Influences of Bimodel Vertical Wind Shear on the Structure and Intensity of Typhoon Fitow (2013)

Simulations of Typhoon（Tropical Cyclone）Fitow(2013) are used in this study to analyze the influence of the vertical wind shear (VWS) on its structure and intensity. A vertical wave-like distribution of VWS is revealed. This wave-like distribution varies at different stage of typhoon Fitow（2013）, and exhibits bimodel structure in the mature stage. It is found that the VWS between the top and bottom of middle troposphere is the major part of the total VWS. The typhoon intensity obviously changes about 6 hours after the distribution mutation of VWS. The different configuration of the secondary circulation induced by bimodel VWS and the typhoon vertical circulation causes the asymmetric structure of the deep convection bands,and they tend to be symmetric gradually with the enhancement of cyclonic circulation. The diagostic analysis also investigate that the wave-like distribution of VWS results in the inhomogeneous feature of vorticity forces in vertical direction. And the vorticity forces in the middle and low troposphere favors the development of convective instability. In accordance with the theoretical models, the maximum vertical velocity appears at the same altitude with the inflection of the vertical wind profile. Therefore, the bimodel VWS is crucial to the strucutre change of deep convection bands and the intensity maintenance in typhoons Fitow(2013). Furthermore, the wave-like distribution of VWS might be the trigger of instability accounting for the convective-rolls in typhoon.

Key words：Typhoon,Vertical wind shear, Bimodel distribution, Mesoscale convection

Acknowledgements: This work is supported by the Natural Science Key Foundation of China (No.41230421) and National Natural Science Foundation of China (No.41175054 and No.41275002)