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| Titel |
Direct and semi-direct radiative forcing of smoke aerosols over clouds |
| VerfasserIn |
E. M. Wilcox |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 12, no. 1 ; Nr. 12, no. 1 (2012-01-03), S.139-149 |
| Datensatznummer |
250010426
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| Publikation (Nr.) |
 copernicus.org/acp-12-139-2012.pdf |
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| Zusammenfassung |
| Observations from Earth observing satellites indicate that dark carbonaceous
aerosols that absorb solar radiation are widespread in the tropics and
subtropics. When these aerosols mix with clouds, there is generally a
reduction of cloudiness owing to absorption of solar energy in the aerosol
layer. Over the subtropical South Atlantic Ocean, where smoke from savannah
burning in southern Africa resides above a persistent deck of marine
stratocumulus clouds, radiative heating of the smoke layer leads to a
thickening of the cloud layer. Here, satellite observations of the albedo of
overcast scenes of 25 km2 size or larger are combined with additional
satellite observations of clouds and aerosols to estimate the
top-of-atmosphere direct radiative forcing attributable to presence of dark
aerosol above bright cloud, and the negative semi-direct forcing
attributable to the thickening of the cloud layer. The average positive
direct radiative forcing by smoke over an overcast scene is 9.2±6.6 W m−2
for cases with an unambiguous signal of absorbing aerosol over
cloud in passive ultraviolet remote sensing observations. However, cloud
liquid water path is enhanced by 16.3±7.7 g m−2 across the range
of values for sea surface temperature for cases of smoke over cloud. The
negative radiative forcing associated with this semi-direct effect of smoke
over clouds is estimated to be −5.9±3.5 W m−2. Therefore, the
cooling associated with the semi-direct cloud thickening effect compensates
for greater than 60 % of the direct radiative effect. Accounting for the
frequency of occurrence of significant absorbing aerosol above overcast
scenes leads to an estimate of the average direct forcing of 1.0±0.7 W m−2
contributed by these scenes averaged over the subtropical
southeast Atlantic Ocean during austral winter. The regional average of the
negative semi-direct forcing is −0.7±0.4 W m−2. Therefore, smoke
aerosols overlaying the decks of overcast marine stratocumulus clouds
considered here yield a small net positive radiative forcing, which results
from the difference of two larger effects. |
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