大屯山區冬季山谷降水加強之個案研究

Abstract

本研究的目的在利用密集的地面觀測、五分山都卜勒雷達與WRF數值模式，來分析探討2018年12月13~14日大屯山區山谷降水加強之機制。大屯山為一座小尺度的凹向性山脈，由兩個山臂與一個山谷所組成，其水平尺度約為15公里，高度約為1公里。地面密集的雨量觀測顯示個案期間降水極值的位置常落在山谷位置，而地面與雷達觀測分析顯示在兩個山臂迎風坡都有明顯的風場分流特徵。 透過WRF模式模擬更進一步發現，山谷降水加強伴隨著明顯的氣流輻合區，此輻合主要與前述山臂前緣分流的側向輻合有關，且其強度隨高度有所變化。山谷地區最顯著的側向輻合是位在山脈中層的高度(~500公尺)，可能由於兩個山臂的距離在此高度恰好有利於引導氣流輻合。垂直速度的定量診斷分析顯示側向輻合在山谷降水極值出現的期間貢獻約38.4%的垂直速度，稍微大於地形強迫舉升之貢獻度(約32.4%)。大屯山區上游的局部Froude number具有顯著的時空變化，當上游Froude number較小時易在山臂前緣出現氣流分流特徵，山谷附近的低層風場與降水的演變受到兩個山臂分流程度差異性的影響甚鉅。

This study investigates the mechanisms responsible for valley precipitation enhancement over Da-Tun Mountain (DT) observed on 13-14 December 2018, using detailed surface and Doppler radar observations and numerical simulations. DT is a small-scale concave ridge comprising two ridge arms and a concave valley. The horizontal scale of DT is around 15 km, and the mountain peak of DT is around 1 km MSL. Rainfall measurements from dense rain gauge network show that local rainfall maximum frequently falls within the valley in the present case. Analyses of surface and radar-observed winds indicate that significant flow splitting signal is present at both ridge arms over the duration of the present case. Case simulations by the WRF model further reveal that the precipitation over the valley is closely related to lateral convergence produced by the splitting flows over the ridge arms. The most prominent lateral convergence is found near the middle of the mountain layer (~500 m MSL), presumably due to a suitable separation between the two ridge arms at this altitude. Quantitative diagnoses of vertical motions indicate that the lateral convergence contributes ~38.4% to the total vertical velocity valid inside the valley, which is slightly greater than that contributed by upslope lifting mechanism (~32.4%). The local upstream Froude number calculated is varied temporally and spatially, and the flow splitting tends to occur in areas with low upstream Froude number. The splitting flows over the two ridge arms, with differential characteristics and intensities, appear to interact with each other to produce evolving, complicated nature of airflow and precipitation over DT.