How Does Asymmetric Heating near the Upstream Peninsula Modulate Extreme Rainfall on the South China Coast?

Southerly flow within marine atmospheric boundary layer (MABL) plays an important role in the production of extreme rainfall (ER) on the South China coast (SCC). This study proposes a new perspective in that the Indochina Peninsula, which is upstream of the northern South China Sea (SCS) in the monsoon airflow, augments the southerlies in the 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 to 2017. The ER days feature strong diabatic heating and downdraft-related adiabatic heating over the Indochina Peninsula, enhancing the southwestward low-level temperature gradient over the northern SCS, which is key to strengthening the southerlies within the MABL. Strong MABL southerly winds produce pronounced water vapor convergence over the SCC. Moreover, the diurnal variation in heating near the peninsula has an impact on the diurnal variation in the MABL wind velocities over the northern SCS, which is closely related to the diurnal ER cycle on the SCC. In numerical simulations of a typical heavy rainfall episode, ER on the SCC is significantly weakened by eliminating diabatic heating or terrain of the Indochina Peninsula in the sensitivity experiments. Weakened heating over the peninsula leads to weaker MABL southerlies and horizontal convergence near SCC, thus substantially inhibiting coastal convection.