Validating precision estimates in horizontal wind measurements from a Doppler lidar

Rob K Newsom,Julie K Lundquist,James M Wilczak,Daniel E Wolfe,W Alan Brewer,Steven P Oncley

doi:10.5194/amt-10-1229-2017

Abstract

Abstract. Results from a recent field campaign are used to assess the accuracy of wind speed and direction precision estimates produced by a Doppler lidar wind retrieval algorithm. The algorithm, which is based on the traditional velocity-azimuth-display (VAD) technique, estimates the wind speed and direction measurement precision using standard error propagation techniques, assuming the input data (i.e., radial velocities) to be contaminated by random, zero-mean, errors. For this study, the lidar was configured to execute an 8-beam plan-position-indicator (PPI) scan once every 12 min during the 6-week deployment period. Several wind retrieval trials were conducted using different schemes for estimating the precision in the radial velocity measurements. The resulting wind speed and direction precision estimates 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 precision fields. The most accurate wind speed and direction precisions were obtained when the radial velocity precision was determined by direct calculation of radial velocity standard deviation along each pointing direction and range gate of the PPI scan. By contrast, when the instrumental measurement precision is assumed to be the only contribution to the radial velocity precision, the retrievals resulted in wind speed and direction precisions that were biased far too low and were poor indicators of data quality.

Highlights

IntroductionCoherent Doppler lidars (CDL) are used in applications ranging from basic atmospheric boundary-layer research to model data assimilation (Riishøjgaard et al, 2004; Chai et al, 2004; Newsom and Banta, 2004a, b; Newsom et al, 2005; Weissmann and Cardinali, 2007; Pu et al, 2010) and to wind resource assessment (Pena et al, 2009; Lang and McKeogh, 2011; Koch et al, 2012; Pichugina et al, 2012; Hsuan et al, 2014; Newsom et al, 2015; Newman et al, 2016; Choukulkar et al, 2016)
This study evaluated the accuracy of wind speed and wind direction precision estimates as computed from Coherent Doppler lidars (CDL) scan data using standard error propagation techniques in a VAD algorithm
Precision estimates were compared to differences in wind speed and direction between the VAD algorithm and the sonic anemometers mounted on the 300 m Boulder Atmospheric Observatory (BAO) tower

Summary

Introduction

Coherent Doppler lidars (CDL) are used in applications ranging from basic atmospheric boundary-layer research to model data assimilation (Riishøjgaard et al, 2004; Chai et al, 2004; Newsom and Banta, 2004a, b; Newsom et al, 2005; Weissmann and Cardinali, 2007; Pu et al, 2010) and to wind resource assessment (Pena et al, 2009; Lang and McKeogh, 2011; Koch et al, 2012; Pichugina et al, 2012; Hsuan et al, 2014; Newsom et al, 2015; Newman et al, 2016; Choukulkar et al, 2016). The International Energy Agency’s recommended best practices (Clifton et al, 2013) state that CDLs should undergo periodic verification using anemometers that have been calibrated against a traceable standard. This enables determination of uncertainty due to both the random error (i.e., precision) and systematic error. The uncertainties determined in this manner may or may not be applicable to the measurements obtained at the resource assessment site It is, possible to obtain “on-site” estimates of the wind speed and wind direction precision using standard error propagation techniques in the CDL wind retrieval algorithm if the variability in the radial velocity measurements can be properly characterized. These precision estimates could be used as measures of data quality

Methods

Results

Conclusion