Most current GCMs have serious deficiencies in their simulation of boundary layers topped by stratocumulus and shallow cumulus clouds, particularly in the subtropics and tropics. Typical problems include underprediction of stratocumulus cloud amount off the subtropical west coasts of the major landmasses and a dry bias at 850 mb in the tropics due to insufficient shallow convection. We discuss a parameterization for handling these problems that works well at typical GCM vertical resolutions.
The scheme is cast in terms of thermodynamic variables conserved in phase changes, unlike the 'dry' boundary layer schemes used by most GCMs. It is based on TKE-based turbulence closure within the boundary layer, with fluxes at the top of the boundary layer specified using a nonlocally determined entrainment rate. The scheme is meshed with a Tiedtke cumulus parameterization to allow shallow stratocumulus and cumulus to interact in a physically realistic way. It allows cloud layers of subgridscale thickness to form and realistically represents their radiative effects.
We show that this scheme is capable of representing stratocumulus clouds and their transition to a trade cumulus layer in a remarkably realistic way, and can simulate the diurnal cycle of stratocumulus cloud properties well