Teasing Out Important Factors in Aerosol Cloud Interactions within Global Models (Invited Presentation)

Aerosol cloud interactions contribute to large uncertainties in estimates of historical responses of the earth system to anthropogenic forcing agents, and confound projections of climate change in the future. Many factors contribute to this uncertainty, including a lack of understanding about some physical processes important to aerosols and clouds, and the approximations and spatial resolution typically employed in global models needed to represent even relatively well understood features of the system. Because of their complexity, it is often unclear which aspects of the interactions between clouds and aerosols are controlling the model climate response to changes in aerosols. I will tease out and discuss a variety of issues that appear important in influencing modeled responses to changing aerosols in the DOE ACME (Accelerated Climate Modeling for Energy) climate model, but are also likely important in models with similar parameterizations. I will touch upon issues that have received some attention in the past (microphysics, emissions, aerosol composition), but spend more time exploring sensitivity to horizontal and vertical resolution, details of the (vertical and horizontal) subgrid distribution of aerosol and cloud, partitioning between convective and stratiform cloud processes, and cloud and rain overlap, which also appear very important in influencing the model response The analysis will be performed in terms of geographic location, and cloud properties (altitude, thickness, cloud type. dynamic regime).