The Atlantic Stratocumulus Transition Experiment (ASTEX)--An Overview

Boundary layer clouds substantially impact on the earth's radiation budget and the visibility in the lower layers of the atmosphere. The Atlantic Stratocumulus Transition Experiment (ASTEX) was designed to 1) address key issues for stratocumulus to trade-cumulus transition and cloud mode selection, 2) improve our understanding of the cloud dynamical, radiative, and microphysical processes important for cloud maintenance, and 3) assess the impact of aerosols, cloud microphysics, and chemistry on cloud properties.

ASTEX is part of the First International Satellite cloud Climatology Program Regional Experiment (FIRE). Scientists from the United Kingdom, France, Germany Netherlands, and Portugal participated in the experiment. Although the office of Naval Research was the lead funding agency for ASTEX, substantial funds were provided by DOE, NASA, NOAA and NSF. The experiment was closely coordinated with a major experiment of the International Global Atmospheric chemistry Program's Marine Aerosol and Gas Exchange: Atmospheric Chemistry and Climate (MAGE) and a French air-sea interaction experiment called SOFLA.

ASTEX involved coordinated measurements from aircraft, satellites, ships, and islands in the area of the Azores and Madeira Islands during June of 1992. These measurements provided a detailed description of cloud and boundary layer structure using surface-based remote sensors and aircraft, a definition of the large-scale thermodynamic and wind fields from radiosondes launched 4-8 times daily from four ships and two islands, and satellite observations in the area of the surface measurements and in coordination with NASA's high altitude research aircraft, the ER-2.

Cloud conditions were excellent during the experiment and several important accomplishments were achieved including the following: 1) frequent sampling of processes associated with cloud transition, 2) measurements in areas with extreme variations in aerosol conditions, 3) successful execution of two Largrangian experiments where the same boundary layer air mass was sampled for two consecutive days with aircraft, 4) sampling the diurnal cycle from the surface sites and by aircraft, 5) assimilation of upper-air data from approximately 650 soundings into the ECMWF analyses, and 6) an unprecedented description of cloud and boundary layer properties using surface-based remote sensors.

Analyses in progress using the ASTEX data will lead to improved representation of cloud processes in climate and mesoscale models and improved techniques for the retrieval of cloud properties from satellites.