Evaporation freezing by contact nucleation inside-out

Ice formation in atmospheric clouds is crucial to our understanding of precipitation and cloud radiative properties. In recent work it was shown that heterogeneous ice nucleation rates can be strongly enhanced by a form of surface crystallization (Shaw et al., 2005). Here we present new laboratory data and consider the implications for contact nucleation and its relevance to ice nucleation in atmospheric clouds. Our observations contradict three leading hypotheses for contact nucleation and suggest, instead, that the notion of contact nucleation should be generalized to include surface crystallization from particles contacting a supercooled drop from the inside out, as well as from the outside in. Our findings lead to the hypothesis that the freezing temperature of an evaporating drop will suddenly become higher once the drop surface contacts an immersed ice nucleus. This mechanism for evaporation freezing is therefore a plausible explanation for the abundant observations of high ice concentrations associated with cloud dilution and droplet evaporation.