Influences of Lateral Boundary Forcings on the 2020 Extreme Meiyu in the Yangtze-Huaihe River Valley

Meiyu occurs in Yangtze-Huaihe River valley (YHRV) every summer. Its intensity, distribution, and intraseasonal variation are greatly influenced by atmospheric forcings from different directions, such as the monsoon southwesterlies from the south, the western Pacific subtropical high (WPSH) in the east, the atmospheric longwave activities in the north, the southwest vortex from the west, and so on. In this study, to explore the contributions of the atmospheric forcings from different directions to 2020 extreme Meiyu, Regional Climate Model version 4.6 (RegCM4.6) is employed. A series of sensitivity experiments are conducted with realistic or climatological averaged lateral boundary conditions. The results show that the monsoon westerlies from the south transport moisture and heat to YHRV, converge with the cold air brought by the frequent atmospheric longwave activities in the north, and result in this extreme Meiyu. The frequent cold air from the north can lift warm air, provide unstable conditions, and make the distribution of precipitation similar to the teleconnection pattern in East Asia. The stable WPSH in the east anchors the 2020 Meiyu rainband to the YHTV for a long time. In addition, the contributions of the atmospheric forcings from different directions to evolution of 2020 Meiyu change with time. Before 10 June, the WPSH in the east mainly contributes to the Meiyu evolution. From 11 June to 3 July, the combined effects of atmospheric forcings from the south and north are dominant. From 3 July to 10 July, the cold air from the north plays a major role. The typical rainy season of the East Asian summer monsoon, Meiyu, occurs in Yangtze-Huaihe River valley (YHRV) in China every summer. Its intensity, distribution and intraseasonal variation are greatly influenced by the monsoon southwesterlies from the south, the western Pacific subtropical high in the east, the atmospheric longwave activities in the north, the southwest vortex from the west and so on. In the summer of 2020, the record-breaking Meiyu rainfall, featuring an early onset, delayed retreat, abundant precipitation, and frequent heavy rainstorm processes, appeared in the YHRV. However, the contributions and roles of atmospheric forcings from different directions to 2020 Meiyu have not been well analyzed. In this study, Regional Climate Model version 4.6 (RegCM4.6) is employed to simulate the 2020 Meiyu, and a series of sensitivity experiments are conducted. The results show that atmospheric forcings from different directions play different roles in the occurrence and movement of the Meiyu rain belt in 2020. In addition, the contributions of atmospheric forcings to the 2020 Meiyu from different directions change with time. The results improve the understanding of the occurrence and maintenance mechanisms of Meiyu. The extreme Meiyu in 2020 occurs due to specific atmospheric forcings which are different from the climatological conditionsThe atmospheric forcings from different directions play different roles in the occurrence and evolution of the Meiyu rain belt in 2020The contributions of atmospheric forcings to the 2020 Meiyu from different directions change with time during the Meiyu period