Diurnal circulations and their multi-scale interactions on rainfall over the South China Sea during monsoon westerly wind bursts

One of key precipitation processes in Asia and other monsoon regions is heavy rainfall concentrated on an upstream coastal sea during monsoon westerly wind bursts (WWB) of the intraseasonal oscillation (ISO). This study investigates the diurnal variation of this coastal rainfall over the South China Sea and explores the related atmospheric circulations via satellite observations and numerical modeling. The TRMM Multi-satellite Precipitation product has a morning rainfall peak over the coastal sea during 17-27 June 2004. The GOES-9 infrared (10.8-µm) brightness temperatures are a minimum in the morning and then spread toward the open sea until the afternoon. This brightness temperature evolution implies that the diurnal rainfall is produced by a life cycle of a mesoscale convective system composed of an early convective region along the coast and then later the stratiform area extends toward the open sea. These observed characteristics are reasonably well simulated in a control run of the PSU/NCAR-Mesoscale Model 5, and especially the morning rainfall peak initiated by a convective updraft. In the model, an enhanced low-level convergence occurs in the morning prior to the convective initiation.

A notable point is that this low-level convergence is produced by the interaction of two diurnal circulations--one on the local scale and a second on the large scale. The local diurnal circulation includes an offshore breeze owing to nighttime differential cooling between the Philippines and the surrounding sea, and this continues weakly until morning. At the same time, the large-scale WWB becomes intensified over the central South China Sea between Vietnam and Luzon in connection with a continental-scale diurnal circulation extending to the southern flank of the Tibet Plateau. In conclusion, the multi-scale interaction of the two diurnal circulations is suggested to modulate the diurnal peak of coastal rainfall in the South China Sea during monsoon westerly wind bursts.