Role of .BETA.-Effect and Vertical Wind Shear on Tropical Cyclone Asymmetry Structure and Eyewall Replacement

A long-standing issue on the role of vertical shear and β-effect on tropical cyclone intensity and structure is studied through a series of numerical experiments under different vertical wind shears(VWS) in f-and β-planes using the fully compressible,nonhydrostatic tropical cyclone model-TCM4.The result shows that:(1) When a TC is embedded in a relatively week VWS,the intensity of this TC will finally become a steady-state,by researching the sensitivity of the steady-state intensity to the VWS,we suggest that it may be more appropriate to discuss the threshold of a VWS for a TC to stay above a certain intensity(typhoon,tropical storm and tropical depression) instead of a threshold to determine whether a TC would weaken or intensify under a VWS.(2) In f-plane,the vorticity advection are changed with height caused by vertical shear,which in turn produces low-level convergence ahead of and to the left of the shear vector,with the air then rising in a cyclonic spiral,correspond to this,the outflow level exist a divergence region,producing convection and rain on the downshear left.(3) Planetary vorticity gradient(β-effect) could also cause a certain asymmetric,when considering the superposition effect of β-effect and VWS,the asymmetric is more than it cause individual by β-effect or VWS,and the severe convection concentrate on the left front of the shear vector.(4) The eyewall replacement phenomena may have a certain degree of predictability because they appear to depend on the β-effect and VWS.