Ice nucleation parameterization for global models

Global modeling of ice clouds and the effects of aerosol-related ice nuclei (IN) changes on clouds and climate remains a significant challenge. Here, we describe a parameterization that accounts for the effects of homogeneous ice nucleation as well as two different modes of heterogeneous nucleation in the upper troposphere. The parameterization treats homogeneous nucleation by sulfate aerosol, deposition nucleation as described by the MEYERS et al. (1992) formulation, and immersion nucleation by soot aerosol. Besides air mass temperature and updraft velocity, number concentration of pure sulfate aerosol acting through homogeneous ice nucleation and number concentration of soot aerosol as heterogeneous ice nuclei are explicitly included in the parameterization so that the indirect effect of aerosols on the cirrus clouds (i.e. the effect of increases in aerosol number concentrations) can be described in a global model. In addition the transition from the heterogeneous to homogeneous-dominated nucleation and the competition between homogeneous and heterogeneous nucleation in the transition regime are considered in the parameterization. The parameterization is compared to the results from air parcel model simulations of uplifting and adiabatically cooling clouds and to other recently developed ice nucleation parameterizations.