The relative importance of heterogeneous and homogeneous nucleation for formation of ice crystals in the Tropical Tropopause Layer (TTL) has implications for the TTL cirrus occurrence frequency, cloud microphysical properties, and the abundance of water vapor entering the stratosphere.

Here, we use numerical simulations do address the competition between homogeneous and heterogeneous ice nucleation in the TTL.  We have developed a general framework for parameterizing heterogeneous ice nucleation rates as a function of temperature and supersaturation.  We apply this parameterization in a one-dimensional cloud model that tracks thousands of individual ice crystals.