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The indirect effect of aerosols on water clouds, whereby aerosol particles change cloud optical properties, is caused by aerosol-induced changes of the size and number of cloud droplets. This affects the lifetime of the water clouds as well as their shortwave radiative properties. In addition, anthropogenic aerosols may change the properties of ice-forming nuclei. To investigate the potential effect of aerosol–ice cloud interactions by contact freezing, a prognostic equation for the number concentration of ice crystals is introduced into the ECMWF–Hamburg (ECHAM4) GCM. A simulation in which the number of contact ice nuclei is considered to be only temperature dependent is compared to simulations in which contact ice nuclei are considered to be dust aerosols. If dust aerosols are assumed to lose their nucleability by forming an internally mixed aerosol with sulfate, then the ice formation is slightly inhibited. On the contrary, if all contact nuclei are assumed to be insoluble carbonaceous aerosols, as found in contrails and some cirrus clouds, then contact nucleation is more important so that the liquid water path is smaller and the ice water path larger. These changes are, however, small compared to the extreme assumptions of having either no ice nuclei at all or so many ice nuclei that no supercooled cloud water exists.
Ulrike Lohmann (Fri,) studied this question.