Numerical Comparison of Two Ice Crystal Formation Mechanisms on Snowfall Enhancement from Ground-Based Aerosol Generators

Abstract

Two mechanisms of ice crystal formation, contact freezing and very rapid condensation freezing, were applied to numerical simulations of ground-based seeding with the Guide Model, an orographic cloud model, to study whether different mechanisms of ice crystal formation substantially affect precipitation patterns and intensities. Although the numerical model has limitations, it was expected to indicate how different ice crystal formation rates lead to differences in precipitation patterns and intensities between the two mechanisms. Numerical simulations of two case studies are presented. One is characterized by moderate wind speeds and colder cloud temperatures, the other by stronger winds and warmer cloud temperatures. The moderate wind field and colder cloud temperatures yielded nearly half an order of magnitude more precipitation than the strong wind field and warmer cloud temperatures. Sensitivity analysis showed that snowfall as a result of forced condensation freezing is strongly dependent on the ambient temperature at the ground-based generator site, while generator site temperature had less effect on the precipitation as a result of contact freezing. Snowfall resulting from contact freezing, however, was found to be strongly dependent on the cloud drop concentration. Liquid water content did not significantly affect the precipitation resulting from ice crystal formation by either mechanism. Precipitation rates induced by forced condensation freezing are about two orders of magnitude greater than those induced by contact freezing in the cases simulated, over the Sierra crest, because of the limited time available for ice particles to grow and precipitate.