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Kaufman, Y. J., I. Koren, L. A. Remer, D. Rosenfeld, and Y. Rudich, 2005:
The Effect of Smoke, Dust and Pollution Aerosol on Shallow Cloud Development Over the Atlantic Ocean.
Proceedings of the National Academy of Sciences, Vol 102 (32), pp 11207-11212.
Clouds developing in a polluted environment tend to have more numerous, but smaller droplets. This 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 satellite data, we study the aerosol effect on shallow water clouds, separately in 4 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¡). All 4 aerosol types affect the cloud droplet size. We also find that the coverage of shallow clouds increases in all the cases by 0.2-0.4 from clean to polluted, smoky or dusty conditions. Co-variability analysis with meteorological parameters associates most of this change to aerosol, for each of the 4 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, though further in situ measurements and model developments is 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 due to aerosol
direct radiative effect.
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