Abstract
Lidar scan techniques for wind profiling rely on the assumption of a horizontally homogeneous wind field and stationarity for the duration of the scan. As this condition is mostly violated in reality, detailed knowledge of the resulting measurement error is required. The objective of this study is to quantify and compare the expected error associated with Doppler-lidar wind profiling for different scan strategies and meteorological conditions by performing virtual measurements implemented in a large-eddy simulation (LES) model. Various factors influencing the lidar retrieval error are analyzed through comparison of the wind measured by the virtual lidar with the ‘true’ value generated by the LES. These factors include averaging interval length, zenith angle configuration, scan technique and instrument orientation. For the first time, ensemble simulations are used to determine the statistically expected uncertainty of the lidar error. The analysis reveals a root-mean-square deviation (RMSD) of less than 1 m s−1 for 10 min averages of wind speed measurements in a moderately convective boundary layer, while RMSD exceeds 2 m s−1 in strongly convective conditions. Unlike instrument orientation and scanning scheme, the zenith angle configuration proved to have significant effect on the retrieval error. Horizontal wind speed error is reduced when a larger zenith angle configuration is used, but is increased for measurements of vertical wind. Results suggest that the scan strategy has a relevant effect on the lidar retrieval error and that instrument configuration should be chosen depending on the quantity of interest and the flow conditions in which the measurement is performed.
Highlights
Profiling Doppler lidars are nowadays widely used for applications like wind energy, airport safety and air quality control (Courtney et al, 2008; Antoniou et al, 2007; Emeis et al, 2007; Nechaj et al, 2019; Cottle et al, 2014)
255 These coherent structures which are present in the turbulent flow field indicate that the requirement of a horizontally homogeneous wind field within the air volume scanned by the lidar is not met by the instantaneous flow field
Comparing the virtual measurements with the truth value of the large-eddy simulation (LES), we are able to quantify the retrieval error that arises due to the violation of the homogeneous wind field assumption
Summary
Profiling Doppler lidars are nowadays widely used for applications like wind energy, airport safety and air quality control (Courtney et al, 2008; Antoniou et al, 2007; Emeis et al, 2007; Nechaj et al, 2019; Cottle et al, 2014). Lidars have become relevant in the field of numerical weather prediction (NWP). State-of-the-art NWP models increasingly require 20 wind profile measurement data of the atmospheric boundary layer (ABL) for assimilation (Knist et al, 2018), to improve the prescription of initial conditions for the simulations. Lidar scan techniques rely on the assumption of a horizontally homogeneous wind field and stationary conditions during the measurement, since a series of wind measurements is performed sequentially along slanted paths at different azimuth directions.
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