Intercomparison of three- and one-dimensional model simulations and aircraft observations of stratocumulus

As part of the EUropean Cloud REsolving Modelling (EUCREM) model intercompa rison project we compared the properties and development of stratocumulus a s revealed by actual observations and as derived from two types of models, namely three-dimensional Large Eddy Simulations (LES) and one-dimensional S ingle Column Models (SCMs). The turbulence, microphysical and radiation pro perties were obtained from observations made in solid stratocumulus during the third flight of the first 'Lagrangian' experiment of the Atlantic Strat ocumulus Transition Experiment (ASTEX). The goal of the intercomparison was to study the turbulence and microphysical properties of a stratocumulus la yer with specified initial and boundary conditions. The LES models predict an entrainment velocity which is significantly large r than estimated from observations. Because the observed value contains a l arge experimental uncertainty no definitive conclusions can be drawn from t his. The LES modelled buoyancy flux agrees rather well with the observed va lues, which indicates that the intensity of the convection is modelled corr ectly. From LES it was concluded that the inclusion of drizzle had a small influence (about 10%) on the buoyancy flux. All SCMs predict a solid strato cumulus layer with the correct liquid water profile. However, the buoyancy flux profile is poorly represented in these models. From the comparison wit h observations it is clear that there is considerable uncertainty in the pa rametrization of drizzle in both SCM and LES.