However, as anthropogenic aerosols decrease the cloud droplet size and the smaller the droplets are the less likely they are to freeze this should lead to a slower or less frequent glaciation of supercooled clouds. In addition anthropogenic aerosols may change the properties of ice forming nuclei. Both effects might change the radiative properties of ice clouds. This effect could have both an effect in the shortwave radiation similar to the effect of water clouds but could have an additional effect in the longwave radiation.
To investigate its potential effect we will include a prognostic equation for the number concentration of ice crystals into the ECHAM GCM and couple the cloud scheme to the aerosol scheme which accounts for sulfate, organic carbon, black carbon, dust and sea salt aerosols. The number of aerosols and the vertical velocity are used to calculate cloud droplet nucleation. Freezing of cloud droplets depends on their size and on the number of ice forming nuclei. Different ways to estimate ice forming nuclei will be compared. In addition to that ice crystal formation will be estimated from different empirical approaches to investigate the sensitivity of ice cloud physical and radiative properties. Results of microphysical and radiative properties of ice clouds from simulations using present-day aerosol and aerosol precursor emissions will be presented.
Reference:
Lohmann, U., J. Feichter, J. E. Penner, and W. R. Leaitch, Indirect effect of sulfate and carbonaceous aerosols: A mechanistic treatment, J. Geophys. Res., 105, 12,193-12,206, 2000.
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