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Measurements of aerodynamic roughness and Bowen ratio using tethersonde and the eddy covariance system in urban, rice paddy and mixed areas
|關鍵字:||aerodynamic roughness;地表粗糙度;tethersonde;Bowen Ratio method;surface energy closure;eddy covariance system;Bowen ratio;rice paddy;CO2;canopy resistance;繫留;探空氣球;複雜地貌;地表能量;平衡;渦流;協變性系統;水稻田;二氧化碳;植物阻抗;Bowen ratio||出版社:||環境工程學系所||摘要:||
本研究利用相似理論搭配地表能量平衡式於台灣中部地區觀測地表粗糙度，計有兩個都會區及三個農地混合住建築物區。在每一個實驗中以繫留探空氣球及地面氣象站觀測長短波輻射、土壤熱通量、及風速、溫度和濕度的垂直剖面。在台中都會區，地表粗糙度為2.1公尺，zero-plane displacement height為35 m；在草屯鎮市區地表粗糙度為1.2 公尺，zero-plane displacement height為11 m；在農地及建物混合區地表粗糙度為0.2-0.5公尺，而在水稻田區為地表粗糙度為0.032公尺。
本研究亦利用渦流協變性系統(Eddy covariance system)觀測地表及大氣之間的各通量，於日間能量平衡率約為94%，並於平衡式中考慮光合作用及平流向對能量的影響。利用渦流協變性系統觀測資料推估日間Bowen ratio約為0.16，利用輻射計推估地表反照率約為0.1。日間的二氧化碳通量約為1.2 mg m-2 s-1而夜晚約為0.12 mg m-2 s-1，同時建立二氧化碳通量與太陽淨輻射、氣溫及葉面積指數的關係式 。植物阻抗在日間呈現”U”型，極大值在中午約為42 s m-1，利用回歸方法建立植物阻抗與太陽淨輻射及葉面積指數的關係式，其模擬值與觀測值得相關係數為0.78。
The similarity theory of the ASL in conjunction with the energy budget equation of land surface under unstable atmospheric conditions was used to determine the aerodynamic roughness lengths for two urban areas and three rice paddies mixed with buildings over a complex terrain in central Taiwan. At each of the sites, surface net radiation, ground heat flux and vertical profiles of wind speed, temperature and humidity within the atmospheric surface layer (ASL) were measured.
Over the Taichung urban area, the roughness was determined to be 2.1 m with a zero-plane displacement height of 35 m. Over the Caotun urban area, the roughness was determined to be 1.2 m with a zero-plane displacement height of 11 m. Over the two mixed farmlands, the roughness values were determined to be 0.2 - 0.5 m and over the homogeneous rice paddy in Wufeng, the roughness values were determined to be 0.032 m that close to the values (0.008 - 0.02 m) for homogeneous rice paddies reported in the literature.
A preliminary relationship for estimating roughness value as a function of residential fraction, farmland fraction and building height (or population density) is derived. The observations show that during the daytime, the mean height range of the ASL was 30<(z-d0)/z0<106 and the fingerprint areas extended 1.5 km upwind from the three profile sites.
During the day, the energy balance ratio measured by an Eddy Covariance (EC) system is found to be 94% after considering the photosynthetic and local advected heat fluxes. The observations by the EC system suggest that the Bowen ratio was about 0.16 during the daytime. Albedo is estimated as 0.1 according to the solar radiation and the reflected show-wave radiation. The EC system also measured the daytime absorbed CO2 flux at 1.2 mg m-2 s-1 and nighttime respiration rate at 0.12 mg m-2 s-1. Relationships of CO2 flux as functions of net solar radiation, air temperature and leaf area index are derived. The diurnal pattern of the canopy resistance for evapotranspiration is found to be a U shape with the minimum value at 42 s m-1 around noon of the rice paddy. A relationship of canopy resistance related to net solar radiation and leaf area index is derived with a correlation coefficient of 0.78.
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