Comparison of methods to derive radial wind speed from a continuous-wave coherent lidar Doppler spectrum

Dominique P Held,Jakob Mann

doi:10.5194/amt-11-6339-2018

Abstract

Abstract. Continuous-wave (cw) lidar systems offer the possibility to remotely sense wind speed but are also affected by differences in their measurement process compared to more traditional anemometry like cup or sonic anemometers. Their large measurement volume leads to an attenuation of turbulence. In this paper we study how different methods to derive the radial wind speed from a lidar Doppler spectrum can mitigate turbulence attenuation. The centroid, median and maximum methods are compared by estimating transfer functions and calculating root mean squared errors (RMSEs) between a lidar and a sonic anemometer. Numerical simulations and experimental results both indicate that the median method performed best in terms of RMSE and also had slight improvements over the centroid method in terms of volume averaging reduction. The maximum, even though it uses the least amount of information from the Doppler spectrum, performs best at mitigating the volume averaging effect. However, this benefit comes at the cost of increased signal noise due to discretisation of the maximum method. Thus, when the aim is to mitigate the effect of turbulence attenuation and obtain wind speed time series with low noise, from the results of this study we recommend using the median method. If the goal is to measure average wind speeds, all three methods perform equally well.

Highlights

Remote sensing is an attractive alternative to traditional in situ measurements of wind speed
We first present an example of the numerical simulation and experimental results and we will compare both to the analytical results
We used an estimation of the transfer function to evaluate the lidar’s attenuation of turbulent fluctuations and the root mean squared errors (RMSEs) to give a metric to the general performance of the methods

Summary

Introduction

Remote sensing is an attractive alternative to traditional in situ measurements of wind speed. Light detection and ranging (lidar) devices can replace the installation of large meteorological masts hosting cup or sonic anemometers in order to meet the constantly increasing measurement height requirements. This flexibility led to a large variety of applications of lidars spanning from lidar-assisted yaw and pitch control (Schlipf, 2015) to site assessment (Sanz Rodrigo et al, 2013) and power curve validation (Borraccino et al, 2017). The maximum value of the power spectral density (PSD) was used Since this gives integer multiples of the frequency step (which depends on the fast Fourier transform set-up) it has the disadvantage of returning a noisy signal. Nowadays most commercial cw lidar systems use the centroid of the PSD above a certain noise level (Harris et al, 2006), while in research instruments (e.g. short-range WindScanner) the median method is implemented (Angelou et al, 2012)

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Conclusion