Precipitation in marine cumulus and stratocumulus. Part I: Thermodynamic and dynamic observations of closed cell circulations and cumulus bands

A case study of vigorous drizzle development in marine boundary layer clouds is presented. The clouds were observed using a research aircraft west of Tasmania during the Southern Ocean Cloud Experiment. A clean marine boundary layer contained both cumulus clouds (some oriented in bands) and an upper level stratus deck organised in a closed cell circulation. Cold pools, that is regions of evaporatively cooled air under mature cumulus clouds, were observed on several occasions. Air in the cold pool centre showed a divergent component of the wind perpendicular to the mature cumulus bands. Ahead of this spreading precipitation downdraft, the wind component perpendicular to the cumulus band was convergent, and new cumulus clouds were subsequently formed over these regions. This dynamical situation is likened to an actively developing squall line of the propagating type. The development of drizzle, and its evaporation in the mixed-layer below the base of mature cumulus bands, is critical to the cloud evolution. It is demonstrated here, that processes resembling deep convection may also be responsible for the cloud development in a marine boundary layer of only 1500 m depth. (C) 2000 Elsevier Science B.V. All rights reserved.