Offshore Strengthen Numerical Simulation Of Tropical Cyclone Structure And Intensity Change Mechanism

In this paper, tropical cyclone "Huang-Feng" is simulated through mesoscale model MM5. We investigate its structure evolution in the processes of intensification in the offshore area and decay after landfall. These processes influence intensity of cyclone lifetime. This study also provides some references to operational prediction.Comparing with observational data, the simulation results have revealed their capability in presenting fine three dimensional structure in movable mesh with a size of 4km. High time and spatial resolution of model data can make up for scarcity of observational data to establish basis for further diagnostic analysis and sensitivity simulations.Analyzing synoptic scale system, we demonstrate that the mid-latitude cold vortex and subtropical high play major roles in the process of tropical cyclone intensification. Variables are transformed from Descartes coordinate to cylindrical coordinate and decomposed into their axisymmetric and asymmetric components. Axisymmetric structure of this tropical cyclone, which possesses its feature, are different from those of typical typhoon. Diagnostic analysis of values in dynamic and thermal fields can help us understand physical mechanism in the period of intensification and decay. Water vapor transported by southwest monsoon and unstable convection caused by warm and cold air can not only induce asymmetric structure, but also impact intensity significantly.During intensification process, mesoscale vortices in the inner core tend to axisymmetrize which induces interaction of wave-mean flow in nonlinear factor to transform energy between disturbance and basic state. Taking advantage of potential vorticity thinking and vortex Rossby wave (VRW) theory, we verify the existence of waves like VRW that is dynamical mechanism for intensity change. Perturbation source forced by convection can produce waves propagating cyclonically, which makes eyewall and spiral rainband enclose eye. After landfall, when vapor transportation is reduced and low troposphere is invaded by cold air mass, it normally results in inactive waves and intensity weakening.Three additional sensitive experiments are carried out to investigate the effect of condensation latent heating, evaporation cooling and surface heat flux. The results show that the positive feedback between condensation latent heat and convective system, and the favorable effect of surface heat flux on thermodynamic structure in planetary layer are helpful to increase the TC intensity. Precipitation evaporation cooling is a inhibitive factor for TC intensification, but its effect is limited.