Large-Eddy Simulation of a Stratus-Topped Boundary Layer. Part II: Implications for Mixed-Layer Modeling

Abstract

Two sets of large-eddy simulation data were used to study some of the assumptions about the cloud-topped boundary layer (CTBL) structure which are used in mixed-layer models. The roles of buoyant production and cloud-top radiative cooling in turbulent kinetic energy generation were examined. The buoyant production in the turbulent kinetic energy (TKE) budgets was partitioned into production and consumption components by grouping the warm-rising and cold-sinking parcels and the warm-sinking and cold-rising parcels, separately. The results indicate that the ratio of the consumption part of the buoyant production to the sum of the shear production and the production part of the buoyant production is 0.15 for the clear convective mixed layer, and 0.22 for the CTBL. Almost all mixed-layer models use the experimentally (from either direct measurement or tank experiment) obtained ratio of the buoyancy flux at the top of the clear convective boundary layer to the flux at the surface for their entrainment constant. Those direct measurement or tank experiment studies adopt the horizontal average in the usual Eulerian coordinates to define their ensemble average. A mixed-layer model, therefore, must also use the same type of averaging process to define the radiative cooling distribution near the cloud top. In that case, the 1arge-eddy simulation results indicate that about 85% of the cooling must occur within the entrainment zone and 15% within the well-mixed layer, for relatively dense clouds.