Freezing Rate Due to Heterogeneous Nucleation

Abstract

The heterogeneous nucleation of ice from supercooled water is influenced by the nature of the foreign nuclei that serve as the sites for ice embryo formation, and by the stochastic nature of the process of embryo growth to critical size. The relative roles of these two factors have been the subject of some debate, especially as they influence the way nucleation of ice is modeled in clouds. ?Freezing rate? is defined as the time-dependent rate at which a population of macroscopically identical samples (e.g., drops in a volume of air) freeze due to the nuclei contained in them. Freezing rate is the combined result of nucleus content and of time dependence. The time-dependent freezing rate model (TDFR) is consistent with available empirical evidence. For droplets cooled at rates of the order of ?1°C per min, the nucleus content, or nucleus spectrum, predicts the freezing rate with reasonable accuracy. For samples exposed to a fixed temperature, the time dependence of the freezing rate becomes important, but the probability of freezing is not the same for each individual of the sample population. Stochastic models are not supported by the results. Application of the TDFR model and use of measured freezing nucleus data for precipitation provide a basis for the description of ice formation via immersion-freezing nucleation in cloud models. Limitations to full development of these models arise from inadequate knowledge about the freezing nucleus content of cloud water as a function of cloud evolution.