地形影響颱風路徑偏折之理想數值模擬及動力機制探討

觀測及數值模擬資料均顯示，當颱風在接近地形時，其路徑會受到地形的影響而產生偏折。然而，在過去的研究中，針對這個問題的原因及其機制探討卻相當有限，因此仍然存在著許多疑問及不確定性。本研究使用MM5中尺度區域模式進行高解析度的理想數值模擬，探討西行颱風遇到地形時向南偏轉的物理機制；結果顯示，與過去地形影響颱風路徑相關的研究結果相比，在控制組實驗中，當颱風接近地形時，即使沒有因颱風與地形間的氣流合流而產生顯著的「通道效應」，颱風仍然會向南偏轉。其地形引起的颱風中層西側風速增加及中層東側風速減少，進一步使得颱風內部不對稱流場其向量指向南方的現象，是使得颱風向南偏轉的主要原因，而此中層東側風速減少的現象與南北向風速的垂直傳送減少有關。另外，藉由一系列的敏感性實驗發現：當不同的參數，如地形較高、較壯，或是颱風較弱、移速較慢時，颱風接近地形時向南偏轉現象會較明顯。控制組實驗及敏感性實驗的結果均顯示，當颱風接近地形時產生的偏轉運動，和地形引起的颱風內部不對稱流場有關，此不對稱流場的改變與不同參數及其大小之關係，亦是探討的重點。透過上述敏感性實驗的分析我們也更了解通道效應發生與否的流制(flow regime)及其形成的關鍵機制，例如地形越高或是颱風是以偏北的位置接近地形，會發生比較顯著的通道效應現象；並針對地形影響颱風路徑偏折問題上，提供新的思路及詮釋。

關鍵字

颱風；地形；路徑偏折；不對稱流場；通道效應；無因次參數

並列摘要

Both observational and numerical studies have shown that tropical cyclones (TCs) tend to deflect when approaching or passing over a mesoscale mountain range. Observations, particularly those from the in-situ radars, have documented a number of typhoons experiencing track deflection near the east coast of Taiwan. In order to further assess the orographic influence on TC track deflection, more parameters and flow regimes are investigated in this study. A full-physics model (MM5) is utilized to construct a set of idealized experiments at 3-km horizontal resolution. In the control experiment, we simulate a westward-moving TC approaching a bell-shape terrain with 3-km maximum height. Different from the results of previous studies, the low-level northerly jet induced by the channeling effect in the western side of TC is not identified. Instead, it is shown that the mid-level wind magnitude is strengthened in the western side of TC, and is weakened in the eastern side of TC, contributing to the southward TC deflection. Further parameters describing the idealized vortex and mountain, including TC intensity (Vmax), translation speed (U), radius of maximum wind (R), initial position, incident angle, mountain height (h), x-direction mountain range (Lx), y-direction mountain range (Ly) and mountain shape, are fully examined. The results suggest that some parameters (such as the TC intensity, translation speed, mountain height and shape) are playing mportant roles in causing the track deflection. By analyzing the results of control experiment and sensitivity experiments, we find that the channeling effect would appear in some specific flow regime (such as TC approaching to the higher terrain or north part of the terrain). The idealized experiments from this study provide better physical insight into how TC movement is influenced by topography. Further researches are needed to investigate the key mechanisms leading to the topographic deflection of TC track.

並列關鍵字

Typhoon ； tropical cyclone ； topography ； track deflection ； asymmetric flow ； channeling effect ； non-dimentional parameter

參考文獻

______, R. A., and T. T. Warner, 1978 Development of Hydrodynamic Models Suitable for Air Pollution and Other Mesometerological Studies. Mon. Wea. Rev., 106, 1045-1078.

Bender, M. A., R. E. Tuleya, and Y. Kurihara, 1987 A Numerical Study of the Effect of Island Terrain on Tropical Cyclones. Mon. Wea. Rev., 115, 130-155.

Brand, S., and J. W. Blelloch, 1974 Changes in the Characteristics of Typhoons Crossing the Island of Taiwan. Mon. Wea. Rev., 102, 708-713.

Chan, J. C. L., and W. M. Gray, 1982 Tropical Cyclone Movement and Surrounding Flow Relationships. Mon. Wea. Rev., 110, 1354-1374.

Chang, S. W., and R. V. Madala, 1980 Numerical Simulation of the Influence of Sea Surface Temperature on Translating Tropical Cyclones. J. Atmos. Sci., 37, 2617-2630.

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地形影響颱風路徑偏折之理想數值模擬及動力機制探討

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