The effect of smoke, dust, and pollution aerosol on shallow cloud development over the Atlantic Ocean

doi:10.1073/pnas.0505191102

The effect of smoke, dust, and pollution aerosol on shallow cloud development over the Atlantic Ocean

^*National Aeronautics and Space Administration Goddard Space Flight Center, Greenbelt, MD 20771; ^‡Joint Center for Earth Systems Technology, University of Maryland, Baltimore County, Baltimore, MD 21228-4664; ^§Institute of Earth Sciences, Hebrew University of Jerusalem, Jerusalem 91904, Israel; and ^¶Department of Environmental Sciences, Weizmann Institute, Rehovot 76100, Israel

Communicated by Veerabhadran Ramanathan, Scripps Institution of Oceanography, La Jolla, CA, June 21, 2005 (received for review January 21, 2005)

Abstract

Clouds developing in a polluted environment tend to have more numerous but smaller droplets. This property may lead to suppression of precipitation and longer cloud lifetime. Absorption of incoming solar radiation by aerosols, however, can reduce the cloud cover. The net aerosol effect on clouds is currently the largest uncertainty in evaluating climate forcing. Using large statistics of 1-km resolution MODIS (Moderate Resolution Imaging Spectroradiometer) satellite data, we study the aerosol effect on shallow water clouds, separately in four regions of the Atlantic Ocean, for June through August 2002: marine aerosol (30°S–20°S), smoke (20°S–5°N), mineral dust (5°N–25°N), and pollution aerosols (30°N– 60°N). All four aerosol types affect the cloud droplet size. We also find that the coverage of shallow clouds increases in all of the cases by 0.2–0.4 from clean to polluted, smoky, or dusty conditions. Covariability analysis with meteorological parameters associates most of this change to aerosol, for each of the four regions and 3 months studied. In our opinion, there is low probability that the net aerosol effect can be explained by coincidental, unresolved, changes in meteorological conditions that also accumulate aerosol, or errors in the data, although further in situ measurements and model developments are needed to fully understand the processes. The radiative effect at the top of the atmosphere incurred by the aerosol effect on the shallow clouds and solar radiation is –11 ± 3 W/m² for the 3 months studied; 2/3 of it is due to the aerosol-induced cloud changes, and 1/3 is due to aerosol direct radiative effect.

Footnotes

↵ † To whom correspondence should be addressed. E-mail: kaufman{at}climate.gsfc.nasa.gov.
Author contributions: Y.J.K., I.K., L.A.R., D.R., and Y.R. designed research; Y.J.K., I.K., and L.A.R. performed research; Y.J.K., I.K., and L.A.R. analyzed data; Y.J.K. wrote the paper; I.K. and L.A.R. also participated in writing the paper; D.R. provided cloud physics context for the analysis; and D.R. and Y.R. participated in data evaluation and writing the paper.
Abbreviations: MODIS, Moderate Resolution Imaging Spectroradiometer; AOT, aerosol optical thickness; LWP, liquid water path; AERONET, Aerosol Robotic Network.
Freely available online through the PNAS open access option.