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| Titel |
Aerosol indirect effects – general circulation model intercomparison and evaluation with satellite data |
| VerfasserIn |
J. Quaas, Y. Ming, S. Menon, T. Takemura, M. Wang, J. E. Penner, A. Gettelman, U. Lohmann, N. Bellouin, O. Boucher, A. M. Sayer, G. E. Thomas, A. McComiskey, G. Feingold, C. Hoose, J. E. Kristjánsson, X. Liu, Y. Balkanski, L. J. Donner, P. A. Ginoux, P. Stier, B. Grandey, J. Feichter, I. Sednev, S. E. Bauer, D. Koch, R. G. Grainger, A. Kirkevåg, T. Iversen, Ø. Seland, R. Easter, S. J. Ghan, P. J. Rasch, H. Morrison, J.-F. Lamarque, M. J. Iacono, S. Kinne, M. Schulz |
| 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 ; 9, no. 22 ; Nr. 9, no. 22 (2009-11-16), S.8697-8717 |
| Datensatznummer |
250007752
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| Publikation (Nr.) |
 copernicus.org/acp-9-8697-2009.pdf |
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| Zusammenfassung |
| Aerosol indirect effects continue to constitute one of the most
important uncertainties for anthropogenic climate perturbations. Within the
international AEROCOM initiative, the representation of
aerosol-cloud-radiation interactions in ten different general circulation
models (GCMs) is evaluated using three satellite datasets. The focus is on
stratiform liquid water clouds since most GCMs do not include ice nucleation
effects, and none of the model explicitly parameterises aerosol effects on
convective clouds. We compute statistical relationships between aerosol
optical depth (τa) and various cloud
and radiation quantities in a manner that is consistent between the models
and the satellite data. It is found that the model-simulated influence of
aerosols on cloud droplet number concentration
(Nd) compares relatively well to the
satellite data at least over the ocean. The relationship between
τa and liquid water path is simulated
much too strongly by the models. This suggests that the implementation of
the second aerosol indirect effect mainly in terms of an autoconversion
parameterisation has to be revisited in the GCMs. A positive relationship
between total cloud fraction (fcld) and
τa as found in the
satellite data is simulated by the majority of the models, albeit less
strongly than that in the satellite data in most of them. In a discussion of
the hypotheses proposed in the literature to explain the satellite-derived
strong fcld–τa relationship, our results indicate that none can be
identified as a unique explanation. Relationships similar to the ones found
in satellite data between τa and cloud
top temperature or outgoing long-wave radiation (OLR) are simulated by only
a few GCMs. The GCMs that simulate a negative OLR–τa relationship show a strong positive correlation between
τa and
fcld. The short-wave total aerosol radiative
forcing as simulated by the GCMs is strongly influenced by the simulated
anthropogenic fraction of τa, and
parameterisation assumptions such as a lower bound on
Nd. Nevertheless, the strengths of the
statistical relationships are good predictors for the aerosol forcings in
the models. An estimate of the total short-wave aerosol forcing inferred
from the combination of these predictors for the modelled forcings with the
satellite-derived statistical relationships yields a global annual mean
value of −1.5±0.5 Wm−2. In an alternative approach, the
radiative flux perturbation due to anthropogenic aerosols can be broken down
into a component over the cloud-free portion of the globe (approximately the
aerosol direct effect) and a component over the cloudy portion of the globe
(approximately the aerosol indirect effect). An estimate obtained by scaling
these simulated clear- and cloudy-sky forcings with estimates of
anthropogenic τa and satellite-retrieved
Nd–τa regression slopes, respectively,
yields a global, annual-mean aerosol direct effect estimate of −0.4±0.2 Wm−2
and a cloudy-sky (aerosol indirect effect) estimate of
−0.7±0.5 Wm−2, with a total estimate of −1.2±0.4 Wm−2. |
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