伴隨信風增強之熱帶氣旋形成過程

Abstract

觀測研究顯示，綜觀環流的變化對西北太平洋之熱帶氣旋形成扮演重要角色，如信風增強（trade-wind surges）或強跨赤道流（cross-equatorial flow）；而伴隨特定綜觀環流變化形成的熱帶氣旋，有70％之個案具顯著中尺度結構特徵。本研究利用全球網格及高解析度衛星資料，針對伴隨信風增強之熱帶氣旋形成個案，分析其氣候特性與形成過程之中尺度結構變化特徵；並利用WRF（Weather Research and Forecasting Model）數值模式，模擬此類型個案之形成過程，探討中尺度過程所扮演之角色與不同尺度系統間之交互作用。 綜合觀測與模擬結果顯示，形成期間伴隨顯著信風增強之熱帶氣旋，形成過程均伴隨類似之中尺度結構變化特徵。強信風區和其南方大範圍弱氣旋式環流間之帶狀渦度區，伴隨出現多個中尺度強渦度塊（vorticity patches）。強渦度塊受氣流影響向偏西南西方移動，透過扭轉與輻合作用增強其渦度，並組織成一較強渦度塊，伴隨之氣旋式環流亦逐漸增強，稱之pre-TC vortex；之後pre-TC vortex取代初始弱氣旋式環流，成為新的環流中心。Pre-TC vortex環流建立後，其環流與北側信風形成帶狀輻合區，激發另一波渦度帶之形成。新渦度帶中伴隨形成之中尺度強渦度塊，受環流導引以偏西路徑移動，於pre-TC vortex之西北側組織為另一渦旋；此渦旋與pre-TC vortex互繞並合併，形成渦度與環流均較強的初始渦旋，跨越隨機階段進入物理決定階段。這種伴隨特定綜觀環流強迫機制下熱帶氣旋形成過程，已具有一定程度準確定性過程（quasi-deterministic process）之特性，意即雖無法確切掌握熱帶氣旋形成期間隨機發生之中尺度系統的形成時間與位置，但整體中尺度過程之重要變化特徵與熱帶氣旋之形成將可預期。

Forty-three out of 638 tropical cyclones (TCs) that formed in the western North Pacific during 1986-2009 have accompanied with trade wind surges located 5-15 lat. to the north of the pre-tropical cyclones disturbance. Composite and empirical orthogonal function analyses indicate that the trade wind surges are related to a midlatitude eastward-moving high pressure system often found during the East Asian winter monsoon. Therefore, these trade wind surge TCs tend to occur in late season (with one-third of them in December), and at lower latitudes (7 degree lat. lower than the climatological average formation position). The evolution of mesoscale features during formation of trade wind surge TCs is examined. Various satellite datasets show similar mesoscale patterns during their formations. A few convective belts form between the trade wind and the original weak cyclonic circulation. Some mesoscale convective systems (MCSs) are embedded in the convective belt. After the merge process of MCSs, stronger convection with cyclonic circulation will be established. The relationships between the temporal variability of synoptic-scale trade wind surges, the mesoscale features and associated vortexes formations are discussed. The formation process is simulated using WRF Model. Results show that WRF can simulate the evolution of the mesoscale convections and the development of the pre-TC vortex and initial vortex reasonably well. The vorticity and the accompanying cyclonic circulation of MCSs increased by the tilting and convergence effects, causing the formation of pre-TC vortex. The enhanced mesoscale cyclonic circulation of pre-TC vortex convergence with synoptic trade wind at the north side of pre-TC vortex at lower level, triggering a new vorticity belt form. Vorticity patches located in the new vorticity belt organize into a new vortex. The merging of vortexes results in the increasing of low-level vorticity as well as the cyclonic circulation, leading the set up of initial vortex. After the initial vortex formation, the disturbance then transform from the stochastic stage to the deterministic stage.