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S. J. Ghan, X. Liu, R. C. Easter, R. Zaveri, P. J. Rasch, J.-H. Yoon, and B. Eaton

influence of anthropogenic aerosol on the optical properties of clouds by serving as the nuclei for droplets and ice crystals and thereby changing droplet and ice crystal number concentration, which changes cloud particle surface area, influences droplet collisions, and changes the accumulation of liquid water and ice in clouds, all of which affect the reflectivity and emissivity of clouds. Semi-direct effects are changes in the planetary energy balance as clouds respond to radiative heating by

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Gerald A. Meehl, Warren M. Washington, Julie M. Arblaster, Aixue Hu, Haiyan Teng, Jennifer E. Kay, Andrew Gettelman, David M. Lawrence, Benjamin M. Sanderson, and Warren G. Strand

the direct and indirect effect for sulfate and black and organic carbon. More specifically, in CAM5 direct effects of aerosols are included in the radiation code. Indirect effects of aerosols are treated such that activated aerosol [as described by Gettelman et al. (2008) ] affects the number concentration of cloud drops and ice crystals. Changing the condensate number impacts the resulting microphysical processes (such as sedimentation and auto conversion) as well as the radiative effects of

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A. Gettelman, J. E. Kay, and K. M. Shell

simulation of low clouds ( Neale et al. 2010 ). The new two-moment stratiform cloud microphysics scheme ( Gettelman et al. 2010 ; Morrison and Gettelman 2008 ) includes aerosol activation of cloud drops/crystals for liquid and ice, explicitly treating aerosol–cloud interactions. The new radiation code, the Rapid Radiative Transfer Model for GCMs ( Iacono et al. 2008 ), is a correlated- K code that compares better to line-by-line calculations than CAM4 ( Iacono et al. 2008 ). The liquid cloud

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J. E. Kay, B. R. Hillman, S. A. Klein, Y. Zhang, B. Medeiros, R. Pincus, A. Gettelman, B. Eaton, J. Boyle, R. Marchand, and T. P. Ackerman

parameterizations in CAM5 and CAM4. Local modifications to the COSP v1.3 code were necessary both to ensure compatibility with the CESM code and software engineering requirements and to incorporate the influence of radiatively active snow, a modification that only influences the ISCCP, MISR, MODIS, and lidar diagnostics in the CAM5 simulations. Snow in this context represents the population of large ice crystals with appreciable fall velocities. Because it incorporates the impact of snow on radiative fluxes

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