選取1995年到2009年間秓季，未受到綜觀擾動直接影響下，在宜蘭蘇澳地區共有14個大豪雨(>200 mm/day)的個案發生，可供研究東北部地形如何影響豪雨之產生及維持。在2009年10月11日，宜蘭地區的日累積雨量為631.5 mm，是14個個案中最大的降雨事件。除了盛行風所帶來的水汽之外，位於南海往海南島前進的颱風芭瑪(PARMA)，在10月11日時距台灣約721 km，雖無直接侵台，但其造成影響台灣東北部降雨仍值得研究。因此10月11日宜蘭地區產生631.5 mm的降雨機制，為本研究的主題。 利用中央氣象局傳統測站資料、五分山雷達回波、衛星資料以及ECMWF/ TOGA ( European Centre for Medium -Range Weather Forecasts/Tropical Ocean and Global Atmosphere)等觀測資料分析10月11日的豪雨特性。吾人發現台灣東北部的上游(125 E，25 N)在1400 LST前都有大量向西傳的水汽通量，同時台灣東北部低層的輻合，高層的輻散，提供豪雨發生及維持的有利環境。另外每6分鐘的雷達資料指出，台灣東北部的降雨多在蘭陽平原南側山區發生，並且從台灣東南部北移的雨帶加強蘭陽平原的降雨。日降雨量631.5 mm發生於蘭陽平原的南側斜坡，而蘭陽平原西側斜坡的降雨最大值僅為南側斜坡的15%。 利用WRF模式2.2.1版(Weather Research & Forecasting Model version 2.2.1)的台灣真實地形(CTL_run)模擬出與10月11日觀測類似的降雨環境。宜蘭平原的南側斜坡除盛行風持續的水汽抬升外，在平原南側低層的偏北風亦加強水汽抬升，故降雨在南側斜坡增加很快，是豪雨產生及維持的機制。降雨受到盛行風東風影響，會往內陸(西邊)移動，但因持續的大雨使得南側斜坡上產生了冷池，且有一擾動壓力較大之區域，降雨隨著冷池及擾動壓力梯度往低處(向東)移動。另外在台灣東南部近海的雨帶隨著時間會往北移動，加強台灣東北部的降雨。在宜蘭平原的西側斜坡因其斜坡走向與盛行風風向大致相同，盛行風的抬升較少，且在清晨低層有輻散，不利地形的抬升，降雨較少。由移除台灣地形(NT_run)的模擬實驗，發現其台灣東南部近海的雨帶仍會出現。但無地形效應，宜蘭蘇澳地區的降雨減少了一個數量級以上。而填平蘭陽平原(L-Y_run)的模擬，則發現在少了蘭陽平原的南側斜坡，在原本的斜坡上降雨減少了相當多，可見南側斜坡抬升來自平原的水氣所產生的降雨，亦是相當有貢獻的。 To isolate the orographic effects over Lan-Yang plain and its surrounding areas of northeastern Taiwan due to the northeasterly prevailing wind leading to produce heavy rainfall in Autumn, 14 extremely heavy rainfall (>200 mm day-1) events over northeastern Taiwan are identified when no synoptic disturbances are within 500 km over Taiwan during the period from1995 to 2009. Among there 14 events the maximum accumulated rainfall was 631.5 mm which appeared on October 11, 2009. The mechanism of generating such a very high accumulated rainfall in Lan-Yang plain will be pursued in this study. The objective of this study is to perform observation analyses of the rainfall data of Central Weather Bureau(CWB) rainfall station, WU-FEN-SHAN(WFS) 6-minute interval radar reflectivity, satellite image and ECMWF/ TOGA ( European Centre for Medium -Range Weather Forecasts/Tropical Ocean and Global Atmosphere) data to examine the heavy rainfall mechanism on October 11 2009. A lots of water vapor flux transported westward before 14 LST to the upstream of northeast Taiwan (at 125 E, 25 N) . The low-level conveged and high-level diverged over northeast Taiwan,also provided a favorable environment for producing and maintaining the heavy rainfall event. Besides, the radar data indicated that northeast rainfall mostly occurred over the southen slopes of Lan-Yang plain and enhanced by east-west oriented rain band from southeast of Taiwan. Daily rainfall maximum 631.5 mm also occurred on southward slope of LY plain and the accumulated rainfall over westward slope of Lan-Yang plain was 15% to that of southward slope. iv Weather Research & Forecasting model (version 2.2.1) is employed to simulate the heavy rainfall event which is initialized on 14 LST 10 October 2009. The mechanism for the development and maintance of heavy rainfall is that the orographic lifting moist air by prevailing easterly wind persistently on the southern slope of Lan-Yang plain. In addition, the low level northeasterly wind due to orographic deflection on prevailing wind over southern Lan-Yang plain also enhances rainfall. Rainfall moves westward following with the prevailing wind, but also moves upstream because of the cool pool which was caused by continuous rainfall on southern slope of Lan-Yang plain and local high pressure disturbance. Over western slope of lan-Yang plain, the orientation of terrain parallels to the prevailing wind. As a result, rainfall accumulation is about 30 % of that in the southern slope of Lan-Yang plain. For the noTaiwan terrain(NT_run) case, there was also an east-west oriented rain band over the adjacent sea of southeast Taiwan. Without the orographic lifting, rainfall is reduced at least one order in Ilan and Suao area. In the filled Lan-Yang plain case in which terrain height lower than 500 mm parallels to coast of Lan-Yang plain. Less rainfall occurred because of the reducing lifting and water vapor from Lan-Yang plain. It is quite obvious that moist air lifted by southern slope of Lan-Yang plain has great contribution to heavy rainfall.
