Aerosol-Cloud-Climate Interactions in the Community Earth System Model

Aerosols affect Cloud properties, with possible significant impacts on climate. The relationship between increased aerosols and smaller, more numerous cloud drops is well observed. More uncertainty remains due to uncertainties in ice nucleation. These changes in microphysics may affect cloud radiative properties, cloud lifetime and precipitation. These effects are examined using the National Center for Atmospheric Research (NCAR) Community Earth System Model (CESM). The global atmospheric model, CAM5, features a new 2-moment microphysics scheme and is linked to a new modal aerosol model using droplet and crystal activation. The model is evaluated against observations of aerosols and cloud properties, and the problem of assessing and constraining effects of aerosols is explored in detail. Basic climatologies and results of sensitivity tests using the model are presented, indicating possible significant global and regional effects of anthropogenic aerosols on clouds, precipitation and cloud feedbacks in the 20th and 21st centuries.