可感熱通量對大氣穩定度應用在空氣品質模式之影響

Abstract

大氣穩定度 (atmospheric stability)是決定空氣品質模式準確度的重要參數，而大氣穩定度是由可感熱通量( sensible heat flux)計算求得的。一般上，可感熱通量是由測站測得的，並且用以模擬特定區域的空氣品質，但由於測站為單點測量，用來模較大空間尺度的區域會產生差異，因此探討擴尺度可感熱通量是一項重要的議題。藉由不同尺度的測量方法測量可感熱通量，會因量測範圍和計算過程的不同而產生差異，本研究利用兩種不同尺度的方法測量可感熱通量：渦度相關法 (eddy-covariance, EC)，及閃爍測量儀 (surface layer scintillometer, SLS)。本研究將SLS架設於關渡平原，是潮濕的草原地表，並於冬天測量3個月。研究發現SLS法低估可感熱通量及摩擦風速，可能是惰性紊流 (inactive turbulence)所造成的。藉由分析可感熱通量與環境因子之間的關係，如摩擦風速及包溫比，有助於了解不同尺度下的差異。在AERMET中，包溫比、地表反照率和粗糙長度，這些地表參數被用以計算求得可感熱通量，所以地表參數對模擬結果十分重要。在本研究中，EC法與SLS法量測的白天平均包溫比分別是0.55和0.42，與AERMET使用手冊的建議值不同。本研究發現：AERMET在計算可感熱通量時，並沒有考慮到能量閉合度，這會造成可感熱通量的高估，進而高估大氣混合的能 力，使模擬結果產生偏差。未來在使用AERMET時，建議修改摩擦風速的計算方式，而在計算可感熱通量時，必須考慮到地表能量平衡，以使模擬結果更為準確。

The determination of atmospheric stability in the air quality models is considered to be one of the key issues to ensure the model performance. Sensible heat flux is an important parameter to quantity the atmospheric stability, which is usually obtained by point-based measurements, and used to quantity the air quality in specific region. However, the gap in spatial scales between the conventional station-measured data and the area-averaged estimation in the larger region makes the upscaling process of the sensible heat flux to be crucial in model simulation. Sensible heat flux measurements by different scale approaches might be distinct because of footprint areas and calculation process. This study measures sensible heat flux by using two different scale approaches: point-based eddy-covariance (EC) method and path-averaged surface layer scintillometer (SLS). SLS was established in winter at Guandu plant, a grassland surface with high moist. The results suggest that the under-estimated sensible heat flux and friction velocity by SLS and might cause by inactive turbulence. By investigating the relationship between sensible heat flux and meteorology parameters such as friction velocity and Bowen ratio would offer a good way to understand the influence of different spatial scale. Surface characteristics parameters such as Bowen ratio, albedo, and roughness length are used to determine sensible heat flux in AERMET, thus, it is important to determine surface parameters for simulations. Averaged Bowen ratio in the daytime are 0.55 and 0.42 which are determined by EC-measurements and SLS-measurements respectively which are different to default suggested from AERMET user’s guide. This study suggests that AERMET does not consider energy balance closure, it results in over-estimating sensible heat flux and then over-estimating ability of convection. For the future works, the calculation process for friction velocity in AERMET should be corrected and energy balance closure should be considered.