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Repeatability and randomness in heterogeneous freezing nucleation

by G Vali
Atmos. Chem. Phys. Atmospheric Chemistry and Physics ()
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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 • C to −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.

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