Flow Distortion by a Solent Sonic Anemometer: Wind Tunnel Calibration and Its Assessment for Flux Measurements over Forest and Field

Abstract

Main flow distortion effects caused by the sonic probe (i.e., deflection and attenuation/amplification of the wind vector) as a function of the azimuth angle of the incoming flow were examined by means of wind tunnel measurements at four different wind speeds and 11 different elevation angles of the flow with respect to the probe. The dependence of the distortion on wind speed and elevation angle turned out to be small compared with the dependence on the azimuth angle. Based on wind tunnel data, a set of calibration coefficients for each azimuth was evaluated. Application of this calibration on wind tunnel data shows an average compensation of the flow distortion of about 61 %. A comparative application of the calibration supplied by the manufacturer leads to an increase of the flow distortion effects, especially at higher wind speeds. Application of the obtained calibration on field data leads to a change of about 20% in momentum flux over forest and grass field. The flux of sensible heat changes by about 4% over forest, whereas it is hardly altered over grass.