Abstract:

To reveal the characteristics and causes of abnormal precipitation in the Sichuan Basin in July of 2018, we analyzed the characteristics of precipitation anomalies and atmospheric circulation, and the effect of water vapour, low system, and cold/warm air masses on precipitation based on statistical method and synoptic diagnostic methods and using precipitation data at meteorological stations and National Centers for Environmental Prediction (NCEP)/National Centers for Atmospheric Research (NCAR) reanalysis data from 1961 to 2018.The results show that rainstorms occur frequently over the Sichuan Basin in July of 2018, which results in anomalously higher precipitation compared with the climate mean state.The abnormal precipitation is characterized by more precipitation days and higher precipitation amounts, especially concentrating in the western basin.Meanwhile, the maximum daily precipitation and continuous precipitation day numbers both increase markedly, as well as the amount and day numbers of over heavy precipitation and rainstorm.Compared with the mean state of atmospheric circulation, the South Asia High is stronger and warmer and moves eastward, and the Subtropical High moves westward and northward in July of 2018, which are beneficial for the divergence of upper-level air over the Tibetan Plateau and its eastern areas.This results in the maintaining of precipitation weather systems and the transport of abundant water vapor to the basin.More water vapor is concentrated in July of 2018 compared with the mean state, coming from the South China Sea and the West Pacific Ocean.Water vapor is transported along the southern side of the Subtropical High, which is closely related to the anomalous northward and westward movement of Subtropical High and tropical cyclones.The period of high precipitable water vapor and high water vapor flux corresponds well with the precipitation process.Low system occurs frequently during precipitation events in July of 2018, with low-level warm and moist flows and middle-level (weak) cold airs, which favors triggering precipitation in the Sichuan Basin.

Key words: Sichuan Basin, Circulation characteristics, Subtropical high pressure, Water vapor transport