南海地區冬季熱帶氣旋形成之分析

Abstract

觀測資料顯示南海地區熱帶氣旋（Tropical Cyclone, TC）逐月形成頻率在11和12月共有16 %，顯著高於同一時期西北太平洋地區者；因此本研究探討南海地區冬季於強東北季風影響下，形成TC之過程。為瞭解此種TC形成過程的重要特徵，本研究同時選取地面天氣圖上具封閉等壓線，但未形成TC的熱帶雲簇，分析其特徵並與形成TC之個案比較。合成分析結果顯示，形成個案具有較顯著的低層風切渦度和高層輻散場等有利TC形成之綜觀環境。值得注意的是，形成與非形成個案最大的差異在於初始擾動上游東北風的變化，在形成個案增強至TC強度(定義為25kt) 前，其上游東北風的風速明顯減弱；此東北風的減弱可減小環境垂直風切並減緩乾冷空氣持續逸入，維持系統附近的對流不穩定度，以利對流的維持發展。統計結果亦顯示，季內震盪MJO （Madden–Julian oscillation）對此類型個案由初始擾動發展為TC之機率高低，似乎未扮演關鍵角色。 透過WRF（Weather Research and Forecasting）數值模式之模擬分析結果顯示，模式可合理模擬TC之形成過程和非形成個案的變化。利用WRF網格納進（grid nudging）方法的敏感度實驗結果顯示，若減弱形成個案初始場北方之高壓，南海地區低層東北風伴隨之風切渦度將減弱，導致系統強度發展受限，甚或無法發展成TC。結果同時顯示，若增強形成個案發展期間上游之東北風，則環境的冷平流作用將變強，降低系統附近之對流不穩定度，系統發展強度則偏弱，甚或未發展為TC；反之亦然。上述實驗結果與合成分析結果一致，驗證了形成期間之環境低層東北風強度的變化，是主導TC形成與否的關鍵因素。

Observations show that the percentage of tropical cyclone (TC) that formed in the South China Sea (SCS) in November and December is 16% of the total number of TCs in the SCS. This number is significantly higher than that in the western North Pacific (WNP). Therefore, this study examines TC formations in the SCS associated with the strong northeasterly monsoons during the late season. In order to understand the characteristics of TC formation process, it is equally important to examine those disturbances that developed to a well-organized stage but not to develop into TCs. The composites of non-formation cases thus are studied and compared with those of the formation cases. The composite analyses show that the formation cases in the late season have larger low-level vorticity and upper-level divergence compared to the non-formation cases. Another major difference between the formation and the non-formation cases is the low-level northeasterly to the northeast (or upstream) of the incipient disturbance, which weakens right before the pre-TC disturbance reaching 25 kt (~13 m s-1). The weakening of the upstream northeasterlies might be a critical factor for TC formation because it decreases the vertical wind shear and reduces the stabilizing effect associated with the cold and dry air intrusion. Results also show that the activity of Madden-Julian oscillation (MJO) seems not to play an important role in the formation rate of a cloud cluster to develop into TC. The Weather Research and Forecasting (WRF) Model is used to simulate the formation and non-formation cases and the WRF grid nudging method is used to perform the sensitivicity tests. Results show that for a formation (non-formation) case, an increase (decrease) in the upstream low-level northeasterlies during the later period of the formation process would increase (decrease) the cold advection leading to the changes of the convective instability. The intensity of the model system will be weaker (stronger) or even become a non-formation (formation) case. These results are consistent with those of composites, and support the argument that the change in the upstream low-level northeasterlies is the key factor to dictate TC formation.