| Summary: | 碩士 === 國立中央大學 === 大氣物理研究所 === 103 === Potencial vorticity contains the dynamic and thermodynamic properties, and has the characteristic of conservation under adiabatic and frictionless conditions, so that it can be used to analyze changes of tropical cyclones. Herein, the purpose is to analyze the potential vorticity change and its distribution of typhoon Megi when it is influenced by topography, as well as discuss its dynamic and thermodynamic characteristics of typhoon before and after it turns northward, to understand the evolution of its dynamic and thermodynamic processes during the entire period. However, the initial intensity of the typhoon is generally weaker than the actual intensity, and typhoon often develops in the vast ocean, where there are few traditional observations. In order to enhance typhoon intensity closer to the observed, we add a symmetric virtual vortex vorticity (Potential Vorticity Bogus Vortex) to the initial field to improve typhoon intensity and structure, and make the typhoon simulation closer to the true, and more conducive to the diagnosis of PV budget.
The study is divided into two parts. The first part we use virtual PV inversion to derive wind, pressure and temperature fields, and then use a three-dimensional variational data assimilation (3DVAR) to implant a virtual PV vortex into initial field and adjust the vorticity perturbation amplitude, and select the cutting radius and potential vorticity perturbation decay rate to obtain the best setting for strong typhoons. Sensitivity experiments show that the higher the potential vorticity perturbation amplitude will also be stronger the typhoon in the initial field, but it will make the typhoon track deflect early. Experiments also show that when potential vorticity disturbance decay rate is 4 and the selection cutting radius is larger, the simulated typhoon tracks have better performance. We also conduct a test that assimilates temperature field as well. The results show that although this additional assimilation has little effect on the lowest center pressure, it will make the overall track of typhoon shift towards the northeast after the assimilation of temperature. Therefore, for the strong typhoon Megi, the weaker potential vorticity perturbation amplitude, the smaller selection cutting radius and potential vorticity perturbation decay rate 4 are the best settings.
The second part is the potential vorticity budget analysis, for which the assimilated initial field is used for 96-h forecast of WRF. In general, in the typhoon entire life cycle, advection term is to redistribute PV in three-dimensional space. In the low level, through the tangential wind and the radial inflow, it advects the higher potential vorticity clockwise downstream and reduces low level potential vorticity. And then through the vertical advection, it transfers PV from the low level to the higher level. Advection effect does not generate or reduce the whole potential vorticity, so diabatic effect is the key to increase or decrease the typhoon intensity. Convection induced by typhoon causes condensation and latent heat release, which provides sufficient energy to maintain typhoon continuously. Frictional effect will be significant only when typhoon is influenced by topography. When the typhoon does not hit the terrain, the front side of the advancing direction of the typhoon has a higher tendency of diabatic effect, which is due to the fact that typhoon is prone to move toward the environment suitable for the development of cyclone. Through the analysis of potential vorticity budget, we can better understand the dynamic and thermodynamic effects and physical processes in various periods.
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