Atmospheric models used to predict precipitation and the effects of clouds on solar and terrestrial radiation are sensitive to variations in the types of ice crystals in clouds and their concentration, yet the processes by which drops freeze in clouds are not understood well enough to make reliable estimates of the ice content. This project is an investigation of the atmospheric relevance of several droplet freezing mechanisms that have been suggested on the basis of laboratory observations or theory. These include homogeneous freezing of solution drops and heterogeneous freezing as a result of evaporation that creates either organic surface films or solid phase inclusions within drops. Numerical modeling of these processes will be compared with existing data from several field programs to determine the extent to which the modeling results can be brought into accord with observations. One of the objectives is to study the conditions under which evaporation at the edges of clouds can promote freezing by heterogeneous nucleation. Another is to attempt to explain recent observations indicating that the homogeneous freezing temperature of solution drops depends only on the water activity, a measure of the reduction in equilibrium vapor pressure of the drop compared to pure water. Results of this research will contribute fundamentally to understanding ice formation in the atmosphere, and thus enable the improvement of weather and climate models in which the details of cloud composition are important.
