Large-Eddy Simulation of a stratocumulus to cumulus cloud transition as observed during ASTEX

Large-eddy simulation (LES) results of the First Lagrangian during the Atlantic Stratocumulus Transition EXperiment (ASTEX) are compared with aircraft observations. In this experiment a column of air was tracked for 48 hours as it moved south-westward over water with increasing sea surface temperatures. The cloud developed from a solid well-mixed stratocumulus layer to a layer filled with cumulus clouds penetrating thin and broken stratocumulus above. The model was initialized in accord with a previous GCSS model intercomparison case that simulated only four hours of the second flight and neglected the effect of drizzle. Taking into account drizzle and cloud droplet sedimentation in the LES tends to diminish the entrainment rate. A key feature of the boundary layer evolution is a gradual weakening of the inversion stability. The observations showed a significant increase of turbulence during the second night, which is well represented by the LES model. The effect of the presence of shallow cumulus on the buoyancy flux profiles and decoupling of the boundary layer will be discussed.