Sonic anemometers tested in a wind tunnel

Ultrasonic anemometers (sonics) are widely used in micrometeorology. The main purpose is to measure fluxes by applying the eddy covariance method. The quantification of the uncertainty of the fluxes due of flow distortion caused by the probe is still an issue. Recently it has been kind of rediscovered, that the inclination of the flow relative to the horizontal plane can be considerably outside the manufacturers specifications, especially in flows above rough surfaces (van der Molen et al., 2004). Five different types of sonics were characterized in a wind tunnel experiment (Gill R2, Gill HS, Campbell CSAT3, Young 81000V, Metek USA1). All sonics were exposed to 4 wind tunnel speeds (2, 4, 6, 8 ms-1) by rotating them continuously around their z-axis at eleven different tilt positions: +-35°, +- 27.5°, +- 20°, +- 12.5°, +- 5° and 0°. Additionally, each sonic was at least at one wind speed rotated around its x-axis at the same tilt positions. This corresponds to a complete tilt resolution in the zy-plane and tilt angles of +- 55°, +- 62.5°, +- 70°, +- 77.5°, +- 85° and 90° in the zx-plane. The flow distortion effects are visualized and feasibility of correction matrices derived from z-axis-rotations are discussed. One CSAT3 was operated with and without rain protection wicks. The results will be presented.

References: van der Molen, M. K., Gash, J. H. C., Elbers, J. A. (2004): Sonic anemometer (co)sine response and flux measurement II. The effect of introducing an angle of attack dependent calibration. Agric. Forest Meteorol., 122,95-109.