CLOUDINESS AND MARINE BOUNDARY-LAYER DYNAMICS IN THE ASTEX LAGRANGIANEXPERIMENTS .1. SYNOPTIC SETTING AND VERTICAL STRUCTURE

A goal of the Atlantic Stratocumulus Transition Experiment (ASTEX) sou theast of the Azores Islands in the east-central Atlantic Ocean during June 1992 was to examine the coupled evolution of cloud, dynamical, a nd thermodynamical vertical structure in a marine boundary layer (MEL) air mass as it advected from cold to warm water in the trade winds. I n two ''Lagrangian'' observation periods during ASTEX, an unprecedente dly complete view of MBL and cloud evolution was achieved by nearly co ntinuous aircraft coverage of such an air mass for 36-48 hours using t hree boundary layer aircraft, supplemented by satellite, ship, and bal loon observations. During the first Lagrangian period, an accelerated stratocumulus to trade cumulus transition occurred in a clean marine a ir mass. In the second Lagrangian period, a 200-hPa-deep decoupled mod ified continental MBL persisted with almost no change in structure. Cu mulus rising into intermittent stratocumulus were observed throughout the period. The two contrasting ASTEX Lagrangians will allow both dire ct comparison with MBL models and budget studies with essentially all uncertainty from poorly measured advective tendencies removed. The aut hors present the synoptic setting and the evolution of cloudiness as s een 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 Lag rangian. In Part II, an analyses of sea surface temperature and surfac e fluxes, cloudiness, drizzle, and entrainment rate during the Lagrang ians are presented.