以渦度相關法驗證包溫比估算式

Abstract

空氣品質模式對於污染物的擴散模擬是被擴散尺度所影響，擴散尺度是由大氣穩定度決定，大氣穩定度的計算又由可感熱通量所計算出，而包溫比是可感熱通量和潛熱通量的比值，所以包溫比決定的合理性，會影響到可感熱通量的值，進而對後續污染物擴散模擬的結果有所影響，因此希望改善不同於目前空氣品質模式裡對於包溫比是以定值作決定之方式。本研究由熱力學的理論推導出一個包溫比估算式，此估算式可由台灣的逐時氣象資料，算出逐時的包溫比。 為了驗證此估算式，本研究使用棲蘭通量站與關渡通量站的數據，也就是Eddy covariance法的數據，利用實際量測的方法探討此估算式在台灣的適用性，並且求出一個修正係數α，藉由此修正係數使理論包溫比更符合實際情況。 由研究結果顯示，在棲蘭通量站理論包溫比有明顯低估，而關渡則沒有，在經過修正後，可以改善低估的情況，而使可感熱通量與潛熱通量更符合EC法所量測。在大氣穩定度方面，是以EC法所計算的大氣穩定度為基準，去比較理論包溫比、修正後的理論包溫比以及定值包溫比三種方法所計算出來的大氣穩定度，理論包溫比和EC法結果一致的比例高於定值包溫比，且修正後的理論包溫比所求之大氣穩定度等級和EC法一致的比例可達75%以上。

The air quality model to simulate pollutants’ diffusion is affected by the diffusion degree that determined by the atmospheric stability, and the calculation of the atmospheric stability can be obtained from sensible heat flux. Bowen ratio, the ratio between sensible heat flux and latent heat flux, and its rational decision will change the value of sensible heat flux as well as subsequently modify the result of contaminants simulation. Therefore, instead of setting Bowen ratio as a constant by recent air quality models, this research applies the theory of thermodynamics to deduce the estimated Bowen ratio formula that could find the time-varying Bowen ratio from time-varying weather information in Taiwan. To verify the estimated formula, this research uses Eddy covariance data from Chilan and Guandu flux site in Taiwan to evaluate the feasibility of the formula by in-situ analysis method and then to propose one modified factorα. It is more consistent with real situation by such modified factor. According to the research result, the theoretical Bowen ratio is significantly underestimated at Chilan flux site but not at Guandu flux site. However, after applying factorα, both sensible and latent heat flux fit data that derived from Bowen ratio method. The atmospheric stability based on Eddy covariance is used to compare the resulting atmospheric stability with the theoretical, the modified and the constant Bowen ratio methods. The ratio of the same stability by the theoretical Bowen ratio and Eddy covariance is higher than the stability by the constant Bowen ratio, and the ratio that both the modified theoretical Bowen ratio and Eddy covariance show the consistent degree of the atmospheric stability is higher than 75%.