楊希颱風(1990)侵台的數值模擬與分析

A Numerical Simulation and Analysis of Typhoon Yancy (1990) during Its Landfall on Taiwan

簡國基(Gou-Ji Jian) ； 李清勝(Cheng-Shang Lee)

《大氣科學》 30卷2期 (2002/06) Pp. 147-170

摘要

於1990年TCM-90國際颱風實驗期間侵台的楊希(yancy)颱風，在侵台前，其路徑出現兩次主要的變化；本文利用PSU/NCAR MM5中尺度模式，模擬楊希侵台前後的結構和路徑變化情形，並利用模擬結果探討導致路徑變化的物理機制。研究結果顯示，控制組實驗在楊希登陸前51小時，即可合理模擬登陸地點、時間及登陸前的主要路徑變化。除控制組實驗外，本研究另設計兩組對照組實驗，分別為移去台灣地形與減弱中緯度槽線強度；透過控制組與對照組模擬結果之比對分析，可合理推論台灣地形作用為導致楊希路徑變化機制。
分析結果顯示，當楊希以西北向路徑移近距台灣陸地約400公里時，因受台灣地形影響，暴風半徑逐漸縮小；然而半徑200~400公里原本較不對稱的風場（東方風速較強），因台灣地形與颱風間水平角動量平流效應的增強，而趨向較為軸對稱，使颱風增加往北運動的趨勢。當楊希移距台灣陸地約230公里以內時，因地形直接破壞颱風暴風圈內環流結構，提高風場的不對稱性（先為東北－西南向，再轉變為北－南向）；其所引發之通風氣流平流效應，為促使颱風向西北轉向西加速、侵襲台灣的主要機制。
此外，當楊希中心位於宜蘭東方約150公里的2小時期間，中心西南側的對流出現短暫增強現象，並伴隨颱風近中心最大風速的短暫增強，此現象在模式中亦可合理模擬。分析模擬結果顯示，颱風西側靠近中心之環流，與近地層繞越台灣地形的外圍環流，交會於颱風中心西南側，促使對流增強。輻合所引發之垂直運動，可透過扭轉效應，增加局部正渦度，更有利於對流之迅速增強。

關鍵字

颱風 ； 颱風登陸 ； 數值模擬

並列摘要

The motion of Typhoon Yancy exhibited two major changes prior to its landfall on Taiwan during the Tropical Cyclone Motion Experiment in 1990 (TCM-90). This study attempted to simulate the changes in motion and structures of Yancy using PSU/NCAR MM5 and to investigate the physical processes leading to the changes of Yancy's motion. Results showed that the control experiment, with the model initial time at 51hours before landfall, simulated reasonably well the landfall point and time as well as the major track changes prior to landfall. Besides the control experiment, two experiments were made for inter-comparison. One was made with Taiwan topography being replaced by ocean condition and the other with a weaker mid- latitude trough. A detail comparison and analysis had revealed that the effect of Taiwan topography was the dominant factor that caused the major changes in Yancy's motion before landfall.
Results showed that the radius of gale force wind decreased due to the influence of Taiwan topography when Yancy moved northwestward to the place 400kilometers off Taiwan coast. The wind field at radii 200-400km,which was axis-asymmetric originally with stronger wind to the east, became more axis-symmetric due to the increased angular momentum flux at region between typhoon and Taiwan. Such change in wind field structure tended to drift typhoon toward northward. When Yancy moved to the place 200km off Taiwan coast, the axis-symmetric structure of inner wind field was destroyed significantly by Taiwan topography. The inner wind field became axis-asymmetric with stronger wind to the northeast and later shifted to the north. The advection effect of the ventilation flow associated with the axis-asymmetric inner wind field caused typhoon to change its moving direction from northward to northwestward and then speedup westward.
In addition, the convection to the southwest of typhoon center was enhanced, accompanied by an increase in typhoon intensity, during a two-hour period when typhoon was located 150km east of I-Lan. Such phenomena were also simulated in the model. An analysis of model results showed that the inner circulation to the west of typhoon center merged with the outer circulation, which moved around Taiwan topography at the region to the southwest of typhoon center. This merging effect enhanced the vertical motion and convection. The enhanced vertical motion, in turn, caused an increase in vorticity via twisting effect and was conducive to the quick increase in convection.

並列關鍵字

Typhoon ； Typhoon Landfall ； Numerical Simulation