近地風場分析方法對氣團軌跡計算結果的影響---CALMET方法、變分分析方法以及MM5輸出結果比對

Abstract

本研究選用變分分析方法、CALMET氣象模式與MM5氣象模式，利用其模擬所輸出的風場資料，去探討各模式對於氣團軌跡計算結果之間的相關性與差異性，以了解空氣污染物在大氣環境中之傳輸情形。 在變分分析方面，採用一代變分為架構(Chang, 1983)，建立台灣地區的三維風場。並且因為台灣地區複雜的地形效應，加入風場繞流調整模組，嘗試將客觀分析後產生之初始風場隨著局部地形加以調整，以充分反映氣流繞山之情形。研究結果顯示，再經過風向的修正後，變分分析水平診斷風場的確較能充分反映出地形效應對近地面層風場的影響;在高空風場方面，也能夠解析出海陸風與山谷風等局部區域環流的變化特性。 利用上述結果產生之風場，對於嘉義地區污染源的探討上，發現層平均風場的二維逆軌跡相較於三維逆軌跡與直角座標風場的二維逆軌跡而言，比較能反映出風場的變化情形，較正確的追溯污染之來源。其軌跡點在可能的污染來源區域的傳送距離與停留時間上較優於另外二種軌跡線的結果，相對的所吸納的前驅物量越多，越有利於污染事件的發生。 在與環保署及中央氣象局觀測之全台風場比對上，變分分析的結果較CALMET所模擬的風場貼近於實際的狀況。且對於測站的風速模擬上，整體而言仍是以變分分析的表現略優於CALMET所模擬的結果。 在竹山地區的逆軌跡模擬結果當中，因為受到中部地區山谷風環流效應與地形的阻滯下，變分分析模擬的二維與三維逆軌跡以及MM5的模擬結果皆會出現打繞的現象，甚至迴流傳輸的情形，但CALMET模式對於這些機制較不明顯，甚至會有軌跡線飛越中央山脈的情況發生。另外也發現MM5的模擬風速會比其他二種模式來得大。綜合上述結果，本研究也建議利用經過繞流調整的層平均風場資料進行二維逆軌跡分析，應該較能正確的推估污染氣團之傳輸路徑。

This study used variational analysis approach, CALMET and MM5 meteorological models, using the wind field data of simulations to discuss the relevance and differences of calculating air mass trajectories in order to understand the transmission of air pollutants in atmosphere. By using the variational analysis of 1st generation scheme (Chang, 1983) to build three-dimensional wind field in Taiwan. Because the effects of the complex terrain area. To add the wind direction shift module to correct the initial wind field by objective analysis according to terrain slope. The results reveal that the horizontal wind field of variational analysis modified by wind direction shift module represents sufficient influence of complex terrain area near the ground level. And the vertical wind field can show the characteristics of local circulations such as sea-land breeze and valley wind. It’s can be found that two-dimensional (2D) back trajectory of level-averaged wind field can reveal the variation of wind field and affiliate the source to pollutants more accurate than 2D back trajectory of rectangular coordinate wind field and three-dimensional (3D) back trajectory in Chiayi area. The transmission distance and retention time of track points in pollution area are superior to others. It’s will absorb more precursor and promote the pollution incidents to occur. The model simulations of wind fields of variational analysis are closer to the observations of EPA and CWB stations than CALMET model. And the simulated performances of station wind speed of variational analysis are slightly superior to CALMET model. The 2D and 3D back trajectories of variational analysis and the outputs of MM5 model will appear the phenomena of flowing around a sharp ridge slope even the circulated flow due to the effects of valley circulations and the blocking effects of terrain in Chushan area in central Taiwan. But the mechanisms of CALMET model aren’t obvious and the trajectory will over fly the Central Mountains. And the simulation speed of MM5 is higher than variational analysis and CALMET model. To synthesize the research results, suggesting that using the data of level-averaged wind field after the wind direction shift with 2D back trajectory analysis will upgrade the accuracy of conjecturing the transmission route of air mass in atmosphere.