Near-ground effects of wind turbines: Observations and physical mechanisms

Wind turbines generate turbulent wakes, which can potentially influence the local microclimate near the ground. To verify and quantify such effects, the VERTical Enhanced miXing (VERTEX) field campaign was conducted in late summer 2016 to measure near-surface turbulent fluxes, wind speed, temperature, and moisture under and outside of the wake of an operational wind turbine in Lewes, Delaware. The main finding is that, in the presence of a wake, vertical mixing (i.e., friction velocity u_*, turbulent kinetic energy, and wind speed) is reduced near the ground, not enhanced as previously thought. Sensible heat flux and moisture near the ground are not significantly affected by the wake. The observed near-ground temperature changes were < 0.3K in magnitude. Near-ground temperature changes due to the wake did not correlate well with surface stability but rather with the temperature lapse rate between hub height and the ground with warming observed during stable and neutral conditions and cooling during unstable conditions. We propose that the mechanism that drives changes in near-ground temperature in the presence of turbine wakes is the vertical convergence of sensible heat flux below hub height. Inside the wake, vertical mixing is enhanced and so is sensible heat flux; near the ground, vertical mixing is reduced and sensible heat flux is unchanged. These conditions cause an increase (during stable/neutral rotor stability) or decrease (during unstable rotor stability) in heat flux convergence, ultimately resulting in warming or cooling near the ground, respectively. Large-eddy simulations (LES) have been performed to validate the mechanism.