2015年6月14日台北盆地劇烈午後雷暴個案研究：對流胞合併機制與強降雨過程探討

Cell Merger and Heavy Rainfall of the Severe Afternoon Thunderstorm Event at Taipei on 14 June 2015

繆炯恩(Jyong-En Miao) ； 楊明仁(Ming-Jen Yang)

《大氣科學》 46卷4期 (2018/12) Pp. 427-454

https://doi.org/10.3966/025400022018124604004

摘要

2015年6月14日台北盆地劇烈午後雷暴個案伴隨對流胞的合併，造成非常顯著的短延時強降雨（最大降雨率198mm h^(-1)）和都會尺度淹水。為了合理地掌握此個案對流胞的發展、合併過程和冷池的特徵，本研究進行次公里網格間距（subkilometer-grid size）WRF模式模擬。WRF模式設定使用四層巢狀網格（最小水平網格間距0.5公里）和垂直55層，希望可以直接解析雷暴系統內的對流胞。模擬對流胞與雷達觀測的比較方面，除了位於台北盆地中央的對流胞沒有模擬到之外，其他4個對流胞WRF模擬皆有合理的掌握。軌跡分析的結果顯示，胞A和胞B進行單次胞合併（single merger）形成胞A＋B，其物理機制為在平地和山麓的對流胞移速相異導致的後方碰撞（rear-end collision）。隨後胞A＋B和胞C進行多重胞合併（multiple merger）形成胞A＋B＋C，其合併之物理機制為對流胞降雨產生方向相反冷池外流的正面碰撞（head-on collision）。發生多重胞合併後，最大上升速度顯著增強，冰相粒子（主要為軟雹）質量成長6倍，冷池厚度由100公尺抬升至750公尺；由於雨滴蒸發與軟雹溶解的作用，多重胞合併後冷池移動速度明顯增大。簡言之，多重胞合併的物理過程對於台北盆地的短時劇烈降雨和都會尺度淹水，扮演著重要的角色。

關鍵字

午後對流 ； 對流胞合併 ； 冷池

並列摘要

On 14 June 2015, a severe afternoon thunderstorm event associated with cell merger developed within the Taipei basin, which produced intense rainfall rate (at the rate of 198 mm h^(-1)) and urban-scale flooding. Cloud-resolving WRF simulations were performed to capture reasonably well the development of convective cells, several cell mergers, and cold-pool characteristics of this event. The WRF model had four nested grids (with the finest grid size of 0.5 km) in the horizontal and 55 layers in the vertical to explicitly resolve deep convective cells within the storm. In comparison with radar observations, five intense convective cells except for one cell in central Taipei were realistically reproduced by the WRF simulation. From trajectory analyses, the single merger between cell A and cell B to generate the merged cell ＂A+B＂ at 1300 LST resulted from rear-end collision associated with different cell propagation speeds over the basin plain and mountain foothill. Later, the multiple merger between the merged cell ＂A+B＂ and cell C at 1320 LST was due to head-on collision between the precipitation-induced outflows in opposite direction. After the multiple merger, the cold-pool depth was elevated from 100 m to 750 m, the maximum updraft significantly increased, ice-phase condensate (mainly graupel) was increased by 6 times, and cold-pool propagation speed was substantially increased by the contribution of raindrop evaporation and graupel melting. The multiple cell merger was responsible for the intense rainfall rate and urban-scale flooding at Taipei.

並列關鍵字

Afternoon thunderstorm ； Cell merger ； Cold pool