A Numerical Study Of Ocean Warm Eddy Impacts On The Rapid Intensification Of Typhoon

Currently,tropical cyclones'(TCs')intensity forecasting are very difficult and important.Apparently,there are many eddies over the ocean,where most Western North Pacific(WNP)TCs' rapid intensification(RI)occur.Previous studies have focused mostly on the role of ocean eddies in the side of quantity analysis,and the role of ocean eddies during TCs' RI have been not clarified theoretically.Then to fill in the gap of ocean warm eddies' impact on TC's RI,this paper has carried out numerical study to explore the dynamical and thermodynamical mechanism of Megi's RI over the ocean eddy.It has been recognized that,an air-sea coupled simulation based on the well TC environment is the basis of TC's RI study.Because there are many ocean eddies in WNP,which are closely connected with sea surface height anomaly(SSHA)in multiple timescale characteristic,and the reproduced ocean eddies depend on the three-dimensional environment construction.So this study uses reanalysis system to evaluate the marine environment,and proposes the optimizing index of Argo data for marine environment,described as an result that,by assimilating Argo observations,the reanalysis quality is much improved: the root mean square(RMS)error of temperature and salinity can be further reduced by about 10% and the RMS error of current can be further reduced by 18%.Consistent with the unique feature of Argo observations,the temperature are largest improved in the upper level and the largest improvement of salinity happens in 500~1000m depth.The result shows that,it is important to assimilate a kind of data homogeneously distributed in temporal and spatial for reproduced marine environment and study more about sea eddies impact on TC's RI.Based on the evolution,by using an atmospheric model coupled to an ocean mixed-layer model in WRF,the study focuses on Megi's high-resolution numerical simulation.Comparied with the observed data,numerical results well reproduced the evolution of Megi's track,intensity and structure.During 96 hours,the reproduced track error is about 55 km,the intensity error is 6.9 hPa,and Tc's structure is reasonable,indicates that this experiment design is effective.In addition,numerical experiments with different oceanic thermal structures are designed to quantitatively investigate the response to the ocean warm eddy.Results show that intensification enhances as Megi encounters the ocean warm eddy with the strengthen rate increasing by 17.6%.Then the study further explore the mechanism through which ocean warm eddy affects the Megi's RI,although little literature focus on the mechanism study.An analysis between sensitivity simulations(with ocean eddy and without ocean eddy)is conducted,to find out the dynamical and thermodynamical process during Megi is over the ocean warm eddy.Thus,we compare the two simulations based on temperature budget equation and tangential wind tendency equation,and recognize the results as follows: First,the potential temperature increases when there is high transfer of heat fluxes from warmer underlying ocean,especially the latent heat,because of the more vertical convection and the sensible heat fluxes due to the ocean warm eddy.Second,warm eddy makes more inward transport of absolute angular momentum by horizontal advection and positive contributions by vertical convection.Third,the changes in inner-core size between the two experiments are found to be primarily caused by the vertical convection.So the processes occur on the convective scale play a vital role during Megi's RI,which is necessary to be studied.Due to the complex nature of the physics in TC's convective scale process,present studies most focus on the convection motion evolution.Here we analysis TC's precipitation involved vertical motion and sensible heat,to identify ocean eddies' positive impact on convection.Also the study discusses the structure and evolution of precipitating areas,and the statistical properties of convective precipitation and stratiform precipitation within a radius of 200 km during RI,and indicates that ocean warm eddy increases the convective precipitation points by 3%~14%.On the one hand more convective precipitation is tied to more convective burst,on the other it increases the mid-tropospheric and lower-tropospheric PV.So,there are more convective burst and stronger upper-level warming core in the warm eddy test.Rather,the extent strong updrafts in convective precipitatin area cause the increase inertial stability,and the more favorable placement of convective heating during RI.In conclusion,the ocean warm eddy's impact on convective-scale structure of Megi's RI is achieved here through classification analysis nethod.