A Numerical Study Of The Role Of Environmental Vertical Wind Shear On Tropical Cyclone

This study simulates an ideal tropical cyclone in the different magnitudes ofenvironmental vertical wind shear and analyzes the vorticity budget and thetemperature tendency for the aim of investigating the response of TC to EVWS,including the track, intensity and dynamic and thermodynamic structure. Analysisindicates that A TC in the easterly shear moves towards northwest at first andturns to southwest after a period of simulation. If the EVWS is as strong as -11ms-1,TC intensity decreases rapidly and the vertical tilt exceeds 160km. Upper-levelcenter and midlevel center become departing from and rotating clockwise about thesurface center. After a period of simulation, the horizontal distance betweenmidlevel and surface centers decreases and the upper-level center also returnsto vertical position later, this is consistent with the Vortex Rossby Waves Dampingmechanism. The secondary circulations at the upshear quadrants are destroyedcompletely. The corresponding low-level inflow becomes outflow. The axisymmetriccomponent of convection diminishes and the maximum falls to 700hPa. As a result,the upper-level warm core descends to a lower level than other two experimentsand becomes much weaker. It is then hypothesized that the vertical tilt developsand the secondary circulations become asymmetric as the EVWS is added. Theaxisymmetric component of convection diminishes and becomes shallow accompaniedby the warm-core structure descending.As EVWS is added, according to the diagnosis of vorticity equation terms, theupper-level vorticity is firstly changed due to the term of horizontal advection,while the midlevel and low-level vorticity is mainly affected by the verticaladvection and tilting term. The term of horizontal divergence changes obviouslyin the early period of simulation and counteracts some part of the term ofhorizontal advection. One-wave asymmetry of each term develops and the initialaxisymmetric component is inhibited. The vorticity tendency on the upshear sideis negative, while the downshear side is positive vorticity tendency. Therefore,vertical vorticity cannot enhance on the upshear side. As a result, the convectionis diminished and then TC intensity weakens. Furthermore, the effect of horizontaladvection plays the main role in the upper-level temperature changes, while inthe midlevel the adiabatic term is the most important.