The Impact Of Vertical Wind Shear Magnitude On Secondary Eyewall Formation Of Tropical Cyclones

The concentric eyewall and eyewall replacement process can significantly impact the structure and intensity changes of tropical cyclones（TCs）.Many previous studies have proposed various physical mechanisms for the formation of the secondary eyewall in a quiescent environment.However,tropical cyclones often experience the influence of environmental vertical wind shear（VWS）.In this thesis,the physical mechanisms of secondary eyewall formation in TCs within VWS of different magnitudes are investigated based on idealized numerical simulations.The results show that when the VWS magnitude is less than 15 m s^-1,the stronger the shear magnitude is,the earlier the secondary eyewall forms,indicating that stronger shear is more conducive to the secondary eyewall formation.Before the secondary eyewall formation,a quasi-stationary rainband complex appears,and the stronger the shear,the more pronounced the rainband complex is,indicating a meaningful physical connection between the quasi-stationary rainband complex and the secondary eyewall formation.Under the influence of VWS,in the downshear-left quadrant,the upper-tropospheric asymmetric outflow forced by the shear promotes the organization of stratiform clouds by transporting water vapor and ice particles from the inner core to the outer core.The stronger the shear,the more water vapor and ice particles are transported outward,so the quasi-stationary rainband complex shows better-organized stratiform clouds in the stronger shear environment.From a dynamical perspective,the diabatic cooling of the stratiform clouds mentioned above can lead to sinking inflow.The inflow increases the tangential wind in the boundary layer,resulting in supergradient winds,which force convergence local convergence and lift in the middle-to-upper part of the boundary layer.From a thermodynamic perspective,low-entropy air generated by the convection in the middle and upwind portions of the outer rainbands is advected to the inner edge of the sinking inflow,causing conditional instability.Ultimately,the convergence and conditional instability allow the low-level convection to be maintained on the inner edge of the sinking inflow.Since stronger VWS can produce stronger mesoscale descending flow,the low-level convection inside the broad stratiform clouds downstream of the quasi-stationary rainband complex is more active.At the same time,under the influence of strong vertical wind shear,inner rainbands tend to form in the downshear quadrant and propagate outward.During the outward propagation,they interact with the low-level convection on the inner side of the rainband mentioned complex and gradually become axisymmetrized to form a secondary eyewall.A potential vorticity tendency diagnosis further indicates that the contribution of the eddy potential vorticity flux convergence associated with spiral rainbands is comparable to that of the symmetric potential vorticity flux convergence during the secondary eyewall formation.Especially in upper layers,the eddy potential vorticity flux divergence mainly contributes to the increase in potential vorticity.A tangential wind tendency diagnosis shows that the symmetric radial advection of absolute vorticity is the leading cause of the formation of the secondary tangential wind maximum,while the eddy radial advection of absolute vorticity plays a secondary role.The study demonstrates that as the environmental shear weakens,the sinking inflow in the stratiform cloud region in the outer rainbands also weakens.In this case,the formation of the secondary eyewall is mainly related to the axisymmetrization of convection upstream of the outer rainbands.In addition,it was also found that in the case of weak sinking inflow,fake concentric eyewalls may appear in the presence of a secondary convective ring but in the absence of the secondary tangential wind peak.This is mainly due to the axisymmetrization of inner rainbands in the inner core.