Evaluation of aerosol indirect radiative forcing in MIRAGE

We evaluate aerosol indirect radiative forcing simulated by the Model for I ntegrated Research on Atmospheric Global Exchange(MIRAGE). Although explici t measurements of aerosol indirect radiative forcing do not exist, measurem ents of many of the links between aerosols and indirect radiative forcing a re available and can be used for evaluation. These links include the cloud condensation nuclei concentration, the ratio of droplet number to aerosol n umber, the droplet number concentration, the column droplet number, the col umn cloud water, the;droplet effective radius, the cloud optical depth, the correlation between cloud albedo and droplet effective radius, and the clo ud radiative forcing. The CCN concentration simulated by MIRAGE agrees with measurements for supersaturations larger than 0.1% but not for smaller sup ersaturations. Simulated droplet number concentrations are too low in most but not all locations with available measurements, even when normalized by aerosol number. MIRAGE correctly simulates the higher droplet numbers and s maller droplet sizes over continents and in the Northern Hemisphere. Biases in column cloud water, cloud optical depth, and shortwave cloud radiative forcing are evident in the Intertropical Convergence Zone and in the subtro pical oceans. MIRAGE correctly simulates a negative correlation between clo ud albedo and droplet size over remote oceans for cloud optical depths grea ter than 15 and a positive correlation for cloud optical depths less than 1 5 but fails to simulate a negative correlation over land.