Analyses On The Secondary Eyewall Formation Of Super Typhoon Haiyan

Secondary eyewall is one of the most common phenomena in mature and severe tropical cyclones.This study chooses Super Typhoon Haiyan(2013)as the case,and then examines the secondary eyewall formation(SEF)process and its internal relation with typhoon intensity by the numerical simulation conducted in the WRF model.Analyses of Haiyan's background environments indicate that the ocean thermal conditions,vertical wind shear,background circulation situation and water vapor distribution are closely linked to the formation and development of Haiyan,and provide favorable conditions for its strengthening to a super landing typhoon.The numerical test through WRF model not only simulates the background conditions,moving path and intensity changes successfully,but also shows several obvious secondary eyewall formation and eyewall replacement processes during the intensification of Haiyan.This paper focuses on the secondary eyewall formation process before Haiyan's landing.The horizontal distribution changes of the surface rain rate suggest that the secondary eyewall formation of Haiyan has close connection with the enhancement and movement of outer rainbands.In the critical stage of secondary eyewall formation,the outer rainband maintains in the southwest quadrant of the outer core region and connect to the inner core convection gradually.The analyses of outer rainband show that cloud features gradually appear in the downstream middle troposphere.The evaporation of stratiform precipitation causes evident cooling effect and then it moves to the boundary layer of inner core region.The dynamic structure also changes under the influence of diabatic cooling.The downward motion and inflow in the middle-low level and boundary layer strengthen significantly by the cooling effect,and develop to the inner core area,enhancing the inner upward motion and convections.Besides,the inward transfer of angular momentum makes the inner core tangential wind enhanced,and the increase of pressure gradient pointing inwards is conducive to the enhancement of boundary layer inflow and convergence.In the boundary layer,the increase of the inflow radial gradient enhances the convergence and moisture transport,providing thermodynamic conditions for inner core convections.The super gradient wind and agradient force increase with the tangential wind,modulating the secondary circulation,and enhance the convergence and upward motion under the joint action of radial inflow.This provides favorable dynamic conditions for the convection development and secondary eyewall formation.The results of sensitivity tests show that the diabatic heating rate in the outer core can influence outer rainbands directly,and then affect the time of secondary eyewall formation and eyewall replacement processes.Reducing the diabatic heating rate can weaken outer rainbands,and delay the time of secondary eyewall formation without eyewall replacements,leading to a small and intense typhoon.Otherwise,strong outer rainbands can make the secondary eyewall formation and eyewall replacement emerge ahead,creating a typhoon of large size and weak intensity.It shows that the secondary eyewall formation is not only the structure change of typhoon,but also closely related to the development of intensity.Eyewall replacement will have a certain negative effect on the typhoon intensification.