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標題: | 非絕熱加熱對侵襲台灣颱風運動之影響 The Influence of Diabatic Heating on Typhoon Motion near Taiwan |
作者: | Li-Huan Hsu 徐理寰 |
指導教授: | 郭鴻基(Hung-Chi Kuo) |
關鍵字: | 颱風路徑偏折,颱風移行速度,地形相位鎖定對流降雨,非絕熱加熱,位渦趨勢診斷, Typhoon track deflection,typhoon translation speed,topographic phase-locked convection,diabatic heating,potential vorticity tendency diagnosis, |
出版年 : | 2012 |
學位: | 博士 |
摘要: | 本研究分別以觀測資料與數值模式模擬,探討受地形激發之非絕熱加熱影響侵襲台灣之颱風通過台灣的移速變化,探討重點在於登陸前的颱風路徑偏折與登陸後颱風移速的改變。在觀測資料部份,研究使用過去51年(1960-2011)84個侵台颱風的資料,在颱風離岸200公里(約登陸前12小時)至登陸之間,以曲率極大值定義颱風路徑之偏折點及偏折角度,研究發現氣旋式路徑偏折且角度大於20°的颱風個案中,有75%的個案其偏折點位於23.5°N以北;而角度小於10°的颱風個案中,有71%的個案偏折點位於23.5°N以南,偏折角度的大小分布呈現南北不對稱的特徵,顯示台灣地形影響的重要性。更進一步的分析顯示,較大路徑偏折角度的颱風,甚或是繞行打轉路徑特徵的颱風,其登陸前12小時平均移行速度較慢,且接近台灣時會加速偏向地形。
分析61個颱風地面中心信號沒有消散之”連續路徑”颱風個案,顯示颱風登陸後移速慢的颱風,在登陸前即有較慢的移速,而登陸後還更進一步發生約3 m s-1的減速特徵,此外移速最慢的颱風有77%登陸在台灣北部,平均在陸滯留時間達到16小時,在台灣帶來極大的降雨,降雨極值則位在台灣中南部山區;而在陸期間屬於移速快的個案裡,平均在登陸後會更加加速,且有60%登陸在台灣南部,累積降雨量較少,降雨地理分布上無明顯極值存在。 本研究利用Weather Research and Forecasting Model (WRF) 來進行理想數值實驗,並以位渦趨勢診斷工具(Potential vorticity tendency diagnosis)進行分析,探討颱風移速受地形降雨的影響。數值實驗顯示,登陸前台灣地形迎風面氣流受地形舉升加強颱風對流,背風面則為下沉氣流使對流受到抑制,此不對稱對流加熱分布會造成颱風路徑向西南偏折。此結果也說明除了動力機制外,對流非絕熱熱力機制在登陸前颱風路徑的偏折也扮演了重要的角色。對於北邊登陸的颱風,由於受到地形相位鎖定的深對流位於台灣中南部,加熱不對稱性分布影響,因此可以減速約3 m s-1,這現象在駛流微弱的環境中更為顯著。南邊登陸的颱風,通過中央山脈後,則會因為和登陸前往西南偏折的原因類似,但方向相反因而使颱風加速;這些模擬為觀測提供物理解釋機制。本研究結果可得知慢速颱風登陸台灣北部時,存在一正回饋效應,慢速颱風滯留台灣時間長,帶來較大降雨量;而降雨量大則使颱風減速效應更為顯著。 The thesis investigates the influence of topographic induced convection on the landfall typhoon motion in Taiwan. We investigate the track deflection by the topographic induced convection before the typhoon landfall and the translation speed change after the landfall. The pre-landfall typhoon track deflection is studied when the typhoon is within 200 km from the coast. The track deflections are investigated in terms of the deflection points and deflection angles, which represent the location where the largest curvature and the most significant deflection occur. Our analysis indicates the geographic asymmetric distribution of deflections. Namely, of the cases with deflection angle greater than 20°, there are 75% (9/12) with deflection points located northern than 23.5°N. For deflection angles smaller than 10 degrees, the ratio of south cases exceeds 71% (5/7). Further analysis of translation speed indicates that cases with larger angle and the looping track type usually has a lower average translation speed than small angle subgroups in the early stage 12 hours before landfall. For the landfall typhoons, our analysis also suggests the geographic asymmetry. Data for 84 typhoons that reached Taiwan’s eastern coast from 1960 to 2010 are analyzed, with motions compared to the long-term average overland translation speed. For 61 continuous track typhoons among all cases, we find that 77% of the slow-moving tropical cyclones (TCs) made landfall on the northern end of Taiwan’s eastern coast, while 60% of the fast storms had southeast coastal landfalls. This geographic asymmetry with respect to typhoon translation speeds widened after landfall, as the slow-movers typically decelerated during the overland period while the faster TCs sped up. In particular, the average overland duration was 16 hours for the slow class, compared to only 3 hours for the fast-moving typhoons. The combination of slower translation with longer duration for the northern class of TCs led to large rainfall on the southwestern slope of the island’s Central Mountain Range. Weather Research and Forecasting Model experiments are used to study the effect of convection on storm motion over a mountainous island resembling Taiwan. The topographic induced asymmetric diabatic heating can alter the typhoon track before and after the landfall. We also find that the topographically phase-locked convection acts to slow down (speed up) the northern (southern) landfalling typhoons. Our model results also suggest a positive feedback mechanism that exists for the slow storms, in which the convective heating pattern forced by topography acts to reduce the TC motion, leading to even more prolonged precipitation and heating, yielding further speed reductions after landfall for northern landfalling typhoons. Our experiments also suggest that the asymmetric diabatic heating may contribute to the cyclonic typhoon track deflections before landfall. The mechanical lifting at windward slope as well as the down slope wind at leeside of terrain both contribute the asymmetric convection and diabatic heating near typhoon before landfall. In addition to the mechanical forcings in the presence of mountain, diabatic heating may play an important role affecting storm motion. In particular, the impact of the diabatic heating is significant when the environmental mean flow is weak. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/6694 |
全文授權: | 同意授權(全球公開) |
顯示於系所單位: | 大氣科學系 |
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