The Evolution Of Rainfall Distribution And Environmental Impact On Landfalling Tropical Cyclones Over China

Tropical cyclone(TC)is one of the complex and intense weather systems in the world.It can bring severe disasters along the coast due to its associated strong wind and heavy rainfall.The Western North Pacific is the most active ocean for TC's genesis and development in the world,thus China is one of the countries that severely suffered from the TC disasters.In our study,Tropical Rainfall Measuring Mission(TRMM)datasets in 10 years from 2005 to 2014,are adopted to explore the evolution of the characteristics of the temporal and spatial rainfall dist:ribution and the vertical structures for TCs that make landfall over China.In addition,from the perspective of synoptic environmental impact,this study analyzes the mechanism that TCs can still produce heavy rainfall and important role of synoptic environment,when the landfalling TC is surrounded in the dry environment or its intensity is not so strong.Analyzed by the TRMM observation data,it is found that the rainfall in the outer core is significantly influenced by the environmental field,especially for the TCs with weak intensity.Although TC intensity decreases after landfalling,the rainfall intensity for weaker TCs has no obvious decreasing tendency,having the similar rainfall intensity as stronger TCs.Under the impact of synoptic environmental pattern,the convective rainfall intensifies in the outer core region during TC landfall,even may produce deep convection,and has the largest percentage of convective rainfall coverage and convective rainfall contribution there.Through the synoptic analysis,it is found that TC circulation can interact with the surrounding environmental field,affected by the dry and warm air from the Subtropical High and dry air from continent when making landfall over China.The dry air is involved into TC circulation and gradually cuts off the midlevel moisture transport from the southern part of TC,leading to the shrink of the rainfall coverage.Because the dry air mainly influences the rainfall distribution in the outer core,the inner core region(eywall and inner rainband)still can produce heavy rainfall If multi-weather systems present the pattern that the low pressure system is located within two high pressure systems,the influence of ambient dry air would be most significant.On one hand,TC is wrapped into the dry environment further,on the other hand,the rainfall distribution becomes more asymmetric for landfalling TC with increasing environmental vertical wind shear(VWS)pointing to the southeast.With the enhancement of VWS,the updraft caused by friction convergence by TC movement is offset by the subsidence anomaly caused by the increasing VWS in the upshear side.Thus,the heavy rainfall concentrates on the downshear side with asymmetric rainfall distribution.The thermal process can also influence the rainfall distribution after TC landfall.In a favorable environment,one landfalling TC with weak intensity still can produce heavy rainfall,although its rainfall coverage is limited.This study shows that these TCs often make landfall over Guangdong and Hainan provinces under the influence of these environmental fields:The Subtropical High is strong with stablely massive structure.The center of South Asian High is northward,which causes the center of landfalling TC is located in the right side of the entrance of upper easterly jet.Through the strong upper divergence and pumping effect,the updraft motion in mid-high level enhances with deep convection,making weak TCs produce heavy rainfall.Meanwhile,the upper easterly jet strengthens the VWS.Therefore,TC vertical structure would tilt to the downshear side and the secondary circulation induced by the VWS would enhance the updraft motion further,making the heavy rainfall concentrating on the limited region in downshear side eventually.