2015年6月14日臺北盆地劇烈午後雷暴個案之高解析度模擬研究

Abstract

2015年6月14日臺北盆地午後暴雨事件為非常顯著的短延時強降雨，公館雨量站測得最大時雨量131毫米，許多其他測站也測得80毫米以上的時雨量，造成臺北市區多處嚴重淹水。五分山雷達(RCWF)觀測到此午後暴雨事件伴隨多重對流胞的合併。本研究使用高解析度的WRF 模式(最小水平網格間距為0.5公里)，討論在此個案中對流胞合併的物理機制，以及造成雙北都會區午後暴雨的物理過程。模擬結果顯示對流胞合併的物理機制為：(1) 海風環流與山區降雨的冷池外流增強低層輻合形成適合對流發展的中尺度環境條件；(2) 多重對流胞的冷池外流相撞增強低層氣流輻合，並且在相撞的冷池外流邊界前緣有顯著的水氣聚集(moisture patch)及水氣通量輻合，環境動力與熱力條件的雙重配合使得連接對流胞的雲橋持續發展，最終完成對流胞合併。 在雲微物理的特徵方面，對流胞合併後，雲內軟雹的混和比顯著增加，顯示存在旺盛的上衝流，並且由暖雨主導的過程轉變為混合相位降雨過程(mixed-phase precipitation process)所主導。除此之外，雲動力的特徵也有顯著改變，對流胞合併後在冷池外流相撞處冷池高度抬升約50%，冷池外衝風場(coldair outflow)增強，進而增強冷池外流風場和海風的低層輻合。 對流胞合併後雷雨胞複合體(merged thunderstorm complex)停留約30分鐘，呈現準靜止(quasi-stationary)的型態。潮濕的海風氣流持續被陣風鋒面所舉升，新的上衝流向後方(陣風鋒面)傾斜，並與舊的上衝流合併，形成更大範圍的上衝流，此時環境乾空氣的逸入作用較小，對流可以在垂直方向發展得更旺盛，因此在對流胞合併發生位置附近產生豪大雨。降雨增強以後，冷池外衝風場增強，此時雷雨胞複合體才開始明顯往北移動。 雲微物理敏感度實驗的測試結果顯示，雨水蒸發冷卻對於冷池強度扮演重要角色，若不考慮雨水的蒸發冷卻效應，則不會發生對流胞合併，強對流也不會由新北市山區移入臺北市平地，降雨區域僅會局限於新北市山區；軟雹溶解冷卻則相對而言影響較小，不過混合相位雲微物理過程仍然重要。地形敏感度實驗的測試結果顯示大屯山的存在透過狹道效應(channel effect)可以顯著增強通過淡水河谷的海風環流，進而增強海風環流與冷池外流的低層輻合，使得降雨增強。

On 14 June 2015, a severe afternoon thunderstorm event associated with cell merger developed within the Taipei basin, which produced an hourly rainfall of 131 mm/h and resulted in urban-scale flooding. Cloud-resolving numerical simulations were performed to capture reasonably well the development and evolution of the afternoon thunderstorms observed on that day. The mesoscale model WRF was used in this study with the horizontal grid size nesting down to 0.5 km and 55 vertical layers in order to explicitly resolve the deep convection. The merging between three intense convective cells was realistically reproduced by the simulations and the model results were in good agreement with radar observations. The low-level convergence was essential to provide the lifting mechanism necessary for the cell merger. The convergence between the cold-air outflow with see-breeze circulation, as well as the interactions between the two cold-air outflows associated with downdrafts, were the main factors that enhanced the low-level convergence. The formation and development of new convection from the cloud bridge was the main reason for the occurrence of the cloud merger. After the convective cells merged, cold pool heights elevated and cloud radii increased, resulting in this severe thunderstorm event. The influence of latent cooling by evaporation and melting on the occurrence of the cell merger was further analyzed. Evaporation cooling played an important role in the cell merger process, whereas melting cooling played a relatively minor role. However, ice-phase microphysics was still important. The experiments with the removal of local topography (Mount Datun) indicated that the channel effect by Mt. Datun intensified the sea-breeze circulation and then enhanced the low-level convergence within the Taipei basin.