Lidar observations of the variation of wind turbine wakes with inflow conditions in an onshore wind farm

Detailed observations of the interactions of wakes of multiple turbines in large onshore wind farms are required for validation of flow simulation models for assessing and forecasting wind power production and loads on individual turbines. The CWEX-13 field campaign was designed to explore the propagation of individual turbine wakes as well as the interaction of multiple wakes in a range of atmospheric stability conditions. CWEX-13 took place between late June and early September 2013 in a 150 MW wind farm in central Iowa, the same wind farm studied in previous CWEX campaigns. The region is characterized by flat topography and enjoys strong diurnal cycles of atmospheric stability as well as regular occurrences of nocturnal low-level jets. Multiple remote sensing systems characterized winds, temperature, and moisture profiles throughout the wind farm, complementing an array of surface-based meteorological stations. Wind turbine wakes effects are clearly visible in both profiling lidar and scanning lidar observations from CWEX-13, which span a range of stability conditions. The attached figure shows a lidar scan at an elevation angle of 1.8 degrees during a case of southeasterly flow. The lidar is located at the origin, and four turbines are located south-south-east of the lidar at a range of approximately 2000. During southeasterly flow of approximately 7 m/s at 80 m above the surface, the four turbine wakes are clearly visible in these lidar scans as plumes of wind speed deficit extending to the northwest. With several hundred scans such as this during southerly flow conditions, we present quantitative analysis of wake variation with inflow wind speed, ambient turbulence, and atmospheric stability.