A Numerical Study Of Tropical Cyclone Rapid Intensification In South China Sea

Forecast skill of tropical cyclone(TC)intensity has been improved slowly in recent years which become an urgent issue in TC research area.Rapid intensification(RI)is one of the biggest challenges in TC forecast.TCs are prone to intensify rapidly in South China Sea(SCS),and influence mainland of China soon which brings huge pressure for disaster prevention and mitigation.Therefore,it is of great importance for RI research in SCS.Moreover,in SCS,RI progress is governed by complicated interactions between environment systems and TC inner structure which contains fruitful knowledge of science.So studying RI in SCS will help to improve our understanding on TC intensification.In this study,Super-typhoon Rammasun(2014)is simulated with high-resolution numerical model to investigate RI trigger mechanism,inner-core structure evolution during RI phase and multi-scale interactions between the environment flow and TC.WRF-ARW and Dynamic Initialization Scheme are used in this 72h-simulation with the finest horizontal resolution of 2km.The simulation begins when Rammasun enters SCS and ends as the TC striking southern part of China.The simulated track and intensity are highly consistent with besttrack records of Joint Typhoon Warning Center(JTWC)with largest track error less than 90 km and intensity error less than 8m/s.Besides,RI progress can be well simulated.Model verifications are conducted using conventional observations and remote sensing data.The result shows that environment systems around TC including tropical monsoon clusters,subtropical high,Meiyu front and Tropical Depression(Matmo,2014)are well simulated as well as TC structure.RI trigger factors are analyzed based on numerical simulation result.Environment flow keeps steady during RI which indicates TC inner-core evolutions should be responsible for RI onset.Key evolutions before RI include TC axisymmetricalization,shrink of radius of maximum wind(RMW),accumulation of Convective Available Potential Energy(CAPE)and vortex alignment.These structure evolutions are in favor of deep convections within TC inner-core as well as RI onset.During RI phase,double warm core(DWC)structure is found with a middle warm core(MWC)between 3 to 9km altitudes and an upper warm core(UWC)between 13 to 17 km altitudes.The MWC emerges at the beginning of RI and grows with TC intensification.The UWC is weak than MWC and generates at the later stage of RI.Using potential temperature budget equation,DWC is proved to be formed by subsidence within TC eye.In particular,high entropy air originates in stratosphere is found at upper layer of TC eye during the development of UWC.The formation of UWC is supposed to be related with convective bursts(CBs).The number of CBs within RMW at 8km altitude increases rapidly at early stage of UWC development.CBs can break through troposphere and the overflow overturns to become compensating subsidence close to inner side of eyewall which lead to upper level warming.Meanwhile,CBs can transport angular momentum upward to increase inertial stability in upper level which is in favor of increasing heat efficiency.However,upper level warming and subsidence lead to an increase of static stability which is negative for CBs activity and UWC keeps growing as CBs decrease which indicates other mechanisms may contribute to UWC formation.Using trace tracking,we find some air within TC eye at upper level comes from stratosphere outside TC circulation.This high entropy air comes with the shallow inflow at low level of stratosphere and sink within TC eye to form UWC.Through diagnostic analysis by Sawyer–Eliassen(S-E)equation,the shallow upper inflow is induced by momentum forcing in outflow.Using diagnostic equation of surface pressure perturbation,we find MWC and UWC are dominant at early and later stage of RI respectively and share equivalent contributions to surface pressure fall.From this point of view,DWC is of great importance for TC intensification.Summer monsoon of SCS has significant influence on RI.Moisture advection by monsoon is the main water vapor source of Rammason.Sensitive experiments demonstrate that changing environment humidity will have strong impact on RI process.Besides,strong environment vertical wind shear(VWS)is found and the average of VWS can exceed 12m/s during RI.Strong VWS can induce remarkable asymmetrical structure of TC.Rainbands are confined in downshear left(DL)and vortex keeps tilting.Precession is the main VWS-defense mechanism within TC inner-core.During RI phase,the upper level vortex rotates clockwise at the beginning.When upper level vortex rotates to upshear,the horizontal distance between upper and low level vortex centers reaches minimum and TC gets intensification simultaneously.As upper level vortex rotates to downshear left,VWS within TC inner-core decreases rapidly and reaches minimum.At this time,CBs increase rapidly and UWC generates.This means interactions between environment VWS and TC inner-core is the key process for TC resisting VWS.The ability of TC self-protection from VWS is related with TC intensity.When TC is weak,the inertial stability is too low to resist ventilation from VWS,and intensification is depressed.Significant diurnal variations of Environment VWS and rainbands are employed.VWS reaches peak at morning and drops bottom before midnight.Rainbands develops after midnight,be active at morning and dissipates at afternoon.Radiation process is the main cause of quasi periodic variations of VWS and rainbands.Sensitive experiments are conducted and show that changing phase of radiation will lead to significant variations of VWS and rainbands activities as well as deep convections.RI process is sensitive to diurnal variation of radiation and TC is prone to intensify at night.At last,balanced dynamics during TC intensification is investigated using S-E diagnostic equation.Latent heat within RMW is proved to be crucial for TC spin-up.Therefore deep convections within RMW are important for RI.Meanwhile,as a part of diabatic heating,radiation cooling at cloud top cannot induce the shallow upper inflow at low layer of stratosphere and eddy advection of vorticity within outflow is the key forcing process.Combined with tangential wind budget analysis,it is confirmed that friction within boundary layer tends to accelerate tangential wind inside TC eye but decelerate it outside.Thus the net forcing of friction is to weaken TC.