妮蔻兒颱風（Nichole,1998）侵台前路徑變化之研究

A Study on the Track change of Typhoon Nichole (1998) Before Making Landfall on Taiwan

李清勝(Cheng-Shang Lee) ； 張龍耀(Lung-Yao Chang)

《大氣科學》 31卷3期 (2003/09) Pp. 285-305

摘要

在1951~2001年間，共有11個颱風由南海北部向東北移動而影響台灣;除1998年妮蔻兒颱風之移速僅約3kt外，其餘颱風均以較快的速度(大都≧5kt，平均約10kt)通過台灣地區。妮蔻兒颱風主要雲區半徑僅100km，最大強度則僅50kt(或熱帶風暴強度);但其由南海北部北移至台灣海峽南部時，卻轉向東北移動並登陸台南。本文使用MM5模擬妮蔻兒侵台過程，並探討登陸前導致妮蔻兒轉向之物理機制。木續示，模式對妮蔻兒侵台前的路徑、綜觀環境場及台灣局部環流特徵，都有合理之模擬。分析結果顯示，由於妮蔻兒為一既小且弱的颱風，較強渦度區(4x10(superscript -4 s(superscript -1))僅向上延伸至600hPa;因此，台灣地形作用所甲起的中低層(Ó=0.65~0.85)環流變化，為導致妮蔻兒於登陸前轉向之主要原因。
渦度收支分析結果顯示，颱風北移接近台灣地形時，系統中低層環流逐漸受地形影響，使系統中心東側有顯著局部輻合與較大正渦度變率。此外，模式渦旋環流呈橢圓形，具類似Vortex Rossby Wave 特徵，槽前之正渦度平流效應且呈氣旋式平移。透過渦度水平平流效應與上述局部輻合作用，使系統中心之東-東北方出現局部渦度變率極大值區，而導致系統轉為向東-東北方移動。另一想面，颱風與地形間合流效應產生的輻合強風速區之西南側(氣旋式風切側)，伴隨有較大範圍之正渦度變率區，似亦有幫助系統向地偏移的趨勢。在颱風即將登陸前，系統中心東南方之輻合區，與台灣西南部沿海受地形影響產生之帶伏輻合區合併，導致中低層輻合效應顯著增強，促使原向東北運動的颱風轉為向東南東方運動。

關鍵字

颱風 ； 颱風登陸 ； 颱風路徑 ； 數值模擬

並列摘要

In 1951-2001, there were 11 typhoons moving northeastward from northern part of South China Sea and affected Taiwan. Except Nichole(1998), with moving speed at about 3kt, other typhoons moved across Taiwan area in a relatively fast speed (mostly above 5 kt and averaged at about 10 kt). Nichole reached a maximum intensity of only 50 kt (or tropical storm intensity) with the radius of major cloudy area of about 150 km. While moving northward to the southern part of Taiwan Strait from the South China Sea, Nichole changed its moving direction to toward the northeast and made landfall on Tainan. This study simulated the motion of Nichole using MM5and discussed the physical mechanisms leading to the change of moving direction before landfall. Results showed that the model could simulate reasonably ewll the motion of typhoon before making landfall, the environmental flow and the local circulation in Taiwan. Analyses showed that since Nichole was a small and weak system, the region with strong vorticity (4x10(superscript -4 s(superscript -1)) extended upward only to 600 hPa. Therefore, the circulation changes at mid-low levels (Ó=0.65~0.85) induced by the Taiwan topography played a major role in the changes of system’s moving direction before landfall. Results of vorticity budget analysis showed that, when typhoon was moving northward approaching Taiwan topography, the mid-low level circulation was gradually influenced by the topography. Such topographical effect resulted in significant local convergence, together with large positive value of vorticity tendency, occurred to the east of the system center. In addition, the circulation pattern of the model vortex was elliptic, similar to that of a vortex Rossby wave. The positive vorticity advection ahead the wave trough tended to move cyclonically. The combined effect of the vorticity advection and the afore-mentioned convergence resulted in a maximum vorticity tendency located to the east-northeast of the system center and made the system to move toward the east-northeast. On the other hand, the confluence between the vortex and the Taiwan topography resulted in a strong wind region there. The area to the southwest (or the cyclonic shear side) of the confluence zone appeared to have large positive vorticity tendency. This might also have helped the system to shift toward the topography. Right before making landfall, the convergent area to the southeast of the system center combined with the topography-induced convergent zone located around the southwest coast of Taiwan. The mid-low level convergence to the southeast of the system center thus was significantly enhanced. Such enhanced convergence would have made the system change its moving direction from northeastward to east southeastward.

並列關鍵字

Typhoon ； Typhoon landfall ； Typhoon track ； Numerical simulation