Ice clouds play an important role in regulating the Earth's radiative budget and water vapor distribution in the upper troposphere. Aerosols can act as solution droplets or ice nuclei that promote ice nucleation. The ice cloud formation and decaying processes are parameterized differently in different global climate models. Large discrepancies have been seen between model simulations in terms of the spatial distribution of ice-nucleating aerosols, relative humidity, and temperature fluctuations, which lead to significantly different estimates of the aerosol indirect effect through ice clouds.
To identify the causes of the discrepancies and improve the estimate of aerosol indirect effect of anthropogenic aerosols, we organized a model inter-comparison project under the AeroCom initiative. The estimated global and annual mean anthropogenic aerosol indirect effect through cirrus clouds (pure ice phase clouds colder than -38°C) ranges from 0.1 Wm-2 to 0.3 Wm-2 in terms of the TOA net radiation flux, and 0.5-0.6 Wm-2 for the TOA longwave flux. Despite the good agreement on global mean, large discrepancies are found at the regional scale. Our analysis suggests that the ice nucleation frequency and relative role of individual ice formation processes play an important role in determining the characteristics of the simulated aerosol indirect effect. In addition to the indirect effect estimate, we also use field campaign measurements and satellite retrievals to evaluate the simulated micro- and macro- physical properties of ice clouds in the four participating models.