Interactions Between Continental Aerosols Particles
and Marine Stratocumulus Clouds During ASTEX

This paper addresses the indirect impact of aerosols on climate through modification of cloud microphysical structure and subsequent radiative characteristics, as observed during the Atlantic Stratocumulus Transition Experiment (ASTEX). During ASTEX a large variation aerosol concentration was observed. The first half of the experiment (1-14 June 1992) was conducted in relatively clean, marine conditions with total aerosol number concentrations of 300-700 cm^-3. From 14-16 June a large plume of continental aerosols (containing urban pollution and desert dust) moved off the Iberian Penninsula into the operating area. The aerosol number concentrations during the remainder of the experiment (through 27 June) were typically 2000-5000 cm^-3. Satellite analysis of aerosol and cloud characteristics from the Advanced Very High Resolution Radiometer (AVHRR) on the NOAA operational polar orbiting satellites (NOAA 10/11/12) will be described in this paper. The aerosol optical depth and size characteristics of the continental plume will be shown along with the resulting cloud characteristics including droplet size and radiative properties. Individual image analysis will be presented with airmass trajectory results to describe the sources of aerosol features and their impact on subsequent cloud formation. An regional analysis will also be presented that will show the regional scale radiative imact of the aerosol particles on cloud reflectance.