Influences of lateral boundary forcings on the 2020 extreme Meiyu in the Yangtze-Huaihe River valley

Abstract In this study, a Regional Climate Model (RegCM4.6) is employed to simulate the extreme Meiyu in the Yangtze-Huaihe River valley (YHRV) from June to July 2020. The influences of lateral boundary forcings in different directions on the 2020 Meiyu are explored through a series of sensitivity experiments with realistic or climatological lateral boundary conditions in the south, north and east directions, respectively. The results show that the 2020 Meiyu occurs under unique atmospheric forcings, and if all lateral boundary conditions are replaced by climatological mean state, heavy rainfall will not occur. Specifically, the south atmospheric signals provide moisture and heat for the occurrence of Meiyu. The atmospheric longwave activities in the north bring cold air to YHRV which can lift the warm air, provide instability conditions, and make the distribution of Meiyu similar to the teleconnection pattern of summer precipitation in East Asia. The western Pacific subtropical high (WPSH) in the east is important in anchoring the rainband locations in YHRV. In addition, the contributions of anomalous atmospheric signals in different directions change with time. Before June 10, the WPSH mainly contributes to the Meiyu evolution. From June 11 to July 3, the combined effects of forcings in the south and north are dominant. From July 3 to July 10, the cold air from the north plays a major role. After July 10, the 2020 atmospheric circulation conditions and Meiyu evolution are similar to the climatological mean state.