Cloud-Top Entrainment in Mixed-Phase Stratocumulus and Its Process-Level Representation in Large-Eddy Simulation

Cloud-top entrainment is a crucial process for the evolution of stratocumulus and is driven by interactions of radiation, microphysics, and turbulence on scales reaching down to less than one meter. Regardless of this fact, most large- eddy simulation studies still apply a horizontal resolution of tens of meters, not resolving these interactions sufficiently. Here, based on an extensive observational campaign, we define a weak- shear benchmark scenario for large-eddy simulation over Arctic ice and for the first time perform large-eddy simulation of mixed-phase stratocumulus with horizontal resolutions of 35, 10, and 3.5 m. Thereby, we investigate the processes contributing to cloud-top entrainment and their role for the evolution of stratocumulus with a particular focus on resolution sensitivity. First, we find that a horizontal grid spacing larger than 10m insufficiently represents the effects of small-scale microphysical cooling and turbulent engulfment on cloud-top entrainment. Indeed, the small size of energy-containing eddies-a consequence of the intense stratification in the vicinity of the cloud-top region-violates the underlying assumptions of subgridscale models by buoyant suppression of eddies at the large-eddy simulation filter scale. Second, the decrease in cloud-top entrainment due to these insufficiently represented processes results in 15% less cloud water after 6 h of simulation and a corresponding optical thinning of the cloud. Third, we show that the applied nonequilibrium microphysics cause microphysical heating beneath the cloud top, which partly counteracts the evaporative cooling.