An Observational Study of Stratocumulus Entrainment and Thermodynamics

Abstract

The marine stratocumulus regime is studied using observational data from the Dynamics and Chemistry of Marine Stratocumulus (DYCOMS) experiment. Such regions are climatically important because of their influence on the earth?atmosphere energy budget. The rate of entrainment of air from the free troposphere into the cloud-topped planetary boundary layer is estimated using a technique based on measurement of ozone flux and mean distribution. This rate is an important parameter for boundary-layer modeling. The average measured value of entrainment velocity is 3.0 mm s?1 with a range of 1.0 to 5.0 mm s?1 for cloudy cases. Variation in the rate between cases is not well correlated with variations in buoyancy or moisture fluxes in the boundary layer. Thermodynamic budgets are constructed for eight DYCOMS cases. Divergence-of the solar radiative flux is an important component of the boundary-layer energetics during midday. This is a significant complication to mixed-layer models. The net longwave (terrestrial) radiative flux profiles consistently show good agreement with theoretical models, but an unambiguous partitioning of the flux divergence between inversion and mixed layers could not be established. Examples of stratocumulus breakup and reformation are also examined. These episodes appear to be in response to differing perturbations.