Analysis Of Concentric Eyewalls Structure Evolution Characteristics In Tropical Cyclone "Lekima"

The intensity change of Tropical Cyclone(TC)is an international research hotspot,the evolution of TC structure has an important influence on the TC intensity change.80% of the strong TCs in the Western Pacific have the concentric eyewalls structure in their life.Compared with the TC with only one eyewall,the dynamic and thermodynamic processes controlling the intensity change of concentric eyewalls are much more complex.Strengthening the research on this type of TC is conducive to improving the understanding of the structure and intensity change of TC.Recently,the process of eyewall replacement cycle of the concentric eyewalls is well understood through observational analysis and ideal numerical experiments,but the mechanism of concentric eyewalls maintenance remains unclear.Therefore,it is necessary to use a variety of observation data,numerical model and data assimilation method to analyze the real TC in order to further explore the mechanism of long-lived concentric eyewalls structure.In this paper,the characteristics of the structure evolution in "Lekima" are analyzed by using satellite and radar data,the process of the formation and extinction of the concentric eyewalls structure in "Lekima" is understood.The analysis found that "Lekima" belongs to CEM(Concentric Eyewalls Maintenance)type,and the duration of the inner and outer eyewalls is up to 35 hours.The scale of the outer eyewall is larger when it is initially established,and there is an obvious weakening and asymmetric process in the TC convection of the outer eyewall in the early stage of concentric eyewalls structure evolution.In addition,WRF model and ensemble Kalman filter assimilation method is used to carry out high-resolution numerical simulation experiments on "Lekima".The result shows that the structure of the concentric eyewalls of "Lekima" is significantly improved after the assimilation of radar radial velocity data.Based on the results of the assimilation analysis field,the environmental conditions and three-dimensional structure characteristics of TC are analyzed,it found that "Lekima" is in the environment of high sea surface temperature,high relative humidity,and strong vertical wind shear in the early stage of concentric eyewalls structure evolution,the TC vortex have large inertial stability in the middle and low levels,as a result,the intensity of the tangential wind at the outer eyewall is maintained during this period.The secondary transverse circulation diagnosed by Sawyer-Eliassen equation shows that the interference between the convection/subsidence couplet of the inner and outer eyewalls transverse circulation is not obvious,the subsidence caused by the outer eyewall mainly occurred in the eye region of TC and the moat region,without directly weakening the convection at the inner eyewall.It indicates that favorable environmental conditions,stable TC structure and special distribution of the transverse circulation play an important role in the maintenance of the concentric eyewalls structure in "Lekima".