Abstract
Wind energy has been growing steadily in the U.S. and worldwide in the past decades. As wind farms are projected to increase in size and number, however, concerns are rising about possible undesirable effects of wind turbines near the Earth's surface. The literature is highly divided about what these effects could be, including warming, cooling, both, or neither. Only one mechanism, however, has been widely accepted (but never tested) to explain how wind turbines affect the lower boundary layer, namely that turbulence generated in wind turbine wakes enhances vertical mixing near the ground. Wakes are plume-like volumes downwind of wind turbines that are characterised by lower wind speeds and higher turbulent kinetic energy (TKE) than the undisturbed upwind flow. The few observational campaigns that have measured changes in near-surface properties by wind turbines have not provided an answer with respect to vertical mixing near the ground. To fill this knowledge gap, the VERTEX (VERTical Enhanced miXing) measurement campaign was conducted in August-October 2016 near a large wind turbine in coastal Delaware, using 15 surface flux towers, a 50-m meteorological tower, a radiometer, and two scanning lidars. During VERTEX, lidar scans and a wake detection algorithm were used to detect wake events and identify which sites were affected by the wake of the wind turbine. TKE, momentum and heat fluxes near the ground were compared between the sites below the wake and those outside of it. Preliminary findings based on two case studies (30 August and 20 September 2016) suggest a lack of enhancement of vertical mixing near the ground.