Effects of resolution on the simulation of stratocumulus entrainment

Three-dimensional large-eddy simulations (LESs) of a smoke cloud beneath a strong inversion are used to investigate how grid resolution affects simula ted 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 bound ary-layers. The simulations are compared to a laboratory analogue for a range of invers ion 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-rate s for strong inversions are two large by a factor of two. Better agreement is obtained when the numerical model resolves the undulations of the invers ion driven by the underlying convection. By this criterion, most LES studie s of subtropical maritime stratocumulus made so far are under-resolved. For determining the bulk entrainment-rate, a nested-grid model which has fi ne resolution only in the entrainment zone is as effective as a model with uniformly fine resolution. If horizontal resolution is much coarser than th e vertical resolution, modelled entrainment-rates are less than those obtai ned with simulations which have comparable vertical and horizontal resoluti ons.