Large wind farms can destroy stratocumulus clouds-evidence from large eddy simulations.

Stratocumulus is one of the most common cloud types found in the atmospheric boundary layer (ABL) and have been shown to have a significant impact on modulating the global radiation and energy balance. In spite of their ubiquitousness however, their shallowness make it particularly hard for regional and global scale atmospheric models to accurately represent this cloud type. For such models, these clouds are usually 'subgrid-scale' and thus parametrizations are the only resort. Most parametrizations only use local thermodynamic information and the role of large-scale vertical shear is almost never taken into account. In study presented herein, results of large-eddy simulations (LES) of stratocumulus-topped boundary layers (STBL) with a very large immersed wind farm are presented. The simulated stratocumulus from the DYCOMS-II case study. It is shown that, in the idealized conditions of these simulations, large wind farms induce significant vertical shear at the interface of the STBL and the free atmosphere resulting in enhanced vertical entrainment. The entrainment is shown to destabilize the cloud, with even moderate wind farm loadings significantly reducing the cloud fraction, the cloud thickness and the liquid water path. With rapidly increasing marine wind energy exploitation and installation of ever-larger wind farms, both in their spatial extent and the geometry of the constituent wind turbines, the effect of wind farms of marine boundary layers and particularly marine stratocumulus is a pertinent question and our results show that due to the destabilizing effect of wind farms, they may contribute to net heating in the global energy balance.