臺北盆地東北季風時期之低空風切研究

Abstract

低空風切是影響飛行起降安全因子之一，其定義為600公尺以下的高度，出現水平方向15 kts/km的風切，並且持續存在10秒鐘以上；部份機場能透過低空風切警示系統(Low Level Windshear Alert System, LLWAS)的建置，來監測低空風切。然而，臺北盆地受到綜觀環境和地形的影響，所產生的低空風切可能不只在松山機場現有的LLWAS監測範圍內，在東北季風的影響之下，低空風切可能會發生在臺北盆地西側和西北側的區域。 為探討臺北盆地西側的低空風切，本研究在臺北盆地西側，新增了四個氣象觀測點和一套雲冪儀，建立臨時性氣象觀測網，並進行60天的高時間解析觀測。觀測資料經過資料處理和推算後，能使用「三角形遞迴運算法」得到達低空風切閾值的輻合、輻散值，藉以觀測250公尺高空的低空風切。在觀測期間該觀測網成功地觀測到低空風切，觀測發生低空風切的時間和松山機場LLWAS警報紀錄相近。此外，藉由輻合和輻散呈現負相關的兩個區域，討論氣象觀測網上空的氣流走向會有偏轉的情況。 為了解數值模式對低空風切的模擬效果，本研究使用中尺度氣象模式WRF和高解析度計算流力模式STREAM，對東北季風期間臺北盆地低空風切事件進行模擬比較。WRF模式利用超過低空風切閾值的渦度場和輻合、輻散場，推測低空風切時空分佈，在1公里解析度的模擬結果中，受綜觀環境條件和地形的影響下，WRF模式能成功偵測低空風切的發生；2010年10月25日東北季風增強的個案，WRF模式成功模擬出和松山LLWAS警報紀錄時間相近的低空風切，而且涵蓋臺北盆地西側氣象觀測網和部份松山機場區域的上空。STREAM模式使用高解析度臺北盆地地形和固定的邊界風場，模擬強盛東北季風進入臺北盆地時的風場結構，能顯示出臺北盆地中的強風速區空間位置，以風場的輻合和偏轉現象，進一步確認低空風切可能發生的區域。

Low-level wind shear is one of the important factors of flight safety factors. Low-level wind shear is defined as 15 kts/km wind vector difference which persists more than 10 seconds between surface and 2,000 feet (600 m) height. At some airports, low-level wind shear could be detected by Low Level Windshear Alert System (LLWAS). However, due to the synoptic environment and topographic effect of Taipei Basin, the low-level wind shear phenomena may occur outside the LLWAS detecting range. Through the influences of northeast monsoon, low-level wind shear might occur at western and north-western regions of Taipei Basin. In order to study low-level wind shear over the western region of Taipei Basin, a field network observation in high temporal resolution with 4 mobile weather stations and one ceilometer lidar lasting for two months were conducted. After data quality control and wind field estimation at 250 m height, the divergence/convergence values in which the strengths are over the low-level wind shear's threshold were estimated by 'triangle recursion calculation method.' During the observation period, the observatory network detected several cases of low-level wind shear which corresponds Song-Sang Airport (RCSS) LLWAS warning record well. Moreover, by means of a negative correlation existed in the divergence/convergence field between two regions, the air streams over the observatory network are deflected. To understand the performance of numerical model simulation on low-level wind shear, Weather Research Forecasting (WRF) model and high resolution computational fluid dynamics STREAM model are used to simulate and discuss cases of low-level wind shear at Taipei Basin during the northeast monsoon period. The WRF numerical model can simulate significant vorticity field and divergence/convergence field in which the strengths are over the low-level wind shear's threshold. The results show that WRF model with 1-km resolution simulation can capture the low-level wind shear successfully by the influence of topography under various synaptic environment conditions. In the strong northeastly wind case on Oct. 25th of 2010, the simulation results show that the hot spots of low-level wind shear locate at western and north-western regions of Taipei Basin including part region of RCSS. The results are consistent with the low-level wind shear alerts of RCSS. The STREAM model uses high resolution topography data and gives stationary boundary wind to simulate the wind structure during the strong northeast monsoon blowing into Taipei Basin. The results show the location of strong wind region at Taipei Basin. With the convergence and deflection of wind, the possible occurring region of low-level wind shear may be further confirmed.