Large-eddy simulations of wind parks in clear and cloud-topped boundary layers

Low clouds like stratocumulus are frequently present off the coast of California and above the North Sea. Experiments with the Dutch Large Eddy Simulation Model (DALES) have been performed to study the effect of boundary-layer turbulence on the power of wind parks. We compare results from the EUROCS/FIRE stratocumulus model intercomparison case with results as obtained for a simulation with identical surface sensible heat fluxes but without clouds. It is found that in particular during night-time, when longwave radiative cooling of the stratocumulus is producing strong turbulence, the velocity deficit in the wake is diminished much faster as compared to the clear sky case. As another example of the interaction of wind turbines with boundary layer turbulence, the triggering of wake clouds as observed during the Horns Rev case will be discussed.