Influence of Monsoonal Wind Speed and Moisture Content on Intensity and Diurnal Variations of the Mei-yu Season Coastal Rainfall over South China

Convection-permitting numerical experiments using the Weather Research and Forecasting (WRF) model are performed to explore the influence of monsoonal onshore wind speed and moisture content on the intensity and diurnal variations of coastal rainfall over South China during the Mei-yu seasons. The focus of the analyses is on a pair of 10-day WRF simulations with diurnally varying cyclic lateral boundary conditions averaged over the high versus low onshore wind speed days of the 2007-09 Mei-yu seasons. Despite differences in the rainfall intensity, the spatial distributions and diurnal variations of rainfall in both simulations verified qualitatively well against the mean estimates derived from ground-based radar observations, averaged respectively over either the high-wind or low-wind days.

Sensitivity experiments show that the pattern of coastal rainfall spatial distribution is mostly controlled by the ambient onshore wind speed. During the high-wind days, strong coastal rainfall is concentrated along the coastline and reaches its maximum in the early morning. The coastal lifting induced by the differential surface friction and small hills is the primary cause for the strong coastal rainfall, while land breeze enhances coastal lifting and precipitation from evening to early morning. In the low-wind days, on the other hand, coastal rainfall is mainly induced by the land-sea breeze fronts, which has apparent diurnal propagation perpendicular to the coastline. With stronger land-sea temperature contrast, the land-sea breeze is stronger during the low-wind days. Both in the high-wind and low-wind days, the coastal rainfall intensity is sensitive to the incoming moisture in the upstream oceanic airflow, especially to the moisture content in the boundary layer.