Field Estimates of Sonic Anemometer Angle of Attack Errors

Turbulent wakes formed downwind of sonic anemometer transducers and their supporting structure can cause underestimation of the vertical and horizontal wind speeds. These errors are referred to as angle of attack or (co)sine errors. The importance of anemometer (co)sine errors in eddy covariance based mass and energy budgets has been established by past wind tunnel studies, but almost no estimates of the anemometer errors that occur in natural boundary layer turbulence are available. In a recent field experiment, sonic anemometer angle of attack errors were estimated for over 100 combinations of angle of attack and wind direction. Corrections to the vertical wind speed estimated using these measurements varied from -3% to 27%. When applied to eddy covariance data, the angle of attack corrections had a significant effect on NEE and the energy balance at two Ameriflux sites.