Cirrus crystal nucleation by homogeneous freezing of solution droplets

A numerical model consisting of a system of ordinary differential equations is used to study the process in a rising parcel containing a distribution of cloud condensation nuclei (CCN). The model traces the evolution of both the particle population (liquid and solid phases) and the thermodynamic variables in the parcel. Droplet growth rates are calculated in 20 size categories assuming ammonium sulfate nuclei; CCN distributions are taken from aircraft data. Homogeneous nucleation rates are derived by classical methods; adjustments for solution effects are made by considering the water vapor pressure over droplet surfaces. Results for convective cirrus simulations indicate that if homogeneous freezing is not considered, liquid water should be detected below -40°C; this is inconsistent with observations. -from Authors