Global modeling of aerosol effects on cirrus clouds

A recently developed parameterization of homogeneous and heterogeneous freezing of ice crystals has been implemented in a global climate model (GCM) to study effects of aerosols on cirrus clouds. With the new parameterization, the competition between homogeneous and heterogeneous freezing can be simulated. This competition effect comes about because aerosols with ice nucleating ability (IN) can lead to ice crystal formation and growth at relatively low supersaturations with respect to ice, potentially preventing supersaturations required for the onset of homogeneous freezing. We show that this has strong implications for cloud microphysical and radiative properties, because homogeneous freezing typically yields ice crystal number concentrations orders of magnitudes higher than heterogeneous freezing. By comparing simulations allowing for homogeneous freezing only to simulations allowing for the competition effects between the two freezing mechanisms, we calculate the climate effects associated with aerosols acting as IN aiding cirrus cloud formation. We also present a sensitivity study varying the heterogeneous freezing parameterization employed, as available parameterizations currently differ substantially.