Abstract
Abstract. This manuscript compiles both theoretical and experimental information on contact freezing with the aim to better understand this potentially important but still not well quantified heterogeneous freezing mode. There is no complete theory that describes contact freezing and how the energy barrier has to be overcome to nucleate an ice crystal by contact freezing. Experiments on contact freezing conducted using the cold plate technique indicate that it can initiate ice formation at warmer temperatures than immersion freezing. Additionally, a qualitative difference in the freezing temperatures between contact and immersion freezing has been found using different instrumentation and different ice nuclei. There is a lack of data on collision rates in most of the reported data, which inhibits a quantitative calculation 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.
Highlights
Clouds play an important role in the global radiative budget (Trenberth et al, 2009) as they cover around 70 % of the Earth’s surface (Stubenrauch et al, 2010)
There is experimental evidence for contact freezing to act as proposed by Cooper (1974), Durant and Shaw (2005) and Djikaev and Ruckenstein (2008), it is still unclear why the available laboratory results indicate that contact freezing is the most efficient ice nucleation mode
More and better controlled experiments are needed to validate the proposed hypotheses by Cooper (1974) and Fukuta (1975a) which so far are the most promising ideas. Both theories are based on the classical nucleation theory (CNT) and include key parameters partially proven by laboratory experiments such as water adsorption on the particle surface, RHw and ice nuclei (IN) size
Summary
Clouds play an important role in the global radiative budget (Trenberth et al, 2009) as they cover around 70 % of the Earth’s surface (Stubenrauch et al, 2010). Ice formation can be enhanced by contact freezing within mixed-phase clouds since both aerosol particles and supercooled cloud droplets may be present. Seifert et al (2011) showed the effectiveness of ash particles to nucleate ice in natural clouds They reported that contact freezing may be responsible for the freezing events that took place at warm. The rates for dust particles are lower than the corresponding values in the deposition nucleation and immersion freezing modes because of the large size of dust particles that renders collisions less likely than in the case of soot In this manuscript we summarize the available theories, instrumentation and laboratory studies on contact freezing with a special focus on the experimental laboratory results and instrumentation but we leave out the field experiments. The limitations of the currently available instrumentation are provided with the aim to build new and better instruments to study contact freezing in the future
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