Lidar arc scan uncertainty reduction through scanning geometry optimization

Hui Wang,Rebecca J Barthelmie,Gareth Brown,Sara C Pryor

doi:10.5194/amt-9-1653-2016

Abstract

Abstract. Doppler lidars are frequently operated in a mode referred to as arc scans, wherein the lidar beam scans across a sector with a fixed elevation angle and the resulting measurements are used to derive an estimate of the n minute horizontal mean wind velocity (speed and direction). Previous studies have shown that the uncertainty in the measured wind speed originates from turbulent wind fluctuations and depends on the scan geometry (the arc span and the arc orientation). This paper is designed to provide guidance on optimal scan geometries for two key applications in the wind energy industry: wind turbine power performance analysis and annual energy production prediction. We present a quantitative analysis of the retrieved wind speed uncertainty derived using a theoretical model with the assumption of isotropic and frozen turbulence, and observations from three sites that are onshore with flat terrain, onshore with complex terrain and offshore, respectively. The results from both the theoretical model and observations show that the uncertainty is scaled with the turbulence intensity such that the relative standard error on the 10 min mean wind speed is about 30 % of the turbulence intensity. The uncertainty in both retrieved wind speeds and derived wind energy production estimates can be reduced by aligning lidar beams with the dominant wind direction, increasing the arc span and lowering the number of beams per arc scan. Large arc spans should be used at sites with high turbulence intensity and/or large wind direction variation.

Highlights

Coherent Doppler lidars have been used to probe a range of atmospheric boundary layer (ABL)phenomena, including nocturnal low level jets in the GreatPlains (Banta et al, 2008), spatial variability of wind in the marine ABL (Pichugina et al, 2012), structures of the urbanABL (Calhoun et al, 2006; Frehlich et al, 2006) and turbulent properties and flow patterns over complex terrain (Krishnamurthy et al, 2011; Choukulkar et al, 2012)
The radial velocity vr is the projection of the wind velocity u on the line of sight (LOS) at the location s = sd for which s is the distance from the lidar along the LOS and d is the unit directional vector determined by the elevation angle φ and the azimuth angle θ of the LOS from north
Wind speeds measured by lidars are subject to uncertainties that originate from prevailing atmospheric conditions, lidar scanning geometry and wind velocity retrieval method

Summary

Introduction

Coherent Doppler lidars (hereafter called lidars) have been used to probe a range of atmospheric boundary layer (ABL). The uncertainty in the wind velocity estimated from arc scans can be derived from the covariance matrix A of the measured radial velocities. Enlarging 1θ increases the spatial coverage, and lowering the number of beams per arc scan increases the separation distance between samples, both of which reduce the correlation between samples and the uncertainty in the estimated horizontal mean wind speed. This sector, which contains 2167 measurements of 10 min mean wind speed (93 % data recovery rate), is chosen for the uncertainty analysis Both observed (εd ) and expected (εd ) RSE for 2◦ bins of β and wind speed between 6 and 14 m s−1 indicate a dependence on wind direction which derives largely from the directional variability of the mean and variance of wind speed (Fig. 8). Note that εl and εc should be lower than the actual values because non-homogeneous horizontal wind fields and non-zero vertical wind speeds over complex terrain violate assumptions in the theoretical models (Bingöl et al, 2009)

Findings

Discussion

Conclusions