Contact freezing: A review of experimental studies
This manuscript compiles both theoretical and ex- perimental information on contact freezing with the aim to better understand this potentially important but still not well quantified heterogeneous freezing mode. There is no com- plete theory that describes contact freezing and how the en- ergy barrier has to be overcome to nucleate an ice crystal by contact freezing. Experiments on contact freezing con- ducted using the cold plate technique indicate that it can ini- tiate ice formation at warmer temperatures than immersion freezing. Additionally, a qualitative difference in the freez- ing temperatures between contact and immersion freezing has been found using different instrumentation and differ- ent ice nuclei. There is a lack of data on collision rates in most of the reported data, which inhibits a quantitative cal- culation of the freezing efficiencies. Thus, new or modified instrumentation to study contact nucleation in the laboratory and in the field are needed to identify the conditions at which contact nucleation could occur in the atmosphere. Important questions concerning contact freezing and its potential role for ice cloud formation and climate are also summarized.