The Rainfall And Structural Changes Of Typhoon Mujigae (2015) During Its Landfall

The rainfall and flow structures of typhoons usually change significantly during landfall,which has been the focus of typhoon research and forecasting in recent years.In this study,the0.667 and 3-km resolution WRF3.9 model simulation results are used to analyze the storm-scale characteristics and possible causes leading to the rainfall and structural changes of Typhoon Mujigae during its landfall on 4 October 2015 in Guangdong Province.The main results can be summarized as follows:The accumulated rainfall after the landfall of Mujigae consists mainly of two parts,one part is the long-maintaining principal rainband rainfall,and the other part is the inner core rainfall that is secondary to the former.The maximum rainfall in the principal rainband differs in intensity and structure between 0-6h and 6-12h after the landfall,and the latter is closer to the coastalline than the former.The long-maintaining rainfall in the principal rainband benefits from a long-lived rainband.In the first stage,the principal rainband structure is maintained by a warm and moist flow around it and a low-level cold pool.The rainfall center is mainly located on the windward slope of the southern Yunwu Mountains（YWM）.Deep convection is generated over the ocean and moves northwestward.There are two rainfall enhancement events,occurring near the coastline and windward slope of the southern YWM,respectively.During 6-12h after landfall,due to the change of the warm and moist airflow direction,the rainband moves more northward.As it moves into the northeast-southwest trumpet-shaped terrain,its intensity and rainfall are suppressed due to the inconsistency between the southeast surface wind and trumpet-shape terrain,so the rainfall center takes place mainly near the coastline.Another part of rainfall after landfall is related to the eyewall in the inner core with more structural changes.Both axisymmetric and asymmetric structures of Mujigae after landfall have obvious differences from that over the ocean.The axisymmetric structure has weaker convection with lower cloud tops,weaker updraft,and weaker tangential wind forced by surface friction,as compared to the oceanic counterpart.The secondary circulation is also significantly weakened,but affected by surface friction,the inflow height in the boundary layer is significantly increased.At the same time,the dry and cold air over the land can destroy the lower layer TC thermal structure,which leads to the formation of an inversion in the lowest boundary layer.In contrast,the asymmetric structure,affected by the coastline convergence and environment vertical wind shear（VWS）,has stronger convection on the north side than that in the south,but with significantly lower height of the northern eyewall than that over the ocean,due to the influence of the continental dry and cold air intrusion.The dry and cold air intrusion plays an important role in maintaining the convection and rainfall in northern quadrant of eyewall.On the one hand,the dry and cold air is wrapped into the inner core,limiting deep convection in the south.On the other hand,it encounters warm and moist air from the ocean in the north,forming a warm front.Furthermore,the strong cyclonic flow and₀)gradient in the lower layer near the eyewall could help frontogenesis.During Mujigae’s landfall,the strong principal rainband could also have certain effects on the asymmetric structures of inner core.It is found that there are some different vertical mass transport characteristics in the upstream,midstream and downstream of the principal rainband.Among them,the net vertical mass flux is positive in the upstream and midstream,with the maximum located in the lower to middle troposphere,while in the downstream,the downward mass flux of the stratiform cloud in the lower layer can have a negative effect on the strength and structure of the inner core vortex with the continental dry and cold air wrapped into the inner core.In terms of the thermodynamic structure,the strong principal rainband could prevent the outer warm and moist airflow from entering the inner core radially so that the northern convection in the inner core could only rely on the tangential₀)advection from the same radius.In terms of dynamic structure,the existence of the principal rainband allows AAM transport to reach the rainband,instead of further moving into the southern eyewall of the inner core,which benefits the development of the principal rainband.Meanwhile,due to the lack of AAM supplies and the negative radial AAM,the collapse of the southern eyewall is accelerated.