Evaluation of the aerodynamic method of determining fluxes over natural grassland

Abstract

AbstractProfiles of wind speed, temperature, humidity, and CO2 content up to a height of 6.5 m were measured over a period of several months above natural grassland in an area with an average fetch of 2km. Profile shapes have been analysed in relation to atmospheric stability.In unstable conditions, temperature and CO2 profiles exhibit similar shapes and depart more from logarithmic than does the wind profile. All profiles are relatively well described by existing empirical stability corrections.In stable conditions all profiles have approximately the same shape: they depart from the logarithmic profile at moderate stabilities and return to it at strong stabilities. However, the oft‐quoted log‐linear profile does not provide a very good fit.Empirical stability corrections have been used to develop a generalized aerodynamic method for flux computation. This method tends to underestimate (by up to 20%) fluxes during unstable conditions and to overestimate (by up to 40%) in moderately stable conditions. These results stem from a comparison of aerodynamic and Bowen ratio estimates of sensible heat flux. They are in essential agreement with the analysis of profile shapes.The drag coefficient of vegetation may be predicted from the leaf area index. This allows the use of a simplified aerodynamic method, based on measurements of wind speed at one level and of temperature (or H2O or CO2) at two levels.