Abstract
Abstract. This study is aimed at clarifying the relative importance of the specific character of the nuclei and of the duration of supercooling in heterogeneous freezing nucleation by immersed impurities. Laboratory experiments were carried out in which sets of water drops underwent multiple cycles of freezing and melting. The drops contained suspended particles of mixtures of materials; the resulting freezing temperatures ranged from −6°C to −24°C. Rank correlation coefficients between observed freezing temperatures of the drops in successive runs were >0.9 with very high statistical significance, and thus provide strong support for the modified singular model of heterogeneous immersion freezing nucleation. For given drops, changes in freezing temperatures 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 alterations of the nuclei, some being permanent and some transitory. The results are used to suggest ways of describing ice initiation in cloud models that account for both the temperature and the time dependence of freezing nucleation.
Highlights
While it is widely recognized that ice nucleation in tropospheric clouds is of critical concern for weather and climate models, there are major shortcomings in our ability to treat this process reliably
One of the areas of uncertainty in the formulation of heterogeneous ice nucleation is the separation between temperature dependence and time dependence
One approach to do that is to examine the dependence of the average freezing temperature on sample volume
Summary
While it is widely recognized that ice nucleation in tropospheric clouds is of critical concern for weather and climate models, there are major shortcomings in our ability to treat this process reliably. One approach to do that is to examine the dependence of the average freezing temperature on sample volume It can be shown (the Appendix in Vali and Stansbury, 1966; page 351 in Pruppacher and Klett, 1997) that both descriptions lead to the same result if the probability density function (pdf) of Tc follows an exponential form. Since most changes were small in comparison to the range of temperatures over which the drops in a sample froze, deviations from the SN subset definition due to this fact, while undoubtedly present to some degree, can be considered to be of minor importance Fuller justification of this statement will emerge from the results to be presented
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