梅姬颱風(2010)快速增強之機制探討

Abstract

在颱風路徑預報有長足進展的近30年間，颱風強度預報進步卻相對有限，其中一個困難在於無法準確預報快速增強(Rapid Intensification, RI)的現象所致，因此研究此現象背後的物理機制有其必要。 本研究使用WRF模式對於梅姬颱風(2010)進行高解析度數值模擬，合理模擬出其快速增強過程，同時進行改變雲微物理參數法的敏感性實驗，將增強速率不同的兩颱風進行比較分析。結果顯示，增強較快颱風開始增強前於中高層有較高位渦、慣性穩定度以及更加的軸對稱，同時暖心發展在更高之高度且強度較強，伴隨近中心有更旺盛之對流發展和更強的次環流，以及更劇烈的眼-眼牆質量交換。 將開始增強前的時間分為強對流旺盛-不旺盛兩種時期，發現強對流較旺盛時會伴隨颱風內核區位渦的大幅增加，顯示旺盛對流之於颱風結構改善的重要性，為了更進一步了解兩者間的因果關係，我們進行了位渦收支分析以及使用熱力風平衡模式進行診斷，收支結果顯示水平平流可能扮演重要角色，而熱力風平衡模式診斷結果則顯示強對流能提供較多潛熱，引起較強次環流，將外圍較大的角動量帶入颱風內部中高層，有助於位渦的增加。另外，我們也發現強對流引起較強的次環流，能有效的幫助眼內增溫，增強颱風暖心。 綜合所看到的特徵以及診斷的結果，我們提出一個可能導致快速增強的概念模式:較強的正壓不穩定導致較劇烈的眼-眼牆交互作用，使得更多高熵空氣由眼傳入眼牆中，刺激眼牆中更旺盛的對流，提供更多潛熱，引起更強的次環流，較強次環流有助於中高層位渦和慣性穩定度的增加，也有助於眼內的增溫，當中高層慣性穩定度增加後，也有助於於較強的暖心維持在較高的高度，如此型態的暖心更能有效的讓颱風中心氣壓下降，導致快速增強的發生。

While TC track forecasts have been improved remarkably during the past 20 years, progress in intensity forecast has lagged significantly behind. One subset of intensity change, rapid intensification (RI), is particularly difficult to predict. Therefore, the objective of this study is to investigate the key mechanisms that lead to the RI of typhoon Megi (2010). This study uses Weather Research and Forecasting Model (WRF) to simulate the RI process associated with Megi. The RI process of typhoon Megi (2010) is simulated reasonably well (by comparing with observations). Furthermore, we carry out a series of sensitivity experiments using different microphysical schemes to evaluate the uncertainty of RI with different model physical processes. Comparisons of different experiments indicate that RI TC has greater potential vorticity (PV), inertial stability (I2), axisymmetricity at mid-upper level. In addition, warmer core located at higher altitude, more active convection near TC center, stronger secondary circulation and more significant interaction between eye and eyewall that can also be identified. These features may be the precursors of RI. The PV budget is conducted to gain more physical insights. Results show that when the convective bursts (CBs) are active, the simulated PV tendency is significantly greater. In addition, horizontal PV advection may play a role in increasing the mid-upper level PV, and this may be a result from the secondary circulation triggered by the heating of CBs. The Sawyer-Eliassen model is utilized to diagnose the balanced response of heating and it shows that when CBs are active, the enhanced latent heat strengthens the secondary circulation and the PV advection due to secondary circulation is greater. In addition, we also found that the strong secondary circulation is beneficial to the increased potential temperature in the eye. In conclusion, this study suggests propose a possible new path leading to RI: more active convection near the TC center which could be triggered by high entropy air transported from the eye which will generate greater latent heat and initiating stronger secondary circulation. The stronger secondary circulation is favorable for the increased PV and I2 at mid-to-upper level, which also facilitate the formation of warm core at higher level. The greater I2 at mid-to-upper level could also sustain the warm core structure at higher level. The development of warm core at higher level induces the surface pressure dropping effectively, greater I2 at mid-to-upper level also enhances the heating efficiency; these can help lead to the onset of RI.