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

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.

Lidar estimation of rotor-effective wind speed – an experimental comparison

“…Additionally, inflow from all yaw position except from the wake sector (195 • ± 30 • ) was considered because the lidar yaw misalignment measurements are biased in wake situations (Held et al, 2018). The yaw position filter led to an additional exclusion of 6.0 % and 6.3 % of the data for the two-and four-beam periods, respectively.…”

“…Equation (18) is assuming that the first statistical moment is used to calculate the dominant frequency of the Doppler spectrum, which is a result of the Fourier analysis of the detected light signal. Different frequency estimators can yield less turbulence attenuation (Held and Mann, 2018). The Fourier transformation of the weighting function (Eq.…”

“…The lidar measurements have been simulated using Eq. (18); however, due to the finite size of the boxes, the integration has been truncated at ±10z R from the focus point; details can be found in Held and Mann (2018). The two lidar systems presented in Table 1 have been used.…”

Characterizing wind gusts in complex terrain

“…Lidars used in atmospheric applications can employ either light emitted in continuous-wave or pulsed forms (Held and Mann, 2018;Vasiljević et al, 2017). Measurements from two ZephIR continuous-wave Doppler lidars (referred to here as "lidars" and by their unit numbers, z447 and z423) are used to extend the analysis of gust parameters to heights above 100 m a.g.l.…”

“…Key features of flow in complex terrain include thermo-topographic flows arising from differential heating (Rucker et al, 2008;Rotach and Zardi, 2007) and lee-side vortices that develop parallel to mountain ridges (Grubišić et al, 2008). Regions with complex topography and land cover heterogeneity also tend to experience more frequent and stronger wind gusts (herein defined as coherent shortterm wind speed maxima) (Letson et al, 2018;Earl et al, 2017;Sheridan, 2011;Hasager et al, 2003) due in part to F. Letson et al: Characterizing wind gusts Wind gusts represent an important source of structural engineering loads for tall buildings, towers, bridges, and wind turbines (Solari, 1987;IEC, 2005;Cheynet et al, 2016), and are known to be of larger magnitude in complex terrain due in part to the factors listed above (Tieleman, 1992;Verheij et al, 1992). A number of numerical wind flow models have been developed for application at high spatial resolution over complex terrain, but model evaluation has been severely constrained by the lack of suitable observational data (Butler et al, 2015;Bechmann et al, 2011;Berg et al, 2011;Suomi and Vihma, 2018).…”