應用CALMET地表能量平衡方程式計算大氣擴散穩定度對ISC模擬結果的影響

Abstract

大氣穩定度是許多空氣品質模式中重要的氣象參數之一，而決定穩定度方法的合理性將對後續進行污染物擴散模擬時的結果影響甚鉅，過去在許多的模式中往往使用Pasquill分類法來決定穩定度，例如ISC3模式，但由於此法僅能考慮氣象因素對於穩定度的影響，無法考量到地表特性亦有可能對於穩定度造成某種程度的影響。因此本研究應用CALMET模式的理論，日間以地表能量平衡法而夜間則以Weil & Brower理論來推估穩定度，且在日間推估可感熱通量的過程中考慮了三個地表參數於其中，即加以考慮到地表特性對於穩定度的影響。 在CALMET模式中對於地表參數包溫比是以定值作設定，而本研究則以推導出的包溫比估算式，利用相對濕度與絕對溫度這兩個氣象資料計算出逐時包溫比再代入日間可感熱通量中運算，藉此可得到一個以台灣本地氣候推估出的包溫比結果，亦可反映出包溫比的改變對於穩定度的影響。 研究結果顯示，若不在特殊情況下，應用CALMET理論計算出來的穩定度結果較ISC模式中以Pasquill法所決定的結果為穩定，這是因為CALMET在日間考慮了地表參數的關係，意即可將地表上所進行蒸發此等削弱可感熱通量的活動予以考慮到，因而使得模擬的結果較為穩定。反觀Pasquill法因為無法考量到地表上所進行的蒸發等作用，所以時常有將不穩定的程度高估的現象。 另外從本研究挑選三個案例日以CALMET理論計算出逐時穩定度的結果可知，絕大部分的白天時段穩定度屬於不穩定狀態，夜間時段穩定度皆屬穩定狀態，而在日夜交替之際大氣穩定度將呈現中性穩定的狀態。而決定日夜間大氣不穩定程度的主要依據，日間仍為熱力紊流，以可感熱通量作為指標；夜間則為機械紊流，以摩擦風速作為指標。

The atmospheric stability is one of important meteorological parameters in many air quality models, and the rationality of its decision method affects extremely the result of contaminants simulation. In the past the decision method of stability was often the Pasquill stability classification, for example ISC3 model. But it could only consider meteorological factor to the influence of stability, could not consider geographical factor. Therefore this research applies the theory of CALMET model. The stability is estimated with surface energy balance method during the daytime and Weil & Brower theory during the nighttime. The results show that the stability which is estimated with CALMET theory is more stable than the results of Pasquill method in ISC model. It was because that, CALMET considered surface parameters during the daytime. It could consider the evaporation which could depress the sensible heat flux. Because Pasquill could not consider the evaporation on surface, the unstable degree could be frequently overestimated. According to the results of three cases, the stability is unstable condition during most daytime; the stability is stable condition during the nighttime; the stability is neutral condition in day and night in turn time. And the main authority of the unstable degree is decided during the daytime and nighttime, it is the thermal turbulence and regards the sensible heat flux as the indicator during the daytime; it is the mechanical turbulence and regards the friction velocity as the indicator during the nighttime.