雙眼牆颱風與西南季風探討

Abstract

透過颱風中心渦旋組織外圍大範圍之不對稱對流雲帶，是颱風雙眼牆結構形成的重要機制之一。2001年利奇馬颱風形成雙眼牆前之雷達回波圖，及2003年尹布都及杜鵑等颱風之微波雲圖，颱風外圍皆有大範圍之不對稱對流雲帶。本研究利用西北太平洋1997年至2009年間69個雙眼牆微波資料，探討雙眼牆生成前外圍對流面積大小及方位之時空與綜觀環境特徵，並探討雙眼牆生成與季風之關係。 我們挑選颱風眼為中心9°×9°範圍內，定義中心渦旋為雙眼牆生成時內眼牆與moat之和，扣除此中心渦旋面積後，依據微波雲圖中小於230K之區域，計算形成雙眼牆前24小時之外圍對流面積，並以平均值及上下一個標準差之統計結果，將外圍對流分成小、中、大三種面積尺度，其大小分別為2~5、6~10、11~17 (萬平方公里)。結果顯示大對流面積之個案好發於6至8月且生成於西北太平洋中123°E ~135°E、13°N ~27°N區域；小對流面積則好發於8至9月且生成於西北太平洋126°E ~154°E、19°N ~27°N區域。進一步於外圍較大面積相對方位上，分成南側及北側主宰兩類，全部個案中，南側個案數約為北側之兩倍，其生成於夏季及西北太平洋135°E以西區域，且外圍通常有較大之對流面積。 SHIPS (Statistical Hurricane Intensity Prediction Scheme)為颱風之強度相關參數統計資料，我們以當中之200-850mb垂直風切向量，針對外圍對流之生成機制進行分析，統計相對於下風切各方位之外圍對流面積。我們發現其外圍對流皆好發於下風切處偏左側。其次在水氣參數上，依據南北側對流主宰，採不同計算範圍，進而分別將外圍對流面積與南側7個區域之水氣參數及北側3個區域之水氣參數作相關分析，結果在南側個案中，對流面積與颱風西南側水氣通量相關性較佳；北側個案中，對流面積與颱風北側環流水氣通量輻合值關係較佳。 在西北太平洋季風指數上，採用110°E ~140°E、5°N ~30°N 範圍之850hPa緯向風場南北風速差，當其值為正，表此區為氣旋式環流型態。13年中，47個雙眼牆個案生成於季風計算範圍內，其中70.5%生成時季風指數達4ms-1以上，尤其在6至9月之個案中，93.5%生成時季風指數達4ms-1以上，顯示於6至9月時，季風指數達4ms-1以上為有利雙眼牆生成之條件。最後，關於雙眼牆生成時強度分析，得知生成強度隨雙眼牆生成區域之上層海洋熱含量(UOHC)之變大，有增加之趨勢，R-square達0.15；而生成強度與SST及垂直風切無明顯關係。

One of the important processes of concentric eyewall formation is the axisymmetrization of outer region asymmetric convection zone by center vortex of typhoon. Before the formation of concentric eyewall, we find a large outer region asymmetric convection zone in every concentric case from radar echoes or microwave images. For instance, typhoon-Lekima(2001) ,Dujuan(2003), and Imbudo(2003). There are 69 concentric cases at North-west Pacific Ocean in the period of 1997 to 2009. We use the cases’ microwave data to discuss about the relationship between outer region convection area and temporal and spatial distribution. Moreover, the study discusses about the relationship between concentric eyewall formation and monsoon index. At first, we calculate the area of outer convection zone that is smaller than 230K in the 9°x9° region of microwave image, and then we subtract the center vortex which is defined as the sum of inner eyewall and moat width at the time of concentric eyewall formation. We also classify the sizes of area into three scale including small(2 to 5 million kilometers), medium(6 to 10 million kilometers), and large(11 to 17 million kilometers) by average and one standard deviation. The result shows that the large convection area cases are mostly happened in June to August and forming in the region between 123°E~135°E , and 13°N~27°N in North-west Pacific Ocean; small convection area cases are mostly happened in August to September and forming in the region between 126°E~154°E , and 13°N~27°N in North-west Pacific Ocean. Moreover, we classify the position of the outer region convection zone into two kinds- south dominate and north dominate. From all cases we choose, the number of south dominate cases is two times greater than the north dominate cases. South dominate cases were mostly formed on the west side of 135°E in North-west Pacific Ocean, and most of them have larger convection area at the outer region than north dominate cases. We use information of vertical wind shear between 200-850hPa from SHIPS (Statistical Hurricane Intensity Prediction Scheme) to analyze the formation mechanism of outer region convections by counting the area of convection at down shear side. We find that the outer region convection happen mostly at down shear left sides. Further, we use water vapor parameters to find the relationship with convection area. For south dominate cases, we find that the area of outer region convection zone has great thing to do with the water vapor flux in the south-west of the typhoon. On the other hand, for the north dominate cases, we find that the area of outer region convections zone is better related to the convergence of water vapor flux in the north part of the typhoon. North-west Pacific monsoon index is defined as the zonal wind speed difference at 850hPa in the 110-140°E, 5-30°N. East Asia region has cyclonic circulation when the index is positive. Among 47 cases of concentric typhoons which happened in the same domain as monsoon index during 13 years, 70.5% of them happened with monsoon index up to 4m/s. Especially during June to September, the percentage of certain cases is up to 93.5%. The fact indicates that during June to September, the environment with monsoon index higher than 4ms-1 is a benefit to concentric eyewall typhoons. Last but not least, the analysis showed that the intensity of concentric eyewall typhoon formation is correlated to the upper ocean heat content (UOHC), with R-square value of 0.15, and it has nothing to do with SST and vertical wind shear.