Coplanar lidar measurement of a single wind energy converter wake in distinct atmospheric stability regimes at the Perdigão 2017 experiment

The understanding of wakes of wind energy converters (WEC) is highly relevant to wind-energy research. In orographically complex terrain, wind is amplified at ridges but also varies significantly, exhibiting strong gustiness and enhanced turbulence levels compared to wind conditions in flat terrain. In this study, long-range lidar instruments are used to detect and analyse the wake of a single WEC in complex terrain in different atmospheric stability regimes. The special orography of two parallel mountain ridges at the Perdigao 2017 experiment allowed to place two lidars in a coplanar set-up and retrieve horizontal as well as vertical wind speed in a cross-section of the terrain which is in-plane with the WEC. In cases of main wind direction, which is parallel to the lidar scans, the wake's propagation can thus be measured far downstream in the valley. A wake tracking algorithm is proposed to automatically detect the wake center in the lidar scans for three periods with distinct atmospheric stability conditions. Wind speed deficits and wake propagation paths are quantified and categorized accordingly. A careful uncertainty estimation is done for the coplanar wind retrieval.