FREEZING RATE DUE TO HETEROGENEOUS NUCLEATION

The heterogeneous nucleation of ice from supercooled water is influenc ed 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 embr yo growth to critical size. The relative roles of these two factors ha ve been the subject of some debate, especially as they influence the w ay nucleation of ice is modeled in clouds. ''Freezing rate'' is define d 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 n uclei contained in them. Freezing rate is the combined result of nucle us content and of time dependence. The time-dependent freezing rate mo del (TDFR) is consistent with available empirical evidence. For drople ts cooled at rates of the order of - 1-degrees-C per min, the nucleus content, or nucleus spectrum, predicts the freezing rate with reasonab le accuracy. For samples exposed to a fixed temperature, the time depe ndence 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 th e TDFR model and use of measured freezing nucleus data for precipitati on provide a basis for the description of ice formation via immersion- freezing nucleation in cloud models. Limitations to full development o f these models arise from inadequate knowledge about the freezing nucl eus content of cloud water as a function of cloud evolution.