NUMERICAL COMPARISON OF 2 ICE CRYSTAL-FORMATION MECHANISMS ON SNOWFALL ENHANCEMENT FROM GROUND-BASED AEROSOL GENERATORS

Two mechanisms of ice crystal formation, contact freezing and very rap id condensation freezing, were applied to numerical simulations of gro und-based seeding with the Guide Model, an orographic cloud model, to study whether different mechanisms of ice crystal formation substantia lly affect precipitation patterns and intensities. Although the numeri cal model has limitations, it was expected to indicate how different i ce crystal formation rates lead to differences in precipitation patter ns and intensities between the two mechanisms. Numerical simulations o f two case studies are presented. One is characterized by moderate win d speeds and colder cloud temperatures, the other by stronger winds an d warmer cloud temperatures. The moderate wind field and colder cloud temperatures yielded nearly half an order of magnitude more precipitat ion than the strong wind held and warmer cloud temperatures. Sensitivi ty analysis showed that snowfall as a result of forced condensation fr eezing 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 result ing from contact freezing. however, was found to be strongly dependent on the cloud drop concentration. Liquid water content did not signifi cantly affect the precipitation resulting from ice crystal formation b y 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, be cause of the limited lime available for ice particles to grow and prec ipitate.