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Sonic Anemometer Tilt Correction Algorithms
Authors:James M Wilczak  Steven P Oncley  Steven A Stage
Institution:(1) Environmental Research Laboratories, Environmental Technology Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO 80303, USA;(2) National Center for Atmospheric Research, Boulder, CO 80303, USA;(3) nnovative Emergency Management, Baton Rouge, LA 70809, USA
Abstract:The sensitivity of sonic anemometer-derived stress estimates to the tilt of the anemometer is investigated. The largest stress errors are shown to occur for unstable stratification (z/L<0) and deep convective boundary layers. Three methods for determining the tilt angles relative to a mean streamline coordinate system and for computing the tilt-corrected stresses are then compared. The most commonly used method, involving a double rotation of the anemometers' axes, is shown to result in significant run-to-run stress errors due to the sampling uncertainty of the mean vertical velocity. An alternative method, requiring a triple rotation of the anemometer axes, is shown to result in even greater run-to-run stress errors due to the combined sampling errors of the mean vertical velocity and the cross-wind stress. For measurements over the sea where the cross-stream stress is important, the double rotation method is shown to overestimate the surface stress, due to the uncorrected lateral tilt component. A third method, using a planar fit technique, isshown to reduce the run-to-run stress errors due to sampling effects, and provides an unbiased estimate of the lateral stress.
Keywords:Anemometers  Coordinate systems  Sloping terrain  Surface layer  Tilt corrections
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