Eeects of Resolution on the Simulation of Stratocumulus Entrainment

Three-dimensional large-eddy simulations of a smoke cloud beneath a strong inversion are used to investigate how grid resolution aaects simulated entrainment rate. These simulations are generalizations of a 1995 GCSS (GEWEX Cloud System Studies) smoke cloud model intercomparison (MacVean et al. 1997). Radiative cooling at the top of the smoke cloud induces convection in the smoke cloud in a manner closely analogous to marine stratocumulus-capped boundary layers. The simulations are compared to a laboratory analogue for a range of inversion strengths and radia-tive forcings. The inverse Richardson number scaling of nondimensional entrainment rate found in the laboratory experiments is largely captured by the numerical model, but the modelled entrainment rates are a factor of two too large for strong inversions. Better agreement is obtained when the numerical model resolves the undulations of the inversion driven by the underlying convection. Most large-eddy simulation studies of subtropical marine stratocumulus to date are underresolved by this criterion. A nested grid model with ne resolution only in the entrainment zone is as eeective as a uniformly ne resolution model in accurately determining the bulk entrainment rate. If horizontal resolution is coarse compared to vertical resolution, the modelled entrainment rate is slightly underestimated compared to simulations with comparable vertical and horizontal resolution.