Effects of resolution on the simulation of stratocumulus entrainment

Abstract

AbstractThree‐dimensional large‐eddy simulations (LESS) of a smoke cloud beneath a strong inversion are used to investigate how grid resolution affects simulated entrainment‐rate. These simulations are generalizations of a 1995 GEWEX Cloud System Studies intercomparison of models of smoke clouds. Radiative cooling at the top of the smoke cloud induces convection in the smoke cloud in a manner closely analogous to that in marine stratocumulus‐capped boundary‐layers.The simulations are compared to a laboratory analogue for a range of inversion strengths and radiative forcings. the inverse Richardson‐number scaling of non‐dimensional entrainment‐rate found in the laboratory experiments is largely captured by the numerical model, but the modelled entrainment‐rates for strong inversions are two large by a factor of two. Better agreement is obtained when the numerical model resolves the undulations of the inversion driven by the underlying convection. By this criterion, most LES studies of subtropical maritime stratocumulus made so far are under‐resolved.For determining the bulk entrainment‐rate, a nested‐grid model which has fine resolution only in the entrainment zone is as effective as a model with uniformly fine resolution. If horizontal resolution is much coarser than the vertical resolution, modelled entrainment‐rates are less than those obtained with simulations which have comparable vertical and horizontal resolutions.