| 摘要: | 位渦包含了動力及熱力特性，且在絕熱無摩擦條件下具有保守特性，故此用來分析熱帶氣旋的變化是個良好的物理量，本篇主旨在於分析梅姬颱風，探討其受到地形影響前後位渦收支變化和分布，以及颱風轉彎前後之動力熱力特性，以了解颱風在這此期間的動力及熱力過程的演變。在進行模擬時，初始場的颱風氣壓強度通常都比實際的氣壓強度來的弱，而颱風生成在廣大的洋面上，缺乏一般的傳統觀測資料，然而為了可以增強颱風強度更接近真實場，會在初始場中加入一個對稱虛擬位渦渦旋(Potential Vorticity Bogus Vortex)來改善颱風的強度以及結構，使颱風在模擬中可以更加貼近真實的颱風狀態，並更有利於位渦診斷分析。
本研究分成二大部分，第一部分使用虛擬位渦反演，反演出風場、壓力場及溫度場，並使用三維變分同化(3DVAR)將虛擬渦旋植入初始場中，調整位渦擾動振幅、選取切割半徑及位渦擾動遞減率進行模擬，得到對強烈颱風之最佳設定。敏感度實驗結果顯示，較高的位渦擾動振幅，會同時增加初始場之動力場及熱力場，然而加強了颱風強度和結構，使其受到綜觀駛流場影響減弱，造成模擬路徑提早北偏。選取切割半徑及位渦擾動遞減率的敏感度實驗顯示，較大的選取切割半徑及位渦擾動遞減率為4時，在預報路徑有較佳的表現，而模擬強度影響不顯著。另外也進行了同化溫度場之測試，結果顯示，同化溫度場後雖然對颱風中心最低氣壓影響不大，但會使颱風整體路徑向東北方偏移，因此，對於強烈颱風梅姬，較弱之位渦擾動振幅、較小之選取切割半徑、位渦擾動遞減率4及不同化溫度場為最佳的設定。
第二部分是位渦收支診斷分析，使用同化虛擬渦旋後之初始場，再利用WRF模擬96小時之輸出，對颱風登陸前後及轉彎前後時期進行位渦收支分析。一般而言，颱風整個生命週期，平流作用是使位渦在三維空間中重新分配，在低層透過切向風，將眼牆中較高的位渦順時針平流至下游，並透過徑向內流，將眼牆外較低的位渦向內輸送，再藉由垂直運動將低層高位渦往上層輸送，因此平流作用扮演了降低眼牆垂直及水平位渦梯度的角色。由於平流作用並不會生成或減少氣旋整體位渦，非絕熱作用才是颱風增強或減弱的關鍵。颱風中的強對流會使水氣凝結，潛熱釋放，提供大量的能量去抵銷平流作用及紊流混合作用在低層的負貢獻，並持續的給予颱風能量供應，而摩擦作用只有在遇到地形時效果才會顯著。當遇到地形時，氣流抬升，潛熱作用增加，進而使徑向內流增加，平流作用也得到提升，受到地形影響，使颱風結構破壞，風速、位渦減弱，當離開地形後，颱風會由原本破碎的眼牆結構逐漸趨於對稱完整，眼牆重建期位渦趨勢也有明顯的極值分布，且在颱風未遭遇地形時，前進方向前側都有較高的非絕熱作用趨勢，這是由於颱風傾向往適合氣旋發展的環境移動，也就是往高位渦的環境前進。經由位渦收支診斷分析，可以更加瞭解颱風在各個時期的動力、熱力作用扮演的角色以及物理過程，相較於渦度收支，其包含了水相作用的過程，而颱風本身的能量來源主要是由非絕熱作用所貢獻，因此使用位渦來分析颱風的動力及熱力過程更加完善。
;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. |
