Atmos. Chem. Phys. Discuss., 10, 7323-7346, 2010
www.atmos-chem-phys-discuss.net/10/7323/2010/
doi:10.5194/acpd-10-7323-2010
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under the Creative Commons Attribution 3.0 License.
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Black carbon absorption effects on cloud cover, review and synthesis
D. Koch1,2 and A. Del Genio2
1Columbia University, New York, NY, USA
2NASA GISS, New York, NY, USA

Abstract. Absorbing aerosols (AA's) such as black carbon (BC) or dust absorb incoming solar radiation, perturb the temperature structure of the atmosphere, and influence cloud cover. Previous studies have described conditions where AA's either increase or decrease cloud cover. The effect depends on several factors, including the altitude of the AA relative to the cloud and on the cloud type. Cloud cover is decreased if the AA's are embedded in the cloud layer. AA's below cloud may enhance convection and cloud cover. AA's over cloud-level stabilize the underlying layer and tend to enhance stratocumulus clouds but may reduce cumulus clouds. AA's can also promote cloud cover in convergent regions as they enhance deep convection and low level convergence as it draws in moisture from ocean to land regions. Most global model studies indicate a regional variation in the cloud response but generally increased cloud cover over oceans and some land regions, with net increased low-level and/or reduced upper level cloud cover. The result is net negative radiative forcing from cloud response to AA's. In some of these climate model studies, the cooling effect of BC due to cloud changes was strong enough to essentially cancel the warming direct effects.

Citation: Koch, D. and Del Genio, A.: Black carbon absorption effects on cloud cover, review and synthesis, Atmos. Chem. Phys. Discuss., 10, 7323-7346, doi:10.5194/acpd-10-7323-2010, 2010.
 
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