Technical Note: A proposal for ice nucleation terminology

Abstract

Abstract. Terminology dealing with ice nucleation in the atmosphere, in biological systems, and in other areas has not kept pace with the growth of empirical evidence and the development of new ideas over recent decades. Ambiguities and misinterpretations could be seen in the literature. This paper offers a set of definitions for various terms in common use, adds some qualifications, and introduces some new ones. Input has been received on the interpretation of various terms from a fair number of researchers; diverse views have been accommodated with some success. It is anticipated that the terminology proposed here will be helpful both to those who adopt it and to those who wish to explain a different perspective.

Refreeze experiments with water droplets containing different types of ice nuclei interpreted by classical nucleation theory

“…The critical size of the embryo is reached when the probability of growth becomes equal to the probability of decay (Vali et al, 2015). Nucleation is described as an activated process with an Arrheniustype equation, which yields nucleation rates as a function of the activation energy needed to form the critical embryo (e.g., Fletcher, 1958;Thomson et al, 2015): Kaufmann et al: Refreeze experiments with water droplets containing different types of ice nuclei…”

“…Ice nucleation is assumed to occur at a random location on this uniform surface with a nucleation rate that scales linearly with surface area. However, there is increasing evidence that preferred locations present on surfaces are responsible for ice nucleation (e.g., Vali, 2014;Vali et al, 2015). Such sites are thought to be special surface regions such as crystal defects (Vonnegut, 1947), pores, cracks, or ledges (Knight, 1979;Sear, 2011;Fletcher, 1969), although direct evidence of the morphology, structure, and chemistry of active nucleation sites is lacking up to now.…”

“…If ice nucleation occurred only at a few specific locations, these have to be highly effective and characterized by high nucleation rate coefficients. While observations of deposition nucleation on a crystal may provide evidence for preferred locations for ice nucleation, only indirect evidence from refreeze experiments exists for immersion freezing (Vali et al, 2015). In deterministic models, active sites are supposed to induce ice nucleation at a characteristic temperature (Vali et al, 2015;Vali and Stansbury, 1966;Vali, 1971).…”

“…While observations of deposition nucleation on a crystal may provide evidence for preferred locations for ice nucleation, only indirect evidence from refreeze experiments exists for immersion freezing (Vali et al, 2015). In deterministic models, active sites are supposed to induce ice nucleation at a characteristic temperature (Vali et al, 2015;Vali and Stansbury, 1966;Vali, 1971). The nucleation rate is equal to zero at temperatures higher than the characteristic temperature of the site and equal to infinity beyond that.…”

“…Its importance for precipitation on the regional scale has been suggested in a number of studies (Pöschl et al, 2010;Prenni et al, 2013;Huffman et al, 2013;Hader et al, 2014). Birch pollen is one of the most efficient pollen species at nucleating ice as high as 264 K (Diehl et al, 2001(Diehl et al, , 2002von Blohn et al, 2005;Pummer et al, 2012Augustin et al, 2013). Pummer et al (2012) have shown that macromolecules on or within pollen grains are responsible for the ice nucleation activity.…”