Initiation of Afternoon Rainfall Over North Central Hainan Island, China: Important Role of Boundary‐Layer Convergence Produced by Differential Downward Momentum Transport by Mixing in a Typical Case

Abstract During the summer monsoon season, with prevailing southwesterly winds, rainfall usually peaks in the afternoon on northern Hainan Island (HNI), China. On days of stronger prevailing winds, convection initiation (CI) often occurs in the north central HNI. Through simulations of a typical case of 20 June 2017 using a 2 km grid, we identify a primary CI mechanism over northern HNI that is driven by daytime boundary‐layer (BL) vertical mixing and the resulting low‐level convergence. Overnight and into the early morning, a wedge‐like wind structure forms in the BL along an east‐west vertical cross‐section over northern HNI, which is characterized by a low‐level southeasterly wind component, deeper in the east and shallower in the west, undercutting the northwesterly wind component aloft. By noon, surface‐heating‐induced BL vertical mixing transports the upper‐level northwesterly component to the surface on the west side, creating a zone of convergence with the low‐level southeasterly component from the east and triggering convection around noon. The wedge structure arises from the orographic effects of the mountains over southern HNI, when southerly flows bypassing the mountains from the west side gain more westerly component, while the flows climbing over the mountains descend on the leeside and cause isentropic surfaces to dip, lowering the layer possessing westerly component. Sensitivity experiments show that sea breezes are not responsible for initiating the convection in north central HNI, contrary to earlier suggestions, while BL horizontal convective rolls play only a secondary role in modulating the timing and locations of CI.