Effects of ice nuclei on cirrus clouds in a global climate model

A multiple-mode ice microphysical scheme is applied in the European Centre/Hamburg (ECHAM) general circulation model to simulate effects of aerosol-ice interactions on global cirrus properties. The different ice modes represent cirrus ice formed by homogeneous freezing of liquid aerosols and heterogeneous nucleation on mineral dust or black carbon particles. A fourth ice mode represents ice from other sources. The competition of these modes for available water is realized in a physical parameterization scheme considering also the effect of preexisting ice on the ice nucleation process. The model is applied to analyze the global characteristics of ice formed by the different aerosol types and to study potential global effects of mineral dust and black carbon particles on cirrus microphysical parameters. The simulations reveal that, on average, ice from heterogeneous nucleation shows fewer but larger crystals and has a smaller contribution to the mean cirrus ice water content than ice from homogeneous freezing. However, heterogeneous ice nuclei may have important effects on the overall cirrus properties. Reductions in zonal mean annual average cirrus ice particle number concentrations induced by heterogeneous nucleation of up to 20% in the tropics and 1%–10% in the midlatitudes are simulated. The effect is further amplified by ice formation on aircraft-generated soot. Significant reductions in the mean ice water content are modeled, which likely result from efficient sedimentation and precipitation of large ice particles generated by heterogeneous nucleation. This leads to reductions in the zonal mean annual average water vapor mixing ratio of up to 5% at cirrus levels