Impact of atmospheric stability and turbulence on wind turbine wake characteristics: a nacelle lidar study

Abstract. Wind turbine wakes reduce the generated power and increase loads on downstream turbines. Their characteristics depend strongly on the atmospheric conditions in the boundary layer. This study addresses the complex turbine-atmosphere interaction specifically in the near wake region up to four rotor diameters downstream of a utility-scale wind turbine. We utilize an exceptionally large database of concise measurements of inflow conditions and wake characteristics collected from November 2023 to June 2024 at the WiValdi research wind farm in northern Germany. The dataset comprises measurements from a downstream-looking Doppler wind lidar mounted on the nacelle, a meteorological inflow mast and wind turbine operational data. Wake characteristics and near wake lengths are deduced from the lidar scanning at multiple horizontal planes and are analyzed across a wide range of atmospheric conditions, including stability, wind shear, veer and turbulence. The wake velocity deficit is observed to be reduced with stronger turbulence and enhanced under stable conditions. Stronger wind veering across the rotor layer, in the absence of yaw misalignment, correlates to intensified wake deflection and to stronger vertical tilting. A high shear exponent and potential temperature gradient are associated with increased lateral asymmetry of the velocity deficit's double Gaussian peaks at one rotor diameter downstream. We find that the near wake extends on average 2.01 rotor diameters downstream, with a standard deviation of 0.41 rotor diameters. The near wake length exhibits greater sensitivity to atmospheric conditions than to turbine operational parameters, with the strongest correlations found for turbulence intensity and static stability. Under strongly stable conditions and weak turbulence, near wake lengths are particularly long reaching up to 3.8 rotor diameters downstream. This study highlights the importance of considering diverse meteorological inflow conditions when refining and validating wind turbine wake models.