Research On The Mechanism Of Rapid Intensification Of Typhoons Under Asymmetric Convection Under High-level Vertical Wind Shea

Previous studies have shown that the vertical wind shear(VWS)has effect on the tropical cyclone(TC)intensitation.The TCs in an upper-level easterly VWS environment experience the atypical evolution,namely,deep convections occur mainly on the upshear side.In this dissertation,the role of the upper-level vertical shear(VWS)on the rapid intensification(RI)of Super typhoon Lekima(2019)is investigated with a high-resolution numerical simulation.Our simulation shows that Lekima experienced a severe RI process from August 6 to 8 before landfall.Under moderate upper-level easterly VWS,the tilting-induced convective asymmetry is transported from the initially downshear quadrant to the upshear quadrant and wrapped around the storm center by the cyclonic flow of the storm while moving inward.The TC occurs the "atypical" convective distribution.This process enhances upward motions at the upshear flank and creates upper-level divergent flow.As such,the establishment of outflow acts against the environmental flow to reduce the VWS,allowing vertical alignment of the storm.The organized outflow plays an important role in sustaining the inner core deep convection by modulating the environmental upper-level thermal structure.Accompanying deep convective bursts(CBs),cold anomalies are generated in the tropopause layer due to the adiabatic cooling by the upward motion and radiative process associated with the cloud anvil.Physically,cold anomalies at the tropopause locally destabilize the atmosphere and enhance the convections and the secondary circulation.The CBs continue to develop episodically through this process as they wrapped around the storm center to form a symmetric eyewall.In order to further investigate the effect of cloud-radiation forcing(CRF)on the "atypical" convective distribution and the TC RI process,the sensitivity experiment without CRF has been designed.The result shows that the CRF acts as positive feedback between the tilting-induced convective asymmetry and the outflow channel.On one hand,the radiative process will induce upper cooling above the cloud anvil of the outflow layer,and the lower warming within the cloud anvil of the outflow layer.This thermodynamical pattern locally destabilizes the upper troposphere,and is conducive to enhance the deep convection.On the other hand,the enhanced deep convection provides the energy source to promote the upper divergent flows.The stronger divergent flow acts efficiently to block the VWS and leads to a stronger outflow channel.In this dissertation,we examine some TCs with different growth rates under the easterly wind shear over the WNP from 2009 to 2019.By comparing the radius of the maximum wind(RMW)of these TCs,it indicates that TCs with RI has a larger size than those with smaller intensification.To investigate the sensitivity of TC intensification to its inner size in a sheared environment,four vortices with different initial RMW are designed.The results show that TC RI is sensitive to the size of the TC disturbance in the presence of a VWS,where a vortex with a larger size intensity fast within a certain scale.When the RMW exceeds a certain range,the TC with larger RMW will intensifies slower.On one hand,the vertical tilt of the larger size TC decreases more quickly.That is conducive to the TC development fast.On the other hand,a TC disturbance with a larger size possesses a larger area of large surface heat flux,favoring the boundary layer recovery for TC development and a large growth.