Synoptic Control on the Initiation and Upscale Growth of Warm-season MCSs over the South China Coast

The South China coast (SCC) experiences frequent heavy rainfall every warm season (May-September). Objective classification analysis 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, we found that mesoscale convective systems (MCSs) play a pivotal role in generating precipitation under all three synoptic patterns, accounting for 60-80% of the total rainfall. In contrast, non-deep convection contributes 10-20%, while non-MCS deep convection makes up less than 20% of the total precipitation. Our analysis also shows that the majority of the MCSs contributing to the precipitation over the SCC are initiated locally under all three synoptic patterns. This underscores the crucial role of locally initiated MCSs in driving warm-season precipitation over the SCC, as opposed to propagating MCSs. MCS precipitation under synoptic patterns P1 and P2 tends to originate along the coastline in the early morning (early afternoon) and then propagates offshore (onshore), likely influenced by the diurnal land (sea) breeze circulation. Conversely, under synoptic pattern P3, nocturnal MCS precipitation initiates near the coastline at midnight and then propagates offshore, merging with the widespread offshore precipitation. Compared to the other two synoptic patterns, the onshore precipitation during the afternoon under P3 is significantly weaker and its onshore propagation distance is shorter. Statistical correlation analysis further shows that the pre-MCS deep-layer wind shear (from surface to 500 hPa) over the SCC, the upstream convective available potential energy, and the lower-tropospheric moisture transport from open ocean play important roles in modulating the maximum precipitation area, maximum precipitation intensity, and maximum hourly precipitation of MCSs under the three synoptic patterns. The potential physical mechanisms will also be discussed in the talk.