AVHRR Observations of the Aerosol Indirect Effect for Summertime Stratiform
Clouds in the Northeastern Atlantic
Mark A. Matheson, James A. Coakley. Jr., and William R. Tahnk
College of Oceanic and Atmospheric Sciences and
Cooperative Institute for Oceanographic Satellite Studies
Oregon State University
Advanced Very High Resolution Radiometer (AVHRR) 4-km imager pixels are collected
over the northeastern Atlantic off the coast of the Iberian Peninsula for May-August,
1992-2001. The AVHRR radiances are used to retrieve aerosol optical depths in cloudfree pixels and cloud properties—visible optical depth, droplet effective radius, cloud
height, and pixel-scale cloud cover fraction in cloudy pixels. Aerosol optical depths in 1
latitude-longitude squares on a given day are compared with the properties of low-level
clouds in the same and adjacent 1 squares on the same day. Results are composited for
small (e.g. 5  5) regions to study the influence of geographically controlled trends.
For most years in most regions, there is a statistically significant decrease in droplet
effective radius and an increase in cloud visible optical depth as aerosol optical depth
increases. These trends indicate that the clouds are responding to increases in particle
concentrations. Depending on the year and region, cloud liquid water path can increase,
remain constant, or decrease as aerosol optical depth increases. The dependence of the
cloud liquid water on the prevailing meteorological conditions is investigated. In
addition, cloud fraction is relatively insensitive to aerosol optical depth suggesting that
the aerosol retrievals are relatively free of cloud contamination. These results offer
observational constraints to simulations of the aerosol indirect effect in climate models.
For the most part, the results suggest that climate models overestimate the magnitude of
the aerosol indirect forcing.