PNNL

Abstract

Results from a recent field campaign are used to assess the accuracy of wind speed and direction uncertainty estimates produced by a Doppler lidar wind retrieval algorithm. The algorithm, which is based on the traditional velocity-azimuth-display (VAD) technique, estimates wind speed and direction uncertainties using standard error propagation techniques. For this study, the lidar was configured to execute an 8-beam plan-position-indicator (PPI) scan once every 12 minutes during the 6 week deployment period. Several wind retrieval trials were conducted using different schemes for estimating the uncertainty in the radial velocity measurements. The resulting wind speed and direction uncertainties were compared to differences in wind speed and direction between the VAD algorithm and sonic anemometer measurements taken on a nearby 300-m tower. All Trials produced qualitatively similar wind fields with negligible bias, but substantially different wind speed and direction uncertainty fields. The most accurate wind speed and direction uncertainties were obtained when the radial velocity uncertainty was determined by direct calculation of radial velocity standard deviation along each pointing direction and range gate of the PPI scan. By contrast, setting the radial velocity uncertainty to the radial velocity precision (thereby ignoring turbulence effects) resulted in wind speed and direction uncertainties that were biased far too low and poor indicators of data quality.