Ultrasonic anemometer angle of attack errors under turbulent conditions

Abstract

Measurements of eddy fluxes are premised on the assumption that wind velocities are measured accurately by an ultrasonic anemometer. Recently, Gill ultrasonic anemometers have been shown to suffer errors depending on the angle of attack, which is the angle between the wind vector and the horizontal. The correction of these errors results in general eddy flux increases. However, since calibration for error dependent on angle of attack was carried out in a wind tunnel experiment under conditions of nearly laminar flow, the applicability of this correction to the field data under turbulent conditions has been questioned. In this study, angle of attack errors from a Gill WindMaster ultrasonic anemometer were assessed by field experiment, over meadow and under turbulent conditions. By using five identical anemometers, two configurations were evaluated: two pairs of anemometers for reference and one between the pairs for tilt. The dependence of anemometer (co)sine response on 0 to −90° angles of attack in 10° steps and at 45° were checked. It was clarified that errors dependent on angle of attack occur even under turbulent conditions, with results similar to the wind tunnel experiments. Sine responses of vertical wind velocities here depended not only on vertical angle of attack but also on horizontal wind direction, a dynamic not considered in previous studies. For a more robust correction, alternative calibration functions were obtained empirically, so as to reasonably explain our field experimental results. Applying this new correction, eddy fluxes increased substantially even over meadow, which is somewhat aerodynamically smooth compared with forests or agricultural fields.