The Synoptic Impacts on the Convection Initiation of a Warm‐Sector Heavy Rainfall Event Over Coastal South China Prior to the Monsoon Onset: A Numerical Modeling Study

A series of convection‐permitting numerical experiments using the Weather Research and Forecasting Model with different model configurations are performed to investigate physical mechanisms governing convection initiation (CI) at the coast of South China (SC) during a warm‐sector heavy rainfall event occurred before the South China Sea (SCS) monsoon onset. The simulation results are more sensitive to initial state from the different reanalysis products than the microphysics schemes, horizontal resolutions, initialization time, and lateral boundary conditions with the variation examined. It is found that the speeds of easterly to southeasterly winds in the boundary layer (BL) over northern SCS determine the strength of horizontal convergence of the warm and moist airflows arriving at the coast. The mid‐tropospheric divergence over the west coastal areas of SC and its southwest upstream impact vertical motion of the mid‐to‐lower tropospheric southwesterly airflows when approaching the coast. Differences in such features of upstream airflows in the experiments lead to distinct temporal evolutions of BL dynamic lifting and temperature stratification in the mid‐to‐lower levels at the CI location, thus the level of free convection and convective available potential energy. The modeling evidence presented shows that the synoptic dynamic and thermodynamic characteristics of the upstream airflows in BL and mid‐to‐lower troposphere, with local modulation by the coastal mountains, play important roles in determining the time and location of CI, and the distribution and amount of rainfall. Studies on more cases are needed to gain more general and robust conclusions about physical mechanism governing the warm‐sector coastal CI. Key Points The synoptic air flows have significant impacts on the time of coastal convection initiation (CI) The speed of the marine boundary layer easterly airflows from northern South China Sea plays a key role in determining the CI time The mid‐troposphere divergence belt over the CI region and its southwest upstream plays a relatively secondary role