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| Titel |
Cirrus clouds in a global climate model with a statistical cirrus cloud scheme |
| VerfasserIn |
M. Wang, J. E. Penner |
| 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. 12 ; Nr. 10, no. 12 (2010-06-21), S.5449-5474 |
| Datensatznummer |
250008564
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| Publikation (Nr.) |
 copernicus.org/acp-10-5449-2010.pdf |
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| Zusammenfassung |
| A statistical cirrus cloud scheme that accounts for mesoscale temperature
perturbations is implemented in a coupled aerosol and atmospheric
circulation model to better represent both subgrid-scale supersaturation and
cloud formation. This new scheme treats the effects of aerosol on cloud
formation and ice freezing in an improved manner, and both homogeneous
freezing and heterogeneous freezing are included. The scheme is able to
better simulate the observed probability distribution of relative humidity
compared to the scheme that was implemented in an older version of the
model. Heterogeneous ice nuclei (IN) are shown to decrease the frequency of
occurrence of supersaturation, and improve the comparison with observations
at 192 hPa. Homogeneous freezing alone can not reproduce observed ice
crystal number concentrations at low temperatures (<205 K), but the
addition of heterogeneous IN improves the comparison somewhat. Increases in
heterogeneous IN affect both high level cirrus clouds and low level liquid
clouds. Increases in cirrus clouds lead to a more cloudy and moist lower
troposphere with less precipitation, effects which we associate with the
decreased convective activity. The change in the net cloud forcing is not
very sensitive to the change in ice crystal concentrations, but the change
in the net radiative flux at the top of the atmosphere is still large
because of changes in water vapor. Changes in the magnitude of the assumed
mesoscale temperature perturbations by 25% alter the ice crystal number
concentrations and the net radiative fluxes by an amount that is comparable
to that from a factor of 10 change in the heterogeneous IN number
concentrations. Further improvements on the representation of mesoscale
temperature perturbations, heterogeneous IN and the competition between
homogeneous freezing and heterogeneous freezing are needed. |
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