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

Abstract

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 aweak-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 35m, 10m, and 3.5m. 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 represents insufficiently 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 sub-grid scale models by buoyant suppression of eddies at the large-eddy simulation filter scale. Second, the decrease in cloud-top entrainment due to these insufficient represented processes results in 15% less cloud water after six hours of simulation and a corresponding optical thinning of the cloud. Third, we showthat the applied non-equilibrium microphysics cause microphysical heating beneath the cloud-top, which partly counteracts the evaporative cooling.