Extreme local mesoscale convective systems over the South China coast during the warm season

The South China coast (SCC) is a climatological rainfall center in East Asia during the warm season. Using a 20-year cloud-tracking and classification dataset, this study shows that mesoscale convective systems (MCSs) contribute approx. 65% of the warm-season rainfall in the SCC. It is found that locally initiated MCSs predominantly contribute to the MCS rainfall, especially those initiated near the coastline of SCC. Notably, about 11.6% of these locally initiated MCSs are characterized by extended lifespans, extensive rainfall areas, and high rain rates, making them responsible for extreme rainfall events in the region. The environments conducive to the development of these local extreme MCSs feature a strong low-level geopotential height gradient over the South China Sea, which leads to strong low-level onshore winds. Due to the land-sea contrast and coastal terrain, the coastal convergence of the onshore winds further leads to a strong moisture convergence near the coastline, which provides a favorable environment for the initiation and development of extreme MCSs. Based on the spatial distribution of their lifetime total rainfall, the extreme MCSs are further classified into onshore, coastal, offshore, and other rainfall types. We found the spatial distribution type is mainly influenced by shifts in the moisture convergence center between onshore and offshore areas, which is modulated by the mid- and low-level synoptic circulations. Among these MCS types, the offshore rainfall type demonstrates a significantly higher lifetime total amount of rainfall compared to the other types, primarily due to its higher lifetime mean rain rate.