The Response and Feedback of Clouds and Cloud Radiative Forcing to the Direct Forcing of Sulfate and Carbonaceous Aerosols

The relationships between aerosol direct forcing and cloud cover as well as associated radiative forcing have been studied using a recently developed aerosol-climate model based on NCAR CAM3. The multi-mode, two-moment aerosol model includes major anthropogenic aerosol constituents of black carbon, organic carbon, and sulfate, and their mixtures. Clear influence of low cloud cover on the direct radiative forcing of aerosols has been found where mixed aerosols with absorbing core and reflecting shell exist. Such a mixture has a single scattering albedo that is often higher than that of black carbon but lower than sulfate. Therefore, the quantity and even sign of its direct forcing are very sensitive to the albedo beneath the aerosol layer. In order to observe the relationships between direct aerosol forcing, cloud cover and cloud forcing, three long- term integrations respectively including direct radiative effects of: 1) scattering aerosols; 2) absorbing aerosols; and 3) both scattering and absorbing aerosols have also been carried out. The climate responses caused by various types of aerosols are derived by comparing the results from these runs with a simulation that does not include direct aerosol radiative effect. We will present and discuss the modeled responses of cloud cover and associated cloud radiative forcing caused by aerosols. The effect of changed cloud cover forced by aerosols on aerosol direct forcing itself will also be discussed.