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| Titel |
A new method for evaluating the impact of vertical distribution on aerosol radiative forcing in general circulation models |
| VerfasserIn |
M. R. Vuolo, M. Schulz, Y. Balkanski, T. Takemura |
| 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 ; 14, no. 2 ; Nr. 14, no. 2 (2014-01-24), S.877-897 |
| Datensatznummer |
250118310
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| Publikation (Nr.) |
 copernicus.org/acp-14-877-2014.pdf |
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| Zusammenfassung |
The quantification and understanding of direct aerosol forcing is essential
in the study of climate. One of the main issues that makes its
quantification difficult is the lack of a complete understanding of the role
of the vertical distribution of aerosols and clouds. This work aims at reducing
the uncertainty of aerosol top-of-the-atmosphere (TOA) forcing due to the
vertical superposition of several short-lived atmospheric components, in
particular different aerosol species and clouds. We propose a method to
quantify the contribution of different parts of the atmospheric column to
the TOA forcing as well as to evaluate the contribution to model differences that
is exclusively due to different spatial distributions of aerosols and clouds.
We investigate the contribution of aerosol above, below and in clouds by
using added diagnostics in the aerosol–climate model LMDz. We also compute
the difference between the TOA forcing of the ensemble of the aerosols and
the sum of the forcings from individual species in clear sky. This
difference is found to be moderate for the global average (14%) but can
reach high values regionally (up to 100%). Nonlinear effects are even
more important when superposing aerosols and clouds. Four forcing
computations are performed: one where the full aerosol 3-D distribution is
used, and then three where aerosols are confined to regions above, inside
and below clouds, respectively. We find that the TOA forcing of aerosols
depends crucially on the presence of clouds and on their position relative
to that of the aerosol, in particular for black carbon (BC). We observe a
strong enhancement of the TOA forcing of BC above clouds, attenuation for BC
below clouds, and a moderate enhancement when BC is found within clouds. BC
above clouds accounts for only about 30% of the total BC optical depth but for
55% of the forcing, while forcing efficiency increases by a factor of 7.5
when passing from below to above clouds.
The different behaviour of forcing nonlinearities for these three components
of the atmospheric column encouraged us to develop the method for
application to inter-model variability studies by reading 3-D aerosol and
cloud fields from different general circulation models (GCMs) into the same model. We apply the method to
the comparison of forcing due to the aerosols and clouds distributions of
the general circulation models LMDz and SPRINTARS. The different amount of
BC above but also within clouds is revealed to play a major role on
the differences of cloudy-sky forcings between the two models, which can
exceed 100% regionally. |
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