Production of ice in tropospheric clouds: A review

Ice in the troposphere affects a variety of processes, including the formation of precipitation, and cloud lifetime, albedo, dynamics, and electrification. A lack of understanding of the ways in which ice is created and multiplied hampers progress in understanding all of these processes. We survey the state of knowledge, starting with homogeneous nucleation, where current formulations for freezing from both pure water and solutions have considerable predictive power. However, debate still exists on the underlying mechanisms of nucleation. Using the concepts and framework that homogeneous nucleation provides, heterogeneous nucleation, where neither a commonly agreed upon theory nor even standard measurement technique exists, is considered. Investigators have established the ice-nucleating characteristics of broad classes of substances, such as mineral dust and soot, which are important ice nuclei in the atmosphere, but a coherent theory of why these substances act as they do has yet to emerge. All ice in clouds is the result of a nucleation event, but its concentration can be enhanced by secondary processes that multiply and magnify the original nucleation events. Riming particles splinter in certain conditions, and this process explains many, but not all, instances of ice concentrations that are greater than those created via primary nucleation. It seems that important secondary processes have not been identified, especally in cases with no liquid water.