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. In the context of the Perdigão 2017 experiment, the German AerospaceCenter (DLR) deployed three long-range scanning Doppler lidars with the dedicatedpurpose of investigating the wake of a single wind turbine at theexperimental site. A novel method was tested for the first time toinvestigate wake properties with ground-based lidars over a wide range ofwind directions. For this method, the three lidars, which were space- andtime-synchronized using the WindScanner software, were programmed to measurewith crossing beams at individual points up to 10 rotor diameters downstreamof the wind turbine. Every half hour, the measurement points were adapted to thecurrent wind direction to obtain a high availability of wake measurements inchanging wind conditions. The linearly independent radial velocities wherethe lidar beams intersect allow the calculation of the wind vector at thosepoints. Two approaches to estimating the prevailing wind direction were testedthroughout the campaign. In the first approach, velocity azimuth display (VAD) scans of one of thelidars were used to calculate a 5 min average of wind speed and winddirection every half hour, whereas later in the experiment 5 minaverages of sonic anemometer measurements of a meteorological mast close tothe wind turbine became available in real time and were used for the scanningadjustment. Results of wind speed deficit measurements are presented for twomeasurement days with varying northwesterly winds, and it is evaluated howwell the lidar beam intersection points match the actual wake location. Thenew method allowed wake measurements to be obtained over the whole measurementperiod, whereas a static scanning setup would only have captured shortperiods of wake occurrences.

Wind turbine wake measurements with automatically adjusting scanning trajectories in a multi-Doppler lidar setup