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| Titel |
Stratocumulus cloud thickening beneath layers of absorbing smoke aerosol |
| 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 ; 10, no. 23 ; Nr. 10, no. 23 (2010-12-10), S.11769-11777 |
| Datensatznummer |
250008951
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| Publikation (Nr.) |
 copernicus.org/acp-10-11769-2010.pdf |
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| Zusammenfassung |
| Marine stratocumulus cloud properties, and the free-tropospheric environment
above them, are examined in NASA A-Train satellite data for cases where
smoke from seasonal burning of the West African savannah overlay the
persistent southeast Atlantic stratocumulus cloud deck. CALIPSO space-borne
lidar observations show that features identified as layers of aerosol occur
predominantly between 2 km and 4 km. Layers identified as cloud features
occur predominantly below 1.5 km altitude and beneath the layer of elevated
smoke aerosol. The diurnal mean shortwave heating rates attributable to the
absorption of solar energy in the aerosol layer is nearly 1.5 K d−1 for
an aerosol optical thickness value of 1, and increases to 1.8 K d−1
when the smoke resides above clouds owing to the additional component of
upward solar radiation reflected by the cloud. As a consequence of this
heating, the 700 hPa air temperature above the cloud deck is warmer by
approximately 1 K on average for cases where smoke is present above the
cloud compared to cases without smoke above cloud. The warmer conditions in
the free-troposphere above the cloud during smoke events coincide with cloud
liquid water path values that are greater by 20 g m−2 and cloud tops
that are lower for overcast conditions compared to periods with low amounts
of smoke. The observed thickening and subsidence of the cloud layer are
consistent with published results of large-eddy simulations showing that
solar absorption by smoke above stratocumulus clouds increases the buoyancy
of free-tropospheric air above the temperature inversion capping the
boundary layer. Increased buoyancy inhibits the entrainment of dry air
through the cloud-top, thereby helping to preserve humidity and cloud cover
in the boundary layer. The direct radiative effect of absorbing aerosols
residing over a bright cloud deck is a positive radiative forcing (warming)
at the top of the atmosphere. However, the greater liquid water path for
cases of smoke overlaying cloud contributes an additional negative
semi-direct radiative forcing (cooling) of climate in locations such as the
southeast Atlantic Ocean owing to the enhanced albedo of the thicker cloud. |
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