Self-maintaining Mechanism of a Large-Scale Persistent Heavy Rainfall Event in Mei-yu Period: Case Study of Yangtze River Heavy Rainfall in 2020

Precipitation is frequent, severe, and persistent and affects a vast area during the Mei-yu period in the Yangtze River basin, often leading to large-scale flood disasters. Its internal dynamic process of persistence deserves further attention. In this study, the Weather Research and Forecasting model is used to undertake a case study on a large-scale persistent rainfall event in the Yangtze River basin from July 5 to July 9, 2020, with an emphasis on the interaction process between the latent heating and the circulation system during the rainfall event. The results show that due to latent heating in the intermediate layers, an anticyclone anomaly forms in the upper layers, supporting the eastward extension of the South Asia high (SAH). The eastward-extending SAH interacts with the western Pacific subtropical high (WPSH), resulting in WPSH’s moving westward, modulating the wind and water vapor fields at lower levels and creating a conducive background field for heavy rainfall. In the lower layers, a cyclone anomaly arises under the center of the condensation latent heating. This system helps to obstruct the WPSH from migrating northward, thereby resulting in a stable WPSH, which is beneficial to maintaining the rainfall system in the Yangtze River Basin. The East Asian summer monsoon’s evolution is characterized by apparent stagnation and a northward jump, which is dominated by WPSH activity. This study shows that releasing large-scale condensation latent heat can help in the formation of a more stable WPSH, which is beneficial to Mei-yu persistency and may provide a self-maintaining mechanism for the East Asian summer monsoon’s stalling during the Mei-yu period.