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

by M. Wang, J. E. Penner
Atmospheric Chemistry and Physics ()
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A statistical cirrus cloud scheme that accounts for mesoscale\ntemperature perturbations is implemented in a coupled aerosol and\natmospheric circulation model to better represent both subgrid-scale\nsupersaturation and cloud formation. This new scheme treats the effects\nof aerosol on cloud formation and ice freezing in an improved manner,\nand both homogeneous freezing and heterogeneous freezing are included.\nThe scheme is able to better simulate the observed probability\ndistribution of relative humidity compared to the scheme that was\nimplemented in an older version of the model. Heterogeneous ice nuclei\n(IN) are shown to decrease the frequency of occurrence of\nsupersaturation, and improve the comparison with observations at 192\nhPa. Homogeneous freezing alone can not reproduce observed ice crystal\nnumber concentrations at low temperatures (< 205 K), but the addition of\nheterogeneous IN improves the comparison somewhat. Increases in\nheterogeneous IN affect both high level cirrus clouds and low level\nliquid clouds. Increases in cirrus clouds lead to a more cloudy and\nmoist lower troposphere with less precipitation, effects which we\nassociate with the decreased convective activity. The change in the net\ncloud forcing is not very sensitive to the change in ice crystal\nconcentrations, but the change in the net radiative flux at the top of\nthe atmosphere is still large because of changes in water vapor. Changes\nin the magnitude of the assumed mesoscale temperature perturbations by\n25% alter the ice crystal number concentrations and the net radiative\nfluxes by an amount that is comparable to that from a factor of 10\nchange in the heterogeneous IN number concentrations. Further\nimprovements on the representation of mesoscale temperature\nperturbations, heterogeneous IN and the competition between homogeneous\nfreezing and heterogeneous freezing are needed.

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