How does asymmetric heating near the upstream peninsula modulate extreme rainfall on the South China coast?

The southerlies in marine atmospheric boundary layer (MABL) play an important role in the production of extreme rainfall (ER) on the South China coast (SCC). This study proposes a new perspective that the Indochina Peninsula, which is upstream the northern South China Sea (SCS) in the monsoon airflow, augments the southerlies in MABL through thermodynamic forcing and thus enhances ER on the SCC. Using the 99th percentile hourly rainfall as the threshold, this study defines 337 coastal ER days in 30 early summers from 1988–2017. The ER days feature strong diabatic heating and downdraft-related adiabatic heating over Indochina Peninsula, enhancing the southwestward low-level temperature gradient over the northern SCS, which is the key cause of the strengthening of southerlies in MABL. The strong MABL southerlies produce a pronounced water vapor convergence over the SCC. Moreover, the diurnal variation in heating near the peninsula has an impact on the diurnal variation in MABL wind velocities over the northern SCS, which is closely related to the diurnal ER cycle on the SCC. In the numerical simulations of a typical heavy rainfall episode, ER on the SCC is significantly weakened by modifying the underlying surface or terrains of Indochina Peninsula in the sensitivity experiments. The weakening of heating over peninsula leads to weaker MABL southerlies and horizontal convergence near SCC, thus the coastal convection is substantially inhibited.