Multiyear Advanced Very High Resolution Radiometer observations of summertime stratocumulus collocated with aerosols in the northeastern Atlantic

Abstract

Advanced Very High Resolution Radiometer (AVHRR) 4‐km data were collected over the northeast Atlantic for May–August 1995–1999. Aerosol optical depth at 0.55 μm was retrieved in pixels identified as being cloud‐free ocean. In pixels identified as containing clouds from single‐layered, low‐level cloud systems over oceans, the following cloud properties were retrieved: 0.64‐μm cloud optical depth, droplet effective radius, cloud layer altitude, pixel‐scale fractional cloud cover, column liquid water amount and column droplet concentration. Aerosol and cloud properties were averaged in 1° × 1° latitude‐longitude regions. Regions that contained clouds were limited to those in which all the clouds were part of a single‐layered, low‐level cloud system. Aerosol and cloud properties were compared only in 1° regions that had sufficient numbers of both cloud‐free pixels that yielded aerosol retrievals and cloudy pixels that yielded retrievals of cloud properties within a single overpass. The comparisons were collected in 5° × 5° latitude‐longitude regions to determine trends. Within each 5° region the cloud properties were similar from year to year, permitting the data to be composited for all 5 years. Aerosol optical depth decreased systematically with time, probably as a result of the increase in solar zenith angle due to the precession of the satellite orbit. Within the 5° regions, as aerosol optical depth increased, droplet effective radius decreased, cloud optical depth increased, and droplet column number concentration increased, qualitatively consistent with the trends expected for the aerosol indirect effect. In some regions, liquid water path decreased as aerosol optical depth increased, contrary to the trends expected for the suppression of drizzle. Within each 5° region, clouds in clean air, as indicated by their collocation with relatively small aerosol optical depths, had larger droplets and smaller cloud optical depths than clouds in polluted air, as indicated by their collocation with relatively large aerosol optical depths. On average, the aerosol indirect radiative forcing for overcast conditions was about twice as large as the direct radiative forcing for cloud‐free conditions. In most of the 5° regions increases in cloud liquid water with increasing aerosol optical depth enhanced the ratios by 20−30% over those calculated from the changes in droplet effective radius and an assumption of constant cloud liquid water. In some 5° regions, however, like those just to the west of the Iberian peninsula, the column amount of cloud liquid water decreased with increasing aerosol optical depth.