The Roles of Local Convergences in the Convection Initiation of a Record-Breaking Rainfall Event at the Coastal Pearl River Delta in South China

A record-breaking rainfall event occurred on the west coast of the Pearl River Delta (PRD) during the onset of the South China Sea Summer Monsoon in 2021. The present study quantitatively investigated the effects of local factors including land-sea contrast, coastal terrain, and nearby convection-induced cold pool on the convection initiation (CI) of the event. The two major local contributors to the CI of the event were the land-sea frictional contrast on the west coast and the low-level outflow of the cold pool produced by the rainfall system on the east coast of the PRD, which result in low-level moisture convergence and lifting on the west coast. The two factors contributed comparably to the moisture convergence. The terrain effect on the west coast of the PRD plays a minor role in modulating the location, time, and intensity of the generated convection cells. Furthermore, the Ʌ-shaped coastline of the Pearl River Estuary (PRE) acts to suppress convection at coastal PRD. On the west coast of the PRE, the north-south-oriented coastline causes a divergence of the low-level southwesterlies, which is not conducive to moisture convergence in the area. In addition, the southwesterlies weakened by the west coast of the PRD are also unfavorable to convection on the east coast. This is distinct from previous studies focused on other areas of coastal south China with relatively straight west-east-oriented coastlines, where the CI can be triggered by the moisture convergence induced by the joint effects of land-sea contrast and coastal terrain.