區域空氣擴散數值模式之應用研究

Application Studies of a Regional Air Dispersion Numerical Model

黃清勇(Ching-Yuang Huang) ； 李坤城(Kung-Cheng Lee)

《大氣科學》 25卷4期 (1997/12) Pp. 511-546

摘要

本研究發展一中尺度空氣擴散數值模式，用於探討複雜地形下區域空氣污染物傳送與擴散問題。台灣海峽為一狹窄的海域，東有台灣高聳的中央山脈，西有中國大陸東南山嶽地形，兩岸空氣污染物在此區域複雜氣流型態下的傳送與擴散過程乃是一值得研究的問題。模擬結果顯示，低層盛行氣流受中央山脈的阻擋作用容易形成繞流現象。若盛行風向與中央山脈走向的夾角較大（如東風、東南風、西南風），可以形成背風渦旋。在複雜地形流場下，台北、台中及高雄三大都會區污染源對其山脈背風區或平地下風區皆有相當的污染潛勢。大體而言，低層盛行風受到台灣海峽渠道效應的影響，其風向受限且風速有加強的現象，造成兩岸污染物不易直接跨越海峽。若中國大陸上的面源污染物有機會進入高層的西風帶，則可快速移至台灣上空，並可能隨著中央山脈下坡風運動向低層輸送。擴散模式中亦進行單點污染物粒子隨平均風場的移動，即拉格朗日軌跡追蹤，同樣地發現在冬夏盛行風場下，低層污染粒不易跨越海峽飄送。另外，本文亦初步探討點源型式的污染粒群受次網格隨機運動影響的分布情形，模擬研究結果顯示，由於受到不均勻平均風場與亂流場影響，污染物濃度產生不對稱擴散現象，但其整體性移送仍依循平均風的拉格朗日軌跡。

關鍵字

中尺度 ； 空氣擴散模式 ； 隨機運動

並列摘要

A mesoscale air dispersion numerical model was developed in this study to investigate regional transport and dispersion problems of air pollutants in complex terrain. Taiwan Strait is a narrow oceanic region with the high and steep Central Mountain Range (CMR) in Taiwan to the east and the mountain topography in southeastern China to the west. It is important to study for the transport and dispersion processes of the air pollutants on both sides of the strait under complex flow patterns.The modeling results show that the low-level prevailing flow is subject to splitting due to the blocking effects of the CMR and tends to induce lee vortices if the angle between the prevailing wind and the CMR is somewhat larger (e.g., easterly, southeasterly and southwesterly). Under the complex topographically-induced flow, the air pollutants of three Taiwan metropolitan cities, Taipei, Taichung and Kuoshiang, are likely to generate remarkable pollution potential over the mountain leeside or downstream regions. In general, the low-level prevailing wind is significantly constrained by the channel orientation and is accelerated by the channeling effect. Hence, the air pollutants on both sides do not tend to move straight over the strait. However, the air pollutants over the Mainland can quickly advect to Taiwan if transferred to the upper westerly and may be transported downward to the lower levels by the CMR downslope wind. The dispersion model also computes the pathways of particles advected by the mean wind, i.e., Lagrangian trajectory. Similarly, the low-level particles do not tend to drift over the strait in cases of prevailing winds in the winter and summer. This study also investigates the distribution patterns of numerous particles (as single point source) as affected by the subgrid random walk motions. The simulation results exhibit local asymmetric dispersions of pollutant concentration. However, the gross movement of most particles remains to follow the Lagrangian trajectory of mean wind.

並列關鍵字

Mesoscale ； Air dispersion model ； Random walk