Radiation feedback and grid convergence in LES of stratocumulus clouds

Stratocumulus clouds (Sc) have a large impact on the Earth's radiative balance and even though is one of the most studied cloud systems, boundary layer parameterizations in global circulation models remain challenging. Typically, large-eddy simulations (LES) are sufficiently reliable and are used to gain insight into boundary layer physics and to inform the development and evaluation of coarse-grained models. However, LES of Sc has been challenging compared to other boundary layer types. The source of difficulty in LES of Sc is studied using a series of numerical experiments. A strong feedback between cloud liquid, cloud top radiative cooling, and turbulence leads to slow grid convergence of the turbulent fluxes. In contrast, when the liquid-radiation-buoyancy feedback is not present in simulations without radiation, the turbulence structure of the boundary layer remains essentially identical for grid resolutions between 20 and 1.25 m. The entrainment rate does not depend on grid resolution but shows strong dependence on physical processes. For fine grid resolutions, the LES results agree with observations, especially with respect to cloud liquid, the vertical velocity variance, and the vertical velocity triple correlation without any model tuning or ad hoc model choices.