A GCSS boundary-layer cloud model intercomparison study of the first ASTEXLagrangian experiment

Three single-column models (all with an explicit liquid water budget and co mpara-tively high vertical resolution) and three two-dimensional eddy-resol ving models (including one with bin-resolved microphysics) are compared wit h observations from the first ASTEX Lagrangian experiment. This intercompar ison was a part of the second GCSS boundary-layer cloud modelling workshop in August 1995. In the air column tracked during the first ASTEX Lagrangian experiment, a s hallow subtropical drizzling stratocumulus-capped marine boundary layer dee pens after two days into a cumulus capped boundary layer with patchy strato cumulus. The models are forced with time varying boundary conditions at the sea-surface and the capping inversion to simulate the changing environment of the air column. The models all predict the observed deepening and decoupling of the boundar y layer quite well, with cumulus cloud evolution and thinning of the overly ing stratocumulus. Thus these models all appear capable of predicting trans itions between cloud and boundary-layer types with some skill. The models a lso produce realistic drizzle rates, but there are substantial quantitative differences in the cloud cover and liquid water path between models. The d ifferences between the eddy-resolving model results are nearly as large as between the single column model results. The eddy resolving models give a m ore detailed picture of the boundary-layer evolution than the single-column models, but are still sensitive to the choice of microphysical and radiati ve parameterizations, sub-grid-scale turbulence models, and probably model resolution and dimensionality. One important example of the differences see n in these parameterizations is the absorption of solar radiation in a spec ified cloud layer, which varied by a factor of four between the model radia tion parameterizations.