Formation Mechanisms of the Extreme Rainfall and Mesoscale Convective Systems over South China during the Dragon Boat Rainy Season of 2022

Abstract

The formation mechanisms of the record-breaking rainfall event during the Dragon Boat Rainy Season (DBRS) of 2022 are comprehensively analyzed from the synoptic scale and the mesoscale perspectives. The extreme rainfall event is characterized by the highest rainfall amount since 1981, and an abnormal spatial distribution with much higher (lower) rainfall amount in the northern (southern) part of South China. The abnormal circulation and thermodynamic conditions are mainly responsible for the extreme rainfall. The favorite synoptic condition for rainfall is the combination of warm advection, frontal forcing, orographic lifting and low-level jet favor the convection development. The similar configurations repeatedly impact South China during the DBRS of 2022, causing multiple heavy rainfall events, leading to the extreme rainfall of the whole period. The abnormal moisture convergence together with the frontal zone, which is stronger than the climatology, results in the rainfall centers over the northern part of South China. 54.35% of the rainfall amount is related to mesoscale convective systems (MCSs) which mainly originate from four regions. The MCSs from the four regions are characterized by different formation peaks, spatial scales, lifetimes and propagations. The large-scale warm and moist air mass, the moistening caused by synoptic advection and the local diabatic heating are responsible for the increasing instability for the MCSs. The low-level jets play an important role in the formation of MCSs by providing moisture. The thermodynamic (dynamic) environmental conditions control the formation of MCSs in the afternoon (night).