梅雨季台灣西南部地形對潮濕氣流影響之初步研究

林秉承(Ping-Cheng Lin)

指導教授 ： 陳景森

國立中央大學/地球科學學院/大氣物理研究所/碩士(2008年)

摘要

台灣梅雨季大約發生在5月下旬至6月上旬（Chen and Chen 2003）。此時台灣海峽及南海北部附近盛行西南~西南西風。台灣是一海島，山區面積佔整個台灣約三分之二。在梅雨季時台灣西南部為迎風面，常發生豪大雨 (Chen et al. 2007)。1997~2007年11年內各年梅雨季月平均雨量大部份降雨之最大值都在台灣西南部地形斜率為0.3以上的山坡地，高度500 m或是在1.5~2 km間。地形對降雨型態影響的數值研究（Chu and Lin 2000, Chen and Lin 2005, Chen et al. 2007），指出Froude number是一個重要的參數。而Smith（1979）指出山的高寬比h/a會影響降雨型態。台灣西南部地形高寬比及地形斜率比Chen and Lin (2005)所用的地形還大一個數量級。本研究利用Weather Research and Forecasting Model（WRF）模式及一個理想探空資料，探討台灣西南部複雜及陡峭的地形如何影響潮溼氣流的特性及降雨分布。
模擬結果發現在低濕夫如數（0.14~0.28）（ ， 為環境風速， 為 、 為虛位溫， 為山高），未飽和氣流受到西南部地形阻擋而繞山，當氣流飽和時浮揚頻率平方值變小或負值，氣流變較不穩定，可以從繞山體系變成爬山的體系。
濕夫如數較大時（0.28）因氣流飽和而變成不穩定，為爬山體系，則降雨大值的位置發生在較高的山坡(1.5 km~2 km)，與2000及2006年梅雨季月平均雨量較大值位置的高度類似，在台灣西南部外海及平原區因氣流受地形影響而產生的輻合有利於降雨的發生。而濕夫如數越小則受地形阻擋效應較明顯，當濕夫如數為0.19時，降雨大值的位置在較低的山坡(500 m)，與2001，2002，2004及2005年梅雨季月平均雨量較大值的位置類似。濕夫如數為0.14時，在平地的較大降雨區域因盛行風西南西風及受地形阻擋轉變成南風的繞山氣流在平地輻合，近似滯留在平地區域累積降雨。在500 m地形前較低位置的較大降雨則是在平地附近發展，隨著盛行風受地形影響轉變成西偏西北風，降雨往東南移動至500 m地形前位置累積降雨，此降雨情形與2008年5月29日台灣屏東地區降雨情形類似。

關鍵字

地形對潮濕氣流影響

並列摘要

The Mei-Yu season over Taiwan occurs during 15 May-15 June (Chen and Chen 2003). During this period, the region of the South China Sea, the Taiwan Strait, and Taiwan is under the influence of the prevailing southwest or west southwest flow. Taiwan is an island, two-thirds of the island of Taiwan is mountainous. The windward side of Central Mountain Range (CMR) locates at southwest of Taiwan during the Mei-Yu season. The occurrences of maximum heavy rain (rainfall rates > 15 mm and daily accumulations > 50 mm) are over the southwestern lowlands and slopes of the CMR (Chen et al. 2007). During 1997 to 2007, the annual maximum averaged rainfall region of Mei-Yu season locates at slope greater than 0.3 in the region of altitude about 500 meters or 2000 meters. The numerical simulation of effects of orography on a rainfall patterns (Chu and Lin 2000, Chen and Lin 2005, Chen et al. 2007), indicate that Froude number is an important index. The aspect ratio (h/a) affects rainfall patterns (Smith 1979). The slope and aspect ratio of southwest of Taiwan terrain are greater than that of Chen and Lin (2005) at least in an order of magnitude. We use WRF model and ideal sounding data in order to realize the effects of southwest of Taiwan terrain on a moist flow and rainfall pattern.
The model result show that low moist Froude number(0.14~0.28) ( ， is wind speed， is 、 is virtual potential temperature， is mountain height), unsaturated moist flow is blocked by southwest of Taiwan terrain and becomes a flow-around regime. When moist flow is saturated, the buoyancy frequency diminish or become negative that moist flow becomes unstable and shift to flow-over regime.
When moist Froude number is 0.28 and flow is saturated over slopes, the maximum accumulation rainfall region over the height about 1500 to 2000 meters region. This kind of rainfall pattern is analogous the maximum average rainfall region of Mei-Yu season in 2000 and 2006. When moist Froude number is 0.19 and orographic blocking is evident, the maximum accumulation rainfall region is over the height about 500 meters. This rainfall patterns resemble the maximum average rainfall region of Mei-Yu season in 2001, 2002, 2004 and 2005. When moist Froude number is 0.14 that maximum accumulation rainfall region is over the plain and the height about 500 meters region. The rainfall over plain of rainfall is influenced by the convergence region of west southwest wind and south wind due to flow-around the terrain. The rainfall over the height about 500 meters region is the rainfall grow up at the plain and move inland by northwest wind. This rainfall patterns may resemble the 29-May 2008 over the Pingdong area.

並列關鍵字

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參考文獻

彭文君，2006：梅雨季台灣西南部豪雨之初步研究（1997年 ~ 2004年）。國立中 央大學，大氣物理研究所碩士論文，122PP。

蘇炯瑞，2007：2005年梅雨季6月12號台灣西南部豪雨之分析。國立中央大學，

Akaeda, K., J. Reisner, and D. Parsons, 1995: The role of mesoscale and topographically induced circulations initiating a flash flood observed during the TAMEX project. Mon. Wea. Rev., 123, 1720-1739.

Chen, C.- S. and Y.-L. Chen, 2003: The rainfall characteristics of Taiwan., Mon. Wea. Rev., 131, 1323-1341.

Chen, C.- S., W.-C. Chen, Y.-L. Chen, P.-L. Lin, and H.-C. Lai, 2005: Investigation of orographic effects on two heavy rainfall events over southwestern Taiwan during the Mei-yu season. Atmospheric Research, 73, 101-130.