凡那比颱風(2010)降雨機制探討

Typhoon Fanapi (2010) Investigate mechanism of rainfall

吳乙昕(I-Hsin Wu)

指導教授 ： 黃清勇

國立中央大學/地球科學學院/大氣物理研究所/碩士(2012年)

摘要

本研究使用WRF模式V3.2版本模擬凡那比颱風(2010)個案，探討台灣南部山區及高屏、高雄沿海地區極端降雨之成因。為了使模式初始場更接近實際大氣，控制實驗中將FORMOSAT-3/COSMIC GPS RO、GTS及DOTSTAR投落送資料同化於中尺度模式，並使用WRF海洋混合層模式，加入海洋混合層對海表溫度的回饋作用，用來避免模式過度預報，並針對臺灣地形，以及臺灣西南方及東南方大尺度海域之水氣進行敏感度模擬實驗。本研究也使用WRF TC bogus方法，拉長模擬的時間並加強颱風初始場結構，接著針對颱風強度進行敏感度測試，結果顯示，雖然TC bogus方法使模擬的颱風強度更接近觀測，但模擬的颱風路徑在登陸後逐漸北偏，造成模擬的降雨分佈相較於控制實驗並不理想。同時，同化各種觀測資料對路徑模擬的影響不大。
由於控制實驗模擬的颱風路徑相當接近觀測，使得模式可以模擬出臺灣南部山區以及高雄沿海地區的極端降雨。在臺灣南部山區的劇烈降雨機制，主要為颱風登陸前後，颱風渦旋暴風圈受到中央山脈的地形抬升作用所致。颱風中心過山後，颱風渦旋內核減弱，但移至海面時，受到海氣交互作用而迅速加強，由流場軌跡回溯發現，此時來自颱風西北方環流的風場分離成西北風及西南風，然後再與台灣西南方的南風於高屏地區輻合，應為造成高雄沿海地區的極端降雨之主要原因。此降雨機制充分反映在雨帶的分佈，當颱風出海之後出現了雨帶分離的現象，強回波帶並隨著颱風西移而移出台灣。
本研究進一步利用水氣敏感度測試，探討西南氣流的水氣對高雄地區的極端降雨是否有所影響。當台灣東南及西南方大尺度的水氣為75%及50%後，模擬結果顯示降低台灣西南方的水氣對此區的降雨量並不明顯，但降低臺灣東南方的水氣會導致降雨明顯減少，由此顯示大尺度的環流並未扮演重要的角色。另外，由降低台灣地形的實驗發現，此高雄沿海的極端降雨，會隨著模式地形的降低而逐漸往內陸移動，及至地形高度約為25%，雨帶即合併而無分離現象。

關鍵字

虛擬渦漩方法 ； 海洋混和層模式

並列摘要

This study used the Weather Research and Forecasting (WRF) Model to simulate Typhoon Fanapi (2010), and investigated the mechanisms of the torrential rainfall in south Taiwan and near the coastal Kaohsiung. In this study, FORMOSAT-3/COSMIC GPS RO、GTS and DOTSTAR dropsonde data were assimilated to improve the model initial conditions. An ocean mixed layer was also employed during the model integration to allow the ocean mixed-layer feedback to modulate sea surface temperature.
By using a tropical-cyclone bogus method, the model results show that the simulated typhoon’s intensity is closer to the observations. However, the track becomes deflected somewhat northward after landfall in Taiwan causing degraded rainfall prediction as compared to the control experiment which matches the observed quite well. Assimilation with the above observational data appears to have no major impact on track prediction. When the typhoon is approaching and passing over Taiwan, its intense circulation interacts with the Central Mountain Range (CMR), leading to the major rainfall production right over the steep slopes. As the typhoon is moving away from south Taiwan, the vigorous rainband is produced near the coastal plains of Kaohsiung, thus appearing as a separation of intense rainfall. It was found that reducing the environmental moisture southwest of Taiwan doesn’t contribute to a noticeable impact on the above rainfall rate, while the rainfall intensity becomes significantly weakened as the environmental moisture southeast of Taiwan has been cut down by half. Finally, the terrain sensitivity experiments indicate that Taiwan terrain height plays an important role in intensity as well as distribution of the rainfall in Taiwan. The double rain bands are no longer existent and become united as the CMR height is reduced by 75%.

並列關鍵字

tropical cyclone bogus scheme ； ocean mixed-layer model

參考文獻

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