Roles of Terrain, Surface Roughness, and Cold Pool Outflows in an Extreme Rainfall Event Over the Coastal Region of South China

An extreme rainfall event with maximum 12 hr accumulated rainfall of 464.8 mm associated with a quasi-stationary mesoscale convective system occurred over the western coastal region of South China in June 2017. An observational analysis shows that early convective storms were initiated near the mountains and moved northeastwards. Moreover, a convergence line between cold northerly winds and warm southerly winds in the lower levels was formed, which favored the development of the quasi-stationary system. Cold northerly winds were associated with land breeze, downslope winds as well as previous rainfall during earlier period, and cold pool outflows during later period. Cloud-resolving simulations were performed to examine the roles of terrain, land-sea surface roughness contrast, and cold pool outflows in the formation of heavy rainfall. Results demonstrate that land-sea surface roughness contrast and mountains in the middle of Yangjiang and Jiangmen facilitated the formation of the convergence line during earlier period, and cold pool outflows sustained it during later period. Besides orographic lifting, coastal hills helped reduce the stability. Mountains in the middle of Yangjiang hindered the movement of the convective system, without which the associated heavy rainfall would shift farther north. Mt. Tianlu with westward concave morphology played a vital role in the formation of local convergence and the concentration of heavy rainfall. Without it, the total rainfall in the region of interest was reduced. This study suggests the importance of representing processes associated with the complex underlying surface in models for the prediction of heavy rainfall in coastal regions.