Activity characteristics of East Asian monsoon depression during a heavy rainfall in Beihai, Guangxi

To further understand the occurrence, development and movement of East Asian monsoon depression (EAMD) and the causes of extreme precipitation events in South China, by using ERA5 reanalysis data and the tracking software named TempestExtremes, the activities of an EAMD that caused the heavy rainstorm in Beihai, Guangxi on June 8, 2023 were analyzed. From the moving trace of the EAMD, it could be seen that during June 7 to 9, the EAMD mainly moved near Beihai, and the large amount of water vapor brought by it had an important impact on the formation of the heavy rainstorm. The structural differences in different development stages of the EAMD show that from the initial stage to the strong moment, the EAMD had an asymmetrical vertical structure, with the inclination of the tilted direction changing from southeast-northwest to northeast-southwest as the height increased. The weakening of the EAMD after its peak may be related to the "filling" effect of cold air entering the EAMD from the north. When the EAMD is about to die out, the high value areas of the deviation from zonal mean in the upper atmosphere have obviously deviated from the center of EAMD, and there are more sinking motions in the deviation from zonal mean of the vertical circulation. It is worth noting that an obvious “cold core” always exists in the middle and lower atmosphere of the EAMD, which is one of the possible reasons for preventing the further development of EAMD. The results also show that the outer flow of the Western Pacific Subtropical high plays a leading role in the moving path of the EAMD, but the indirect influence of Typhoon “Guchol” cannot be ignored. In addition, after the EAMD landed, the northerly cold air hindered the movement of the EAMD to a certain extent.