Synoptic Control on the Initiation and Rainfall Characteristics of Warm-Season MCSs Over the South China Coast

The South China coast (SCC) experiences frequent heavy rainfall during the warm season (May–September). Objective classification analysis on 925-hPa geopotential height shows that the majority of warm-season precipitation (>80%) occurs under three typical synoptic patterns: the southerly monsoon pattern (P1), the southwesterly monsoon pattern (P2), and the low-level vortex pattern (P3). Using 20 years of satellite observations and cloud tracking, this study highlights that mesoscale convective systems (MCSs) play a pivotal role in generating precipitation under all three synoptic patterns, while the initiation and rainfall characteristics of MCSs are strongly modulated by the background synoptic circulations. The diurnal MCS precipitation under P1 and P2 is predominantly influenced by land-sea breeze circulation, which is characterized by a morning offshore propagation and an afternoon onshore propagation. The rainfall propagation speeds, however, are strongly modulated by the prevailing low-level monsoonal flows. Under P3, MCS precipitation initiates near the coast around midnight and then propagates offshore, merging with the widespread offshore precipitation. The analysis also shows that the majority of MCSs contributing to SCC warm-season precipitation were locally initiated. This underscores the critical role of locally initiated MCSs in driving the SCC precipitation characteristics, as opposed to propagating MCSs. Statistical correlation analysis further indicates that the precipitation area and intensity of locally initiated MCSs are closely related to the lower-tropospheric moisture transport and the convective available potential energy over the upstream South China Sea, and the deep-layer wind shear (from surface to 400-hPa) over the SCC.