Sonic Anemometer Vertical Wind Speed Measurement Errors

In eddy covariance studies, errors in the measured vertical wind speed cause errors of a similar magnitude in the vertical fluxes of energy and mass. Several recent studies on the accuracy of sonic anemometer measurements indicate that non-orthogonal sonic anemometers used in eddy covariance studies underestimate the vertical wind speed. It has been suggested that this underestimation is caused by flow distortion from the interference of the structure of the anemometer itself on the flow. When oriented ideally with respect to the horizontal wind direction, orthogonal sonic anemometers that measure the vertical wind speed with a single vertically-oriented acoustic path may measure the vertical wind speed more accurately in typical surface-layer conditions. For non-orthogonal sonic anemometers, Horst et al. (2014) proposed that transducer shadowing may be a dominant factor in sonic flow distortion. As the ratio of sonic transducer diameter to path length and the zenith angle of the three transducer paths decrease, the effects of transducer shadowing on measurements of vertical velocity will decrease. An overview of this research and some of the methods available to correct historical data will be presented.