Abstract
The reason why ice nucleation is more efficient by contact nucleation than by immersion nucleation has been elusive for over half a century. Six proposed mechanisms are summarized in this study. Among them, the pressure perturbation hypothesis, which arose from recent experiments, can qualitatively explain nearly all existing results relevant to contact nucleation. To explore the plausibility of this hypothesis in a more quantitative fashion and to guide future investigations, this study assessed the magnitude of pressure perturbation needed to cause contact nucleation and the associated spatial scales. The pressure perturbations needed were estimated using measured contact nucleation efficiencies for illite and kaolinite, obtained from previous experiments, and immersion freezing temperatures, obtained from well-established parameterizations. Pressure perturbations were obtained by assuming a constant pressure perturbation or a Gaussian distribution of the pressure perturbation. The magnitudes of the pressure perturbations needed were found to be physically reasonable, being achievable through possible mechanisms, including bubble formation and breakup, Laplace pressure arising from the distorted contact line, and shear. The pressure perturbation hypothesis provides a physically based and experimentally constrainable foundation for parameterizing contact nucleation that may be useful in future cloud-resolving models.
Highlights
Ice particles in the atmosphere are important for many cloud processes
We address the following questions: Can pressure perturbation explain previous experiments related to contact nucleation? How large of a pressure perturbation is needed to have a noticeable effect on ice nucleation and to account for the contact nucleation efficiency observed? Is the pressure perturbation realistic and achievable? It should be mentioned that the purpose of this study was not to verify the pressure perturbation hypothesis, because only lab experiments can verify theory
We show that the pressure perturbation hypothesis can explain, qualitatively, all existing lab experiments related to contact nucleation
Summary
Ice particles in the atmosphere are important for many cloud processes. Growth and sedimentation of ice particles under different environmental conditions can cause various types of surface precipitation, such as snow, hail, and graupel (e.g., [1]). Even for liquid precipitation, a rain drop might have started as an ice particle in the upper atmosphere (e.g., [2]). Ice particles in mixed-phase cloud can absorb extra sunlight and reduce surface shortwave irradiance (e.g., [3]). Besides their impacts on precipitation and radiation, ice particles can create halos by reflecting and refracting sunlight (e.g., [4]). Because of the importance of ice particles to so many processes, it is important to understand the origin of ice particles in atmospheric clouds
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