| 摘要: | 台灣位於亞洲大陸東南方，夏天盛行西南風，容易帶來豐沛的水
氣，加上白天海風發展，兩者與地形交互作用容易產生午後熱對流。
然而，因複雜的地形又缺乏密集的觀測，過去對午後熱對流發展與演
變的相關研究相當有限，故本研究利用WRF 以及2017 雙北都會區
夏季暴雨觀測預報實驗(TASSE)的觀測資料來分析與模擬2017 年7
月1 日午後熱對流強降雨事件的發展及其相關的物理過程。
觀測資料顯示此個案之對流系統在中央山脈及雪山山脈迎風處
開始發展並逐漸的往北移動發展，為北部地區帶來降雨，時累積雨量
超過100 mm，造成許多地區的道路淹水。
本研究發現，受到綜觀環境的影響，偏南風的盛行風將暖濕的空
氣輸送至台灣附近，提供大氣不穩定條件。TASSE 觀測資料分析顯
示，發生午後熱對流時海風發展的高度較高，且有明顯的對流不穩定
層。比對觀測資料與模擬結果顯示，受環境風場與地形產生的繞流及
海風發展的影響，台北盆地與北部山區有明顯風場輻合，午後對流系
統於13LST 在山區形成，透過系統發展位置的垂直剖面可以看到，
風場在山區迎風面與地形交互作用，產生輻合與抬升有利於對流之形
成，從本個案的分析模擬發現海陸風發展及地形效應是對流系統生成
的重要關鍵。
接著透過模式進行地形與地表熱通量的敏感度測試，發現當缺少
地形阻擋，環境風場能夠直接影響陸上風場，使得系統發展的位置較
靠近沿岸，同時因為少了地形抬升，降水強度也弱了許多。當地表熱
通量移除後，海陸溫差變小，熱力環流減弱，故海陸風發展不易，對
流系統主要來自環境風場與地形的交互作用而形成，導致降雨區只集
中在山區迎風面，故透過模式的敏感度測試可以了解地形與地表熱通
量對於午後熱對流的重要性。;Taiwan is located in Southeast of Asia, prevailing southwest monsoon wind, and developing sea breeze usually affect local weather during daytime. Both factors interact with terrain providing a favorable condition to develop afternoon convection. However, due to lack of intensive observation stations and long-term data, related studies on the afternoon convection and local circulation are limited. In this study, WRF and observation data of TASSE were used to simulate and analyze the key factors resulted in the development of the afternoon convection occurred on July 1, 2017.
According to the composite reflectivity images on July 1, 2017, convective system began to develop in the windward of the Central Mountain Range and Snow Mountain Range, and gradually moved and spread northward. It brought rainfall to the north of Taiwan, and the maximum hourly accumulated rainfall exceeded 100 mm, caused flooding in many roads of towns. This paper found that under the influence of weak synoptic environment, the south component of wind brought warm moist air to Taiwan, provided the conditions of the atmospheric instability. The analysis of TASSE observation data showed that the height of the development of the sea breeze was higher,
also had obvious convectively unstable layer. After comparing observations with simulation results showed that convective systems organized at 13 LST in the mountains. Meanwhile, the model result showed that topographic affected surrounding flow and interacted with the development of the sea breeze, there is obvious
convergence wind flow in Taipei basin and northern mountain. We can see the wind field on the windward interacts with terrain and cause uplifting through the cross section analysis of the convective system position. From the analysis of this case, the key factors responsible for the generation of the system is the development of sea breeze and topographic effects.
Based on the sensitivity tests of terrain and surface heat flux, we found that the environmental flow can directly affect the onshore wind field when there is no
terrain blocking, and make the position of convective system closer to the coast. Meanwhile, because of the lack of the terrain, the precipitation intensity is much smaller. After the surface heat flux being removed, the thermal circulation is weakened. Thus, the sea and land breeze is difficult to develop, most of the convective system is resulted from the interaction between the environmental wind field and the terrain, leading to
the rain area only concentrated in the windward side of the mountain. Through the sensitivity test of the model, we could understand the importance of these two factors
for afternoon heat convection. |
