瑞伯颱風(1998)之數值模擬探討

The Evolution of Typhoon Zeb(1998)in a Non-Hydrostatic Mesoscale Model

周昆炫(Kun-Hsuan Chou) ； 吳俊傑(Chun-Chieh Wu) ； 顏自雄(Tzu-Hsiung Yen)

《大氣科學》 29卷4期 (2001/12) Pp. 291-313

摘要

瑞伯颱風為1998年侵台颱風中造成最大災情之颱風，也造成菲律賓、台灣及日本等地區嚴重災情。此外，其生命期中亦包含一些值得深入研究的科學問題，如：1．與亞力士颱風的互繞、合併作用；2．其眼牆結構受地形影響所產生之變化；3．呂宋、台灣地形對其路徑、強度之影響；4．中緯度斜壓系統的牽引與交互作用。由於受限於觀測資料與全球模式分析資料（例如ECMWF）在空間、時間上解析度的不足，本研究採用數值模式（美國國家大氣科學研究中心與賓州大學合作發展的第五代中尺度靜力／非靜力模式－MM5）之模擬結果來探討部分上述科學問題。
本研究首先使用MM5三層巢狀之網格設定來模擬瑞伯颱風侵台期間之情形，以檢測模式對瑞伯颱風之掌握情形；另外亦針對地形影響的部分進行深入探討，我們設計移除呂宋、台灣地形的一系列實驗來凸顯地形對於颱風移動路徑、強度演變、降雨分佈的重要性；此外本文亦針對模式所模擬結果進行位渦反演，藉以瞭解中緯度槽線系統在瑞伯颱風運動過程所扮演的角色。
數值模擬結果顯示，本數值實驗可以反應出瑞伯颱風在侵台期間主要的生命期特徵，即登陸呂宋、轉向往台灣方向北移及槽線牽引加速往東北方向移動；此外，模式亦能呈現瑞伯颱風在登陸呂宋前後其眼牆碎裂與再生成之過程，另外亦能顯示當颱風北行至宜蘭外海時，在台灣東南部所生成之焚風現象。至於地形影響的部分，模擬結果顯示，台灣、呂宋等地形的存在對瑞伯颱風的路徑、強度、降水分佈具有相當程度的影響。就路徑而言，呂宋島的存在，會使得颱風移動變慢、路徑會有往移動方向左偏的現象，此路徑偏折現象與Bender et al.(1987)的研究一致；至於台灣島的存在，則同樣會讓颱風移動變慢，但路徑會有往移動方向右偏的現象。在強度方面，台灣、呂宋的存在皆導致瑞伯颱風的強度減弱。至於降水方面，模式模擬的降水分佈明顯受到地形解析度高低影響；至於在降水量的掌握上，模式所模擬的降水量與模式中颱風移動速度的快慢有顯著的相關性。
最後在模擬資料的位渦反演分析上，我們發現颱風在朝東北方向遠離台灣的加速過程中，中緯度斜壓系統一槽線，對瑞伯颱風有牽引的作用，此槽線系統所貢獻之流場提供瑞伯颱風一往北移動的分量，約佔環境駛流場之20%。

關鍵字

瑞伯颱風 ； MM5數值模式 ； 眼牆碎裂 ； 位渦反演

並列摘要

Typhoon Zeb was one of the most damaging typhoons that affected Taiwan, as well as Phillipine and Japan, in 1998. Besides the serious damages, several interesting scientific questions are also identified associated with Zeb:1)the merger process of Zeb and Typhoon Alex;2)eyewall evolution during Zeb's landfall at Luzon;3)effects of Luzon and Taiwan topography on Zeb; and 4)interaction between Zeb and the midlatitude baroclinic trough. Numerical integrations using the fifth-generation Penn. State/NCAR Mesoscale Model(MM5)were performed to address some of the above scientific issues.
In order to investigate the capability of MM5 model, we used triply nested grids to simulate typhoon Zeb as it approached Taiwan. A series of experiments with or without the terrain of Luzon or/and Taiwan was conducted to investigate the impact of the terrain of Luzon and Taiwan on the track, intensity and rainfall simulation. Potential vorticity inversion were also performed to evaluate the impact of the mid-latitude trough on Zeb's movement. Results indicate that MM5 can capture several features during Zeb's life period as Zeb approached Taiwan, such as Zeb's making landfall over Luzon, turning northward to Taiwan, and accelerating northeastward while interacting with the mid-latitude upper-level trough. Detailed analyses also indicate that the model simulation produced the main feature of eyewall breakdown and reformation during the period when Zeb moved over Luzon. The mesoscale phenomenon, Foehn, was also simulated over southeastern Taiwan when Zeb approached Taiwan.
It is shown that the presence of both Taiwan and Luzon affected Zeb's track, intensity and rainfall distribution over Taiwan. As to the track and intensity, both the presence of Taiwan and Luzon terrain tended to decelerate the movement of Zeb and to reduce its intensity. For rainfall simulation, the model produced similar rainfall distribution as compared to observation. It is demonstrated that the simulated rainfall distribution is affected by the model resolution, while the amount of simulated rainfall correlated negatively with the translation speed of Zeb. Finally, the PV diagnostics indicate that the mid-latitude upper-level trough led to a northward steering flow during the periods when Zeb accelerated northward and left Taiwan area. This flow accounted for about 20% of the total steering of Zeb.

並列關鍵字

Typhoon Zeb ； MM5 ； Eyewall breakdown ； PV inversion