Stability Criteria for Stratocumulus-Topped Boundary Layers Evaluated ThroughConditional Sampling of Turbulence Elements

摘要

The stability of stratocumulus layers to breakup is in large part determined bythe nature of turbulent exchanges across cloud base and across the capping inversion. Conditional sampling techniques have been applied to aircraft measurements of boundary layer turbulence in the stratocumulus regime off the southern California coast in order to evaluate processes believed important to the breakup of stratocumulus. Results show that cloud layers can persist despite the production of evaporatively cooled, negatively buoyant parcels near cloud top, a condition known as cloud top entrainment instability. The drying effect produced by the sinking of such parcels through the cloud layer is compensated for by the supply of moisture from surface-based plumes, contrary to implications that the cloud layer was decoupled from the subcloud layer as indicated by weak mean buoyancy flux at cloud base. Measurement and analysis of individual turbulence elements will continue to assist in the testing of proposed theories of stratocumulus breakup as well as help guide the formulation of new theories. (KR)