2000: Eddy Covariance Measurements on Mountain Slopes: The Advantage of Surface-normal Sensor Orientation Over a Vertical Set-up

Abstract. The measurement of scalar fluxes employing the eddy covariance method is a widely used experimental approach, for which the flow distortion due to obstacles (e.g., sensor mounts and mast) is a well-known but not fully solved problem. In order to reduce flow distortion we installed a sonic anemometer in a surface-normal orientation relative to the terrain slope, and a second instrument in a vertical position at a horizontal distance of 1.54 m from the first instrument We found a significant reduction in the rotation angle necessary for the coordinate rotation procedure in the x-z plane when computing 30-minute flux averages with the surface-normal orientation. In 91 % of all cases this rotation angle remained within the angle of incidence of ±10 ◦ recommended by the manufacturer. In contrast, only 24 % of the measurements taken with the vertically mounted anemometer were obtained at an angle of incidence within ±10 ◦ , and 3 % were outside the ±30 ◦ range specified for an acceptable operation. A data quality test based on the variance of vertical wind speed normalized with friction velocity (σw/u∗) revealed problems for application under stable conditions due to large uncertainties in the determination of the Monin–Obukhov stability parameter z/L. An alternative test using the bulk drag coefficient CD revealed other problems related to the dependence of CD on z/z 0, the measurin