caa a22 4500 47577714X CHVBK 20180406123634.0 cr unu---uuuuu 170329e20000901xx s 000 0 eng 10.1023/A:1002660521017 doi (NATIONALLICENCE)springer-10.1023/A:1002660521017 Eddy Covariance Measurements On Mountain Slopes: The Advantage Of Surface-Normal Sensor Orientation Over A Vertical Set-Up [Elektronische Daten] [Peter Geissbühler, Rolf Siegwolf, Werner Eugster] 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 orientationrelative to the terrain slope, and a second instrument in a verticalposition at a horizontal distance of 1.54 m from the first instrument We found a significant reduction in the rotation angle necessary for the coordinaterotation procedure in the x-z plane whencomputing 30-minute flux averages with the surface-normal orientation. In 91% of all cases this rotation angleremained within the angle of incidence of ±10° recommended bythe manufacturer. In contrast, only 24% of the measurements taken with the vertically mounted anemometer were obtained at an angle of incidencewithin ±10°, and 3% were outside the ±30° range specified for an acceptable operation. A data quality test based on the variance of vertical windspeed 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/z0, the measuring height normalized by the roughness length, which do not appear to be constantin complex terrain. With both tests, a tendency for a slightly improved dataquality was found for the surface normal set-up, which, however, proved statistically insignificant. It is concluded that the surface-normal set-up of a sonic anemometer significantly reduces flow distortion by thesensor head. Although the surface-normal mounting position therefore appears to be the preferred one, with decreased flow distortion and a slightly improved data quality, no significant differences in turbulent quantities were found between the two set-uppositions. Hence, the consequences for short-term measurements of massand energy fluxes with a surface-normal set-up in complex terrain appearto be relevant only if single flux events are to be inspected, while for long-term measurements of integrated fluxes both the surface-normaland vertical installation of the sonic anemometer are adequate,indicating that eddy covariance measurements in complex terrain are lessdelicate than expected. Kluwer Academic Publishers, 2000 Atmospheric turbulence nationallicence Complex terrain nationallicence ECOMONT nationallicence Flow distortion nationallicence Micrometeorology nationallicence Geissbühler Peter Paul Scherrer Institute, Villigen PSI, CH-5232, Switzerland aut Siegwolf Rolf Paul Scherrer Institute, Villigen PSI, CH-5232, Switzerland aut Eugster Werner Institute of Geography, University of Bern, Hallerstrasse 12, CH-3012, Bern, Switzerland aut Boundary-Layer Meteorology Kluwer Academic Publishers 96/3(2000-09-01), 371-392 0006-8314 96:3<371 2000 96 10546 https://doi.org/10.1023/A:1002660521017 text/html Onlinezugriff via DOI 1 research-article jats NATIONALLICENCE 856 40 https://doi.org/10.1023/A:1002660521017 text/html Onlinezugriff via DOI NATIONALLICENCE 700 1- Geissbühler Peter Paul Scherrer Institute, Villigen PSI, CH-5232, Switzerland aut NATIONALLICENCE 700 1- Siegwolf Rolf Paul Scherrer Institute, Villigen PSI, CH-5232, Switzerland aut NATIONALLICENCE 700 1- Eugster Werner Institute of Geography, University of Bern, Hallerstrasse 12, CH-3012, Bern, Switzerland aut NATIONALLICENCE 773 0- Boundary-Layer Meteorology Kluwer Academic Publishers 96/3(2000-09-01), 371-392 0006-8314 96:3<371 2000 96 10546 Metadata rights reserved Springer special CC-BY-NC licence nationallicence BK010053 XK010053 XK010000 NATIONALLICENCE NATIONALLICENCE NL-springer