A Numerical Investigation of the Formation Mechanism for the Extremely Heavy-Rain Event in Southern Fujian Induced by Westward-Moving Typhoon Soulik in 2013

The extremely heavy rain event induced by the westward-moving typhoon Soulik during 13-14 July 2013 is investigated through numerical simulation, diagnostic analysis, and sensitivity experiments using a mesoscale numerical model. The results show that the superposition of strong positive differential vorticity advection and strong low-level warm advection in the typhoon circulation is the large-scale environment favorable for the formation of heavy convective rainfall. After the typhoon landed, its spatial structure of wind, pseudo equivalent potential temperature (θ_se), high moisture, convective available potential energy (CAPE), convective instability and positive vorticity, and convergence area all varied with the typhoon rotation during the period of moving and rotating. The combined effect of high θ_se air mass, the inflow of instable air, the low-level convergence or wind speed convergence in typhoon circulation, the convective instability and local topographical forcing was the main reason for the formation of the extremely heavy rainfall in southern Fujian Province. The heavy rainfall was mainly located in the downstream and left side of the environmental vertical wind shear. In this case, the west-moving typhoon passed north of Taiwan, where the topography might affect the moving path and structure of the typhoon through changing the typhoon rainfall distribution and associated diabatic heating, eventually affected intensity and location of the heavy rainfall. The local topography in southern Fujian Province might exert an enhancing influence on the typhoon extremely heavy rainfall. The wind speed convergence induced by differences in friction between the land and sea surface played a critical role in the rainfall within the easterly flow to the north of the typhoon during its moving-close-to-land and landing period.