Coordinate Rotation–Amplification in the Uncertainty and Bias in Non-orthogonal Sonic Anemometer Vertical Wind Speeds

Recent research indicates that non-orthogonal sonic anemometers underestimate vertical wind velocity and consequently eddy-covariance fluxes of mass and energy. Whether this is a general problem among all non-orthogonal sonic anemometers, including those calibrated for flow-shadowing effects, is unknown. To investigate this, we test two sonic anemometer designs, orthogonal (3Vx-probe, Applied Technologies, Inc.) and non-orthogonal (R3-50, Gill Instruments, Ltd.), in a series of field manipulation experiments featuring replicate instruments mounted in various orientations, and use a Bayesian analysis to determine the most likely posterior correction to produce equivalent measurements. The 3Vx-probe experiment was conducted on a 24-m scaffold at the Glacier Lakes Ecosystem Experiments Site (GLEES), Wyoming, USA AmeriFlux site while R3-50 anemometer experiments were conducted at the GLEES field site and on a 2.9-m scaffold at the Pawnee National Grassland, Colorado, USA. Without applying a shadowing correction to the 3Vx-probe, the posterior correction significantly increases the standard deviation of the horizontal velocity component by 5–15% (95% Bayesian credible interval) but without a significant change in the horizontal temperature flux; with the shadowing correction applied neither of these have significant changes. Similarly, for the R3-50 GLEES experiment, the standard deviation of the vertical velocity and vertical temperature flux significantly increase by 13–18% and 6–10% (95% credible intervals); results from the Pawnee experiment are contradictory and inconclusive. The reason for the underestimated vertical velocity is undetermined, though a mathematical by-product of the non-orthogonal geometry is that small systematic measurement biases can become large uncertainties in the vertical velocity. This could affect all non-orthogonal designs.