Modes of Ice Nucleation
Ice is formed in clouds either by homogeneous freezing of water and solution droplets at temperatures below about -35°C or by heterogeneous ice nucleation processes induced by insoluble aerosol particles. The rates of ice formation in supercooled water is formulated in the so-called classical nucleation theory (see e.g. Pruppacher and Klett, 1997, or Jeffery and Austin, 1997). Different approaches have been developped to relate the homogeneous freezing rates of solution droplets to those of the pure water phase (Sassen and Dodd, 1988; Koop et al., 2000).
Heterogeneous ice nucleation is induced by so-called ice nuclei (IN), a subset of (insoluble) aerosol particles which nucleate ice under certain conditions of supercooling between 0 and -35°C or ice supersaturation at temperatures below 0°C.
According to the definition by the Committee on Nucleation and Atmospheric Aerosols (Vali, 1985), one can distinguish different heterogeneous ice nucleation modes. The basic distinction is made whether water nucleates ice from the vapour or the (supercooled) liquid phase:
- Deposition nucleation is the formation of ice in an ice supersaturated vapour environment
- Freezing nucleation is the formation of ice in a supercooled liquid environment.
Further distinction is made between the following freezing nucleation modes:
- Immersion freezing is induced by a particle immersed in the body of supercooled water.
- Condensation freezing is similar to immersion freezing, but occures when a particle acts as cloud condensation nucleus (CCN) and immediately induces freezing of the water condensate.
- Contact freezing is induced by a particle upon contact with the supercooled liquid water phase.
Jeffery, C. A., and P. H. Austin (1997), Homogeneous nucleation of supercooled water: Results from a new equation of state, J Geophys Res-Atmos, 102, 25269-25279.
Koop, T., B. P. Luo, A. Tsias, and T. Peter (2000), Water activity as the determinant for homogeneous ice nucleation in aqueous solutions, Nature, 406, 611-614.
Pruppacher, H. R., and J. D. Klett (1997), Microphysics of clouds and precipitation, Kluwer Academic Publishers, Dordrecht, The Netherlands.
Sassen, K., and G. C. Dodd (1988), Homogeneous Nucleation Rate for Highly Supercooled Cirrus Cloud Droplets, Journal of the Atmospheric Sciences, 45, 1357-1369.
Vali, G. (1985), Nucleation Terminology, Bull. Am. Meteorol. Soc., 66, 1426–1427.