2009年8月7日莫拉克颱風侵襲台灣地區，7日16Z颱風中心由台灣東部花蓮登陸，8日06Z於桃園出海，但累積降雨最大值卻發生於台灣中南部山區。根據台灣氣象局阿里山傳統測站資料顯示72小時(8月7日00Z至8月10日00Z)累積雨量高達2682.5mm，造成台灣中南部、東部多處雨量刷新歷史紀錄，引發多處土石流災害，為臺灣地區50年以來最嚴重的水災。根據天氣分析結果指出，颱風引發之環流、地形抬升作用和水氣充沛的西南氣流之交互作用是造成此劇烈降水的主要機制。再加上颱風緩慢的移動速度，長時間影響台灣地區而產生極端降雨量。 本研究利用美國國家大氣研究中心(NCAR)發展的WRF(Weather Research and Forecasting)數值模式進行72小時的模擬，使用水平網格解析度18、6、2公里為此高解析度巢狀網格之設定。控制組(CTRL)結果顯示模式可以掌握到台灣中南部山區觀測到的累積雨量最大值與降雨分佈。在模擬結果中，颱風強度在模擬24小時至72小時之間接近實際值，另外模擬颱風路徑亦接近觀測，得模擬累積雨量最大值和分佈接近觀測結果。本研究另外設計台灣地形作用、西南氣流水氣傳送以及網格解析度三個敏感度實驗，探討對此劇烈降水事件的影響。在降低台灣地形高度至50%(ter50)以及1%(ter01)之敏感度實驗中，模擬累積雨量最大值明顯減少50%以及80%，ter01實驗之降雨量最大值則分佈於南部沿岸地區。證明台灣地形是影響降雨最大值與分佈的重要因素之一。此外計算台灣西南部地區之平均上升速度顯示，CTRL與ter01實驗之低層平均上升速度相差約5倍，顯示地形抬升作用的重要性。將一部分西南氣流之相對濕度降低50%，得72小時累積降雨量最大值減少約20%，台灣西南部山區的降雨強度均有減少，由此顯示西南氣流水氣的傳送亦是造成莫拉克颱風劇烈降水的重要因素之一。此外透過低水氣量區域的移動軌跡，可以了解台灣西南方海面上水氣經過台灣南部傳送至莫拉克颱風環流，進一步降低莫拉克颱風之水氣含量。將模式網格加粗2倍以及4倍之敏感度實驗，粗的解析度模擬難以解析小尺度對流胞的發展以及降水。較粗的地形解析度造成較弱的上升速度，進一步導致粗的解析度敏感度實驗之72小時累積雨量值低於CTRL。 Typhoon Morakot struck Taiwan on 7 Aug 2009. Although the center made landfall in Hualien county along the central east coast of Taiwan and passed over the central northern part of the island, it was southern Taiwan that received the storm where locally as much as 3000mm of rain were reported resulting in the worst flooding there in 50 years. The Weather Research and Forecasting model (WRF) is used to conduct a 72 hours model integration for high resolution simulation of this high-impact weather event. The horizontal grid resolution of nested domains is 18, 6 and 2 km. The result indicated that the model is able to capture the observed surface rainfall both the total amount and the location. The simulate typhoon intensity was close to the observation between 24 - 72 hours. Three sensitivity experiments were designed for Taiwan terrain height, southwest water vapor transmission and model grid resolution. The results were shown that the 72 hours maximum accumulated rainfall of simulation decreased to 50% and 80% as terrain height was reduced to 50% and 1%. The average vertical velocity on southwest region of Taiwan indicated that the vertical velocity of CTRL is 5 times the magnitude of ter01 in the low level. It confirmed that terrain height is an important factor for rainfall distribution and dynamic process. As relative humidity of southwest flow was reduced 50%, the rainfall intensity was decreased and the 72 hours maximum accumulated rainfall was declined about 20%. It demonstrated that the water vapor transmission from southwest flow is also an important factor of heavy rainfall causing from Typhoon Morakot. When model grid resolution was reduced to 1/2 and 1/4 times from CTRL, the coarse-resolution simulations was difficult to resolve small-scale convection and precipitation. On the other hand, the simulation of coarse-resolution has weak vertical motion caused by terrain-resolution and led the 72 hours accumulated rainfall of simulation was less than CTRL.
