data with a resolution of 2.5°×2.5°and the six-hourly reanalysis data with a resolution of 1°×1°from National Centers for Environmental Prediction/National Center for Atmospheric Research(NCEP/NCAR)in July from 1981 to 2010 and 2019, the causes of precipitation anomaly in southern China in the first half of July in 2019 was analyzed by using ageostrophic wet -vector. The results are as follows: (1) The abnormal precipitation in the southern China occurred under the conditions that the south Asia high in the high-level was eastward and southward, the west Pacific subtropical high was significantly strong and westward and southward in the mid-level, and the geopotential height field in South China and the south of the Yangtze River was weak (cyclonic circulation anomaly) in the low-level in the first half of July in 2019. (2) The diagnosis of ageoserophic wet -vector showed that the upward movement corresponding to abnormal precipitation was mainly caused by the strong diabatic heating, and the diabatic heating was mainly induced by strong convergence of water vapor flux in the middle and low level. (3) The larger horizontal gradient of sea surface temperature (SST) in the equatorial western Pacific was favorable to strengthen the cross-equatorial flow, forming a zonal circulation similar to Walker cell over the north of the equator, and its descending branch was conducive to occurrence of anticyclonic circulation anomaly near the Bay of Bengal and the Philippines, and increasing the northward water vapor transport from the Bay of Bengal and the South China Sea. It was found that the change of SST horizontal gradient in the western equatorial Pacific would affect wind field and water vapor flux in the southern China by affecting the cross-equatorial flow, and then affect the precipitation in the first half of July in the southern China.
