Aerosol Influence on Cloud Optical Depth and Albedo Over the North Atlantic Shown by Satellite Measurements and Chemical Transport Modeling Stephen E. Schwartz, Carmen M. Benkovitz(1) and Harshvardhan(2). American Geophysical Union Fall Meeting, December 10-14, 2001, San Francisco.

(1) Atmospheric Sciences Division, Brookhaven National Laboratory, Upton NY 11973

(2) Dept. of Earth and Atmospheric Sciences, Purdue University, West Lafayette IN 47907-1397

The Twomey effect of enhanced cloud droplet concentration, optical depth, and albedo due to anthropogenic aerosols is thought to contribute substantially to radiative forcing of climate change over the industrial period. However, present model-based estimates of this indirect forcing are highly uncertain. Satellite-based measurements would provide global or near-global coverage of this effect, but previous efforts to use satellite observations to identify and quantify enhancement of cloud albedo due to anthropogenic aerosols have been limited, largely because of strong dependence of albedo on cloud liquid water path (LWP), which is inherently highly variable. Here we examine satellite-derived cloud radiative properties over a one-week episode for which a chemical transport and transformation model indicates that sulfate aerosol in a remote area of the North Atlantic experienced a substantial excursion due to transport from Northern Europe. Despite the absence of discernible dependence of optical depth or albedo on modeled sulfate loading, examination of the dependence of these quantities on LWP readily permits detection and quantification of increases correlated with sulfate loading which are otherwise masked by variability of LWP, demonstrating brightening of clouds due to the Twomey effect on a synoptic scale. Median cloud-top spherical albedo was enhanced over the episode, relative to the unperturbed base case for the same LWP distribution, by 0.04 to 0.15. Examination of the dependence of cloud optical depth and albedo on LWP should be broadly applicable to identification of enhancement by anthropogenic aerosols and well suited for global-scale characterization by satellite measurements.

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