Cloudiness and Marine Boundary Layer Dynamics in the ASTEX Lagrangian Experiments. Part I: Synoptic Setting and Vertical Structure

A goal of the Atlantic Stratocumulus Transition Experiment (ASTEX) southeast of the Azores Islands in the east-central Atlantic Ocean during June 1992 was to examine the coupled evolution of cloud, dynamical, and thermodynamical vertical structure in a marine boundary layer (MBL) air mass as it advected from cold to warm water in the trade winds. In two “Lagrangian” observation periods during ASTEX, an unprecedentedly complete view of MBL and cloud evolution was achieved by nearly continuous aircraft coverage of such an air mass for 36–48 hours using three boundary layer aircraft, supplemented by satellite, ship, and balloon observations. During the first Lagrangian period, an accelerated stratocumulus to trade cumulus transition occurred in a clean marine air mass. In the second Lagrangian period, a 200-hPa-deep decoupled modified continental MBL persisted with almost no change in structure. Cumulus rising into intermittent stratocumulus were observed throughout the period. The two contrasting ASTEX Lagrangians will allow both direct comparison with MBL models and budget studies with essentially all uncertainty from poorly measured advective tendencies removed. The authors present the synoptic setting and the evolution of cloudiness as seen from satellite for both Lagrangians, and vertical sections of wind, temperature mixing ratio, liquid water, droplet concentration, and ozone formed from time series of 17 aircraft soundings during each Lagrangian. In Part II, an analyses of sea surface temperature and surface fluxes, cloudiness, drizzle, and entrainment rate during the Lagrangians are presented.