Comparison between SODAR and anemometer-based turbulence measurements

Abstract

Sonic detection and ranging (SODAR) is a technique for measuring wind speed and turbulence parameters from backscattered sound waves. The SODAR projects a beam of sound straight up, as well as at angles slightly off vertical. Sound waves are scattered by variations in the density of the air and are then received back at the SODAR, the time of flight giving the height being probed. Doppler shifts provide information about the wind velocity. Since larger variations in the local density of the atmosphere imply higher turbulence, backscatter strength is related to turbulence. The instrument used here was a Scintec MFAS flat array SODAR. While the backscatter strength thus appears to be a direct indicator of the turbulence strength, calibration and an estimate of the variation of temperature with height is needed to process this strength into values for CT2 and Cn2. Consequently, it is interesting to compare measurements from this technique with results from other turbulence measurement approaches. A sonic anemometer measures the wind velocity and temperature over the volume of air between its probes. From this instrument, turbulence is estimated by the temperature variations in the air moved past the instrument by the wind. The sonic anemometer measures turbulence at a single location, while the SODAR measures turbulence as a function of height (up to about 400 meters above ground). Thus these comparisons aren’t really looking at the same thing. By mounting the sonic anemometer on a small UAV, this difficulty can be overcome.