Direct Sampling of Entrainment Events in a Marine Stratocumulus Layer

Abstract

Entrainment mixing and the stability of a marine stratocumulus layer are investigated using aircraft data gathered during the FIRE marine stratocumulus experiment. Direct measurement of entrainment events is accomplished through conditional sampling based on ozone mixing ratio. This enables the identification of evaporatively cooled, negatively buoyant events that could otherwise only be inferred by thermodynamic jump conditions at cloud top. Evidence for evaporation cooling within entrainment events is obtained with aerosol and cloud droplet spectra. An increase in the entrainment of dry air into the cloud layer due to sinking of negatively buoyant downdrafts is offset by an increased flux of moisture through cloud base. The positive flux of moisture at cloud base is driven, in part, by the sinking of evaporatively cooled parcels through the entire cloud layer. Despite a minimum in buoyancy flux near cloud base, the cloud layer was not decoupled from the subcloud layer.