Air temperature equation derived from sonic temperature and water vapor mixing ratio for turbulent airflow sampled through closed-path eddy-covariance flux systems

[摘要] Air temperature ( T ) plays a fundamental role in manyaspects of the flux exchanges between the atmosphere and ecosystems.Additionally, knowing where (in relation to other essential measurements)and at what frequency T must be measured is critical to accurately describingsuch exchanges. In closed-path eddy-covariance (CPEC) flux systems, T can becomputed from the sonic temperature ( T s ) and water vapor mixing ratiothat are measured by the fast-response sensors of a three-dimensional sonicanemometer and infrared CO 2 – H 2 O analyzer, respectively. T is thencomputed by use of either T = T s 1 + 0.51 q - 1 , where q isspecific humidity, or T = T s 1 + 0.32 e / P - 1 , where e is watervapor pressure and P is atmospheric pressure. Converting q and e / P into the samewater vapor mixing ratio analytically reveals the difference between thesetwo equations. This difference in a CPEC system could reach ± 0.18 K,bringing an uncertainty into the accuracy of T from both equations andraising the question of which equation is better. To clarify the uncertaintyand to answer this question, the derivation of T equations in terms of T s and H 2 O-related variables is thoroughly studied. The twoequations above were developed with approximations; therefore, neither oftheir accuracies was evaluated, nor was the question answered. Based onfirst principles, this study derives the T equation in terms of T s andthe water vapor molar mixing ratio ( χ H 2 O ) without any assumption andapproximation. Thus, this equation inherently lacks error, and the accuracyin T from this equation (equation-computed T ) depends solely on themeasurement accuracies of T s and χ H 2 O . Based on currentspecifications for T s and χ H 2 O in the CPEC300 series, andgiven their maximized measurement uncertainties, the accuracy inequation-computed T is specified within ± 1.01 K. Thisaccuracy uncertainty is propagated mainly ( ± 1.00 K) from theuncertainty in T s measurements and a little ( ± 0.02 K) from theuncertainty in χ H 2 O measurements. An improvement in measurementtechnologies, particularly for T s , would be a key to narrowing thisaccuracy range. Under normal sensor and weather conditions, the specifiedaccuracy range is overestimated, and actual accuracy is better.Equation-computed T has a frequency response equivalent to high-frequency T s and is insensitive to solar contamination during measurements.Synchronized at a temporal scale of the measurement frequency and matched at aspatial scale of measurement volume with all aerodynamic and thermodynamicvariables, this T has advanced merits in boundary-layer meteorology andapplied meteorology.

[学科分类] 内科医学