Repeatability and randomness in heterogeneous freezing nucleation
This study is aimed at clarifying the relative im- portance of the specific character of the nuclei and of the duration of supercooling in heterogeneous freezing nucle- ation by immersed impurities. Laboratory experiments were carried out in which sets of water drops underwent multi- ple cycles of freezing and melting. The drops contained sus- pended particles of mixtures of materials; the resulting freez- ing temperatures ranged from−6◦Cto−24◦C. Rank correla- tion coefficients between observed freezing temperatures of the drops in successive runs were >0.9 with very high sta- tistical significance, and thus provide strong support for the modified singular model of heterogeneous immersion freez- ing nucleation. For given drops, changes in freezing tem- peratures between cycles were relatively small (<1◦C) for the majority of the events. These frequent small fluctuations in freezing temperatures are interpreted as reflections of the random nature of embryo growth and are associated with a nucleation rate that is a function of a temperature difference from the characteristic temperatures of nuclei. About a sixth of the changes were larger, up to ±5◦C, and exhibited some systematic patterns. These are thought to arise from alter- ations of the nuclei, some being permanent and some transi- tory. The results are used to suggest ways of describing ice initiation in cloud models that account for both the tempera- ture and the time dependence of freezing nucleation.