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Cirrus clouds in a global climate model with a statistical cirrus cloud scheme

Wang M, Penner J ...see all

Atmospheric Chemistry and Physics, vol. 10, issue 12 (2010) pp. 5449-5474

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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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  • M. Wang

  • J. E. Penner

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