Based on precipitation data from 7213 weather stations in Shaanxi province and its surrounding areas, the spatial distributions of precipitation from September 1 to 18, 2011 in the West China were analyzed. The results indicate that accumulated precipitation during the period is twice to threefold as much as that during the same period of history. Precipitation is characterized with long duration and serious disaster. This precipitation event can be divided into three processes, i.e. the first process is from September 4 to 6, the second on September 11 and the third from September16 to 18. According to 1°×1° NCEP reanalysis data, atmospheric circulation characteristics of these three processes suggest that major water vapor transport of lower level in rainstorm region has two branches. One is the southern air flow from eastern Sichuan province to north, and the other is the eastern air flow from southern North China plain to west. Corresponding to these three precipitation processes, three westerly troughs influence the circulation in 500 hPa, and 700 hPa southerly and 850 hPa, and northerly winds appear in three obvious control periods over West China. Unstable stratification is formed because southerly airflow in 700 hPa transports a large number of warm-moist airflow to West China in advance. When dry-cold air transported by northern wind enters into the bottom of warm-moist air mass and lifts it, dynamic lift action in the front of the superadded trough and wind field convergence uplift caused by the terrain occur, which makes the convective activity strengthened and potential unstable energy released, so appears heavy rain. Continued unstable stratification and dynamic lifting caused by stable circulation situation (such as blocking high) and specific terrain condition, which benefits to release unstable energy, are the essential reason forming steady and continuous rainstorm in West China.
