梅姬颱風(2010)—路西行，自菲律賓登陸不久後，即急遽轉彎向北移動。本文主旨在於探討其受到 地形影響前後之位渦收支分布與變化，以及轉彎前後之動力與熱力特性與演變。為增強初始颱風強度， 使更接近真實觀測，本研究利用位渦反演得出颱風渦旋風場、壓力場及溫度場，並利用WRF三维變分 同化將此虛擬渦旋同化於初始場。在本研究，由不同初始位渦擾動設定測試得到最為理想的強烈颱風分 析場。實驗結果顯示，較高的位渦擾動振幅，會同時增強初始颱風之動力場及熱力場，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WRF預報路徑有較佳的表現， 而對模擬強度的影響則不顯著。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，但會使颱風向東北方偏移。 我們由敏感度實驗得到最佳模擬，進行位渦收支診斷。診斷分析指出，在颱風低層，位渦水平平流 透過切向風，將眼牆中較高的位渦逆時針平流至下游，並透過徑向內流，將眼牆外較低的位渦向內輸送， 而位渦垂直平流則藉由上升氣流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。位渦的非絕熱作用則為颱風增強或減弱的關鍵角色，旺盛對流提供大量潛熱釋放，抵銷 在低層平流作用及紊流混合作用的負貢獻，而位渦的摩擦作用只有在地形才具顯著的負貢獻。當颱風即 將登陸菲律賓時，潛熱作用增加，進而使徑向內流增強，亦提升平流作用，登陸後受到地形影響，使颱 風結構破壞，風速、位渦迅速減弱。在離開菲律賓後，梅姬颱風由原本較為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，在渦旋西北前側有較高的非絕熱作用，使梅姬颱風 傾向往此高位渦趨勢移動。因此，從位渦觀點來看，為反應於較大的正位渦趨勢，眼牆對流於轉彎前 後的順時鐘旋轉可能為梅姬北轉的關鍵因素。

Westbound Typhoon Megi (2010) made landfall at the Philippines and then took a sharp north turn shortly. This paper aims to investigate the changes and distributions of potential vorticity budget before and after the influence of the terrain, and the associated dynamic and thermodynamic characteristics and their evolution. For enhancing the initial typhoon intensity closer to the observed, this study utilizes PV inversion to obtain the wind, pressure and temperature of the typhoon vortex, and applies WRF three-dimension variational assimilation to ingest this bogus vortex into the initial condition. In this study, we obtain the optimal analysis for intense typhoon thru tests on different settings of initial PV perturbations. Results of numerical experiments indicate that larger PV perturbation amplitude will enhance both dynamic and thermodynamic fields, thus resulting in intensification and consolidation of the typhoon but leading to an earlier northward track deflection due to reduced influence by the environmental steering flow. Using the optimal experiment among the sensitivity tests, we conduct PV budget diagnostics. Diagnostics indicates that at lower levels of the typhoon, PV horizontal advection transports larger PV in the eyewall counterclockwise to downstream through tangential wind, and lower PV outside the eyewall to the inner core, while PV vertical advection transports high PV at low levels upward; advection effects thus play the role in reduction of vertical and horizontal gradients of PV in the eyewall. On the other hand, diabatic effect in PV is crucial for intensification and weakening of the typhoon. Vigorous convection offers a huge amount of released latent heating that offsets the negative effects of advection at lower levels and turbulent mixing; frictional effects on PV is more prominent only over the topography. As the typhoon is near landfalling at the Philippines, latent heating is increasing to enhance radial inflow and advection effect. After landfall, PV and wind speed are quickly reduced with a more destructive typhoon due to topographic effects. After departure from the Philippines, Megi reorganizes into a symmetric, consolidated typhoon from the somewhat broken eyewall. During the eyewall re-construction, PV tendency exhibits prominent concentrations with stronger diabatic effects to the northwest that thus force Megi to move to such high tendency. From PV perspectives, the clockwise rotation of eyewall convection before and after turning may be the key mechanism responsible for the north turning of Megi in response to large positive PV tendency.