Evaluation of the Flow Distortion around the Campbell Scientific CSAT3 Sonic Anemometer Relative to Incident Wind Direction

Abstract

In June 2002, a high-frequency air?sea momentum system was deployed in the surf zone for 3 days as part of an experiment to quantify air?sea momentum transfer when the wind and wave direction were at angles. The system obtained measurements in the nearshore via a high-resolution Campbell Scientific CSAT3 3D sonic anemometer and five high-frequency saltwater wave staffs. An advantage of the air?sea momentum system is that direct measurements of the atmospheric turbulent fluctuations can be obtained and applied to the calculation of momentum transfer at the air?sea interface. The Campbell Scientific CSAT3 sonic anemometer was postcalibrated under turbulent wind conditions to determine incident wind direction measurements influenced by the geometry of the instrument. Measurement results are compared to a pre-established benchmark, constant tow speed; and the mean wind speed, incident wind direction, and spectral density characteristics are evaluated to resolve specific instrument orientations in which the measurements are corrupted by the head and probe supports of the sonic anemometer. Calibration testing of the sonic anemometer determined that the mean wind speeds are reduced by 16% over a 40° range for incident wind angles of 160°?200° relative to the head of the anemometer. Tilting the anemometer is found to decrease mean wind speed reduction influenced by the geometry of the anemometer. Variations in the measured wind directions were found to be greater than 1° for incident wind angles between 160° and 200° for 0° and 10° of tilt. Spectral characteristics were highly repeatable for all wind angles except for incident wind angles of 180° for 0° and 10° of tilt.