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THREE-DIMENSIONAL WIND SPEED AND FLUX MEASUREMENTS OVER A RAIN-FED SOYBEAN FIELD USING ORTHOGONAL AND NON-ORTHOGONAL SONIC ANEMOMETER DESIGNS

The eddy covariance method for estimating fluxes of trace gases, energy and momentum in the constant flux layer above a plant canopy fundamentally relies on accurate measurements of the vertical wind speed. This wind speed is typically measured using a three-dimensional ultrasonic anemometer. Previous studies comparing anemometers with orthogonal transducer sets to those with non-orthogonal transducer sets suggest differences in measured 3D wind speed components, particularly for a vertical component. These differences, attributed to additional flow distortion caused by the non-orthogonal transducer arrangement and support structure, directly affect fluxes of trace gases, energy and momentum. A field experiment was conducted over a rain-fed soybean field at the AmeriFlux site (US-Ne3) near Mead, Nebraska to quantify these differences. Ultrasonic anemometers featuring orthogonal transducer sets (ATI Vx Probe) and non-orthogonal transducer sets (Gill R3) collected high frequency wind vector and sonic temperature data. The non-orthogonal Gill R3 models underestimated sensible heat flux by 11% and friction velocity by 5% relative to the ATI Vx orthogonal design under the same atmospheric conditions. For two versions of an angle of attack correction developed for this non-orthogonal anemometer, neither adequately corrected the Gill R3 sensible heat fluxes compared to those measured using the orthogonal ATI Vx probe.