季風環流圈環境下之熱帶氣旋形成過程

Abstract

本研究之目的在分析於季風環流圈中形成熱帶氣旋之過程，並探討其中的重要物理機制。利用NCEP-GFS經緯網格資料，分析顯示在2000~2009年間西北太平洋，共有290個熱帶氣旋形成，其中10個熱帶氣旋於季風環流圈中形成，佔所有個案之3.4％，且於季風環流圈中形成之熱帶氣旋，其暴風半徑一般都偏大。此外，在所選取的季風環流圈個案中可分為兩類，第一類為一個季風環流圈形成單一熱帶氣旋；第二類為一個季風環流圈形成兩個熱帶氣旋。透過渦度場之緯向平均隨時間的變化圖中發現，尺度大於2500公里的季風環流圈，逐漸東西分為兩個颱風尺度的低壓擾動，若無其他因素影響，兩者將以間隔48小時內、約20個經度的距離先後形成熱帶氣旋。 本研究挑選兩組無其他因素影響之個案（2004年Mindulle、Tingting，2006年Bebinca、Rumbia）利用WRF模式進行模擬及分析。結果顯示，先形成之個案，最初環流中心位於強風軸之北側，其西側伴隨許多對流，隨著時間往環流中心靠近，近中心風速也逐漸的增強，當對流中心與擾動中心接近後，可透過對流使渦度在近中心附近集中，快速發展成熱帶氣旋；而後形成之個案，出現於原先季風環流圈東南側之對流區中，當環流中心形成後，南側的強西風帶伴隨著對流及正渦度以氣旋式方向，逐漸延伸至擾動東側的合流區，使渦度值逐漸增加。最後，中心附近的深對流在形成前出現，讓系統可以於季風環流圈東南側發展成熱帶氣旋。

This study focus on the process of tropical cyclones formation inthemonsoon gyres in the western North Pacific, and discuss the important physical mechanisms. From 2000 to 2009, there are 10 out of 290 (3.4 %) tropical cyclones (TCs) formed in the monsoon gyres environment in the western North Pacific. Based on QuikSCAT oceanic wind field analysis (Lee et al. 2010), the monsoon gyre is favourable for forming larger TCs. In Hovmöller diagram analysis, the spatial scale of positive vorticity is from about 2500km. As time goes by, it becomes two small but strong positive vorticity area. If TC forms in the western side of the monsoon gyre, another TC will form in the eastern side of the monsoon gyre in less than forty-eight hours. This study usesWRF model to simulate and analysis the process of Mindulle(2004), Tingting(2004), Bebinca(2006) and Rumbia(2006) formation. The results show that, the first formation cases have more convection on the western side of the circulation center. When thecenter of vorticityapproaches the center of circulation, the tropical cyclone will form quickly. The second formation cases are located in the convective zone on the south-eastern side of the monsoon gyrein the beginning. After the circulation center formed, the strong westerly wind with deep convection on the south side of monsoon gyre will gradually merge the circulation.In the end, tropical cyclone formon the eastern side of monsoon gyre.