[A-AS04] Extreme Events and Mesoscale Weather: Observations and Modeling

[AAS04-P13] Anomalous Features of Extreme Meiyu 2020 Over the Yangtze-huai River Basin and Attribution to Large Scale Circulations

*Ruoyun Niu¹ (1.National Meteorological Center)

Extremely anomalous features of Meiyu in 2020 over the Yangtze-Huai River Basin (YHRB) and causes of the associated large scale circulation are investigated. The main conclusions are as follows: (1) The extremely anomalous features of the 2020 Meiyu season over the YHRB are prominent since at least 1980s. They are characterized by the fourth earliest onset, the third latest retreat, the longest duration, the maximum Meiyu rainfall, the strongest mean rainfall intensity, and the maximum number of stations/days of rainstorm and heavy rainstorm in the Meiyu season. (2) The extremely long Meiyu season is attributable to the combined effects of the extremely early onset and late retreat of the Meiyu. The extremely early onset of Meiyu is mainly due to the earlier-than-normal first stage northward seasonal shift and migration simultaneously of the key influence systems including the Northwest Pacific subtropical high (NWPSH) and the South Asian High (SAH) along with the East Asian summer monsoon, induced by weak cold air activities from late May to early mid-June. However, the extremely late retreat of Meiyu is because of later-than-normal second stage northward seasonal shift of the associated large scale circulations systems accompanied with strong cold air activities, and extremely weak and southward located ITCZ over the Northwest Pacific in July. (3) The extremely more than normal Meiyu rainfall is caused by the concurrence of the extreme long duration and strong rainfall intensity. The extremely strong Meiyu rainfall intensity is likely attributable to extreme anomalies of the strong water vapor convergence and vertical ascending motion over the YHRB, resulting from the compound effects of the extremely westward extended and enlarged NWPSH, the extremely eastward extended and expanded SAH, and the extremely strong water vapor transportation associated with the low-level southerly wind.