台北盆地午後熱對流降水集中區形成機制之探討

Abstract

本研究使用三維渦度向量方程式的雲解析模式(VVM)，搭配500公尺的水平解析度來探討台北盆地午後熱對流降水集中區的形成機制。主要的研究目標集中在研究從盆地兩側的河谷進來的海風以及熱島效應，對於降水發生區域與強度變化的影響。在理想化地形幫助下，我們設計了三組實驗，以突顯河谷與熱島效應對於形成台北盆地南邊降水集中區的重要性。控制組實驗有兩個河谷，以及在盆地有較高的Bowen ratio，結果顯示控制組實驗能掌握在盆地南邊觀測到的降水集中特徵。另一方面，在河谷封住的實驗中，因為海風無法在盆地中間輻合，使得此降水特徵無法在此實驗被觀察到，這個結果也顯示單純的熱力作用無法在盆地中激發對流，進而產生降水集中的現象。而在移除熱島效應的實驗當中，盆地降水因為海風環流的減弱而減少。另外，本研究對於降水時間以及背景風風向所造成的影響也有更詳細的討論。

In this study, we use a three-dimensional Vector Vorticity Equation Cloud-resolving Model (VVM) with high horizontal resolution (500 m) to investi¬gate precipitation hotspots of afternoon thunderstorms in Taipei Basin. The major focus is on the changes of locations and intensity of surface precipitation caused by the sea breezes from the two valleys and the urban heat island effect on locations and intensity of sur-face precipitation. Three experiments with idealized topography are performed to un-derstand the importance of valleys and the urban heat island effect on the precipitation hotspot at the south of Taipei Basin. The control experiment has two valleys and higher Bowen ratio in the basin. The results of the control experiment are comparable to obser-vation in that the precipitation hotspots occur at the south of the basin. On the other hand, the hotspot diminishes when we close the valleys because the convergent zone cannot be established by sea breezes suggesting that the thermal instability alone cannot produce precipitation hotspots in the basin. Furthermore, when the urban heat island ef-fect is reduced from control experiment, the result shows that the surface precipitation in the basin decreased through weakening the sea-breeze circulation. Detailed analyses on the timing of precipitation and impacts of wind direction are also discussed.