奈格颱風引起遠距降雨之個案研究

Abstract

秋颱奈格(Nalgae)於2011年10月1日至3日間影響臺灣，颱風中心距離臺灣甚遠，但臺灣東北部地區降下超大豪雨，排汛不及造成水災。本研究利用WRF模式模擬，藉以分析臺灣東北部劇烈降雨成因、測試颱風存在與否對環境氣流及水氣傳輸的影響，並改變臺灣地形高度進行敏感度實驗，此外，亦選取同年份相似路徑之納莎(Neseat)颱風做環境場之對照比較，討論環境場與臺灣地區降雨的關聯。 模擬結果顯示，臺灣東半部發生強降水可分為兩個時期，第一為颱風環流與來自太平洋的東風輻合後形成對流，對流隨氣流方向進入臺灣陸地，在迎風面降下豪雨；第二為颱風東側的南風環流與東風氣流匯合後，兩者北偏與乾冷之東北季風輻合，發展出旺盛對流，加以宜蘭地區的地形效應，使該處出現劇烈降水。另外，若將颱風移除，臺灣南側暖溼氣流北送的情勢減弱，使累積雨量降低、降水區域改變。而將奈格颱風與納莎颱風的環境場相互對照後發現，東北季風對於東北部出現超大豪雨有重要貢獻。 歸納上述結果，本個案造成臺灣出現遠距降水之原因有：(1)颱風環流將南邊的暖濕水氣北送。(2)南來的暖溼氣流與乾冷的東北季風交會提供對流發展之有利條件。(3)對流系統移入時，受到地形抬升而增強。(4)宜蘭地區之地形利於氣流匯集，輻合現象顯著。Typhoon Nalgae (2011) causes extremely torrential rain and flood to northeast of Taiwan, though the center of Nalgae is far away from Taiwan. We used WRF model to analyze the factor of torrential rain, tested whether the existence of typhoon affects synoptic-scale environment and moisture transmission, then did experimentation by changing the topography height; additionally, we also analyzed typhoon Neseat (2011), which has the similar track and intensity, and told what difference between synoptic-scale environment and the rainfall in Taiwan. The simulation result indicates that the torrential rain happened at east of Taiwan can be divided into two periods. One is the convection that formed by the convergence of typhoon circulation and east wind from the Pacific Ocean. Convections entered Taiwan by the direction of airstream, and caused torrential rain at windward side. The other formed by the above northward and converged with northeast monsoon, which is dry and cold, added the terrain effect of Yi-Lan, then torrential rain happened. On the other hand, when the typhoon removed, the northward conditions of warm moisture became weak. The rainfall became less and the precipitation area has been changed, either. After comparing typhoon Nalgae with Neseat, the northeast monsoon plays an important role of torrential rain to northeast of Taiwan. To conclude the results above, there are four reasons that cause remote rain event: 1.Typhoon circulation transport warm moisture northward. 2. The convergence of warm moisture (from south) with cold and dry northeast monsoon sustained deep convection. 3. Convection system reinforces because of topography raise. 4. The topography of Yi-Lan supports low-level airflow convergence.