Parcel cloud model simulations have explored the feasibility of a unified approach to describe the homogeneous and heterogeneous ice nucleation mechanisms in cirrus clouds. It was assumed that the ice crystal generation process involved the freezing of ammonium sulfate haze particles, as modulated by an effective temperature proportional to molality in both cases. Treated as variables in the -36¿ to -60 ¿C model domain are the updraft velocity, and the terms in a modified Fletcher equation used to approximate heterogeneous freezing. Parameterizations drawn from the homogeneous freezing runs are provided for the threshold relative humidity and number of crystals nucleated, as functions of both temperature and updraft velocity. We find that heterogeneous nucleation can compete or dominate at relatively weak updrafts, but only for unusually high ice nuclei concentration and temperature activation terms. ¿ 2000 American Geophysical Union