Summary of the NOAA/Utah Atmospheric Modification Program: 1990-1998

This paper summarizes many previously published physical investigations into the artificial nucleation (``seeding'') of winter mountain clouds in central Utah during 1990-1998. Program goals were to evaluate the effectiveness of the Utah operational cloud seeding program and to recommend improvements. Field programs employed a wide variety of instrumentation systems. Sophisticated numerical modeling was used in conjunction with the observational programs. Amounts and distributions of supercooled liquid water (SLW) cloud were investigated, as was transport and dispersion of ground-released seeding agents and tracer gases. Several experiments directly monitored ice crystals and snowfall rates resulting from either silver iodide (AgI) or liquid propane seeding. Results showed frequent SLW in excess of natural conversion to snowfall, suggesting significant seeding potential. The SLW was concentrated near the terrain where temperatures were relatively warm. When valley-released AgI was transported to cloud levels, resulting ice crystal formation was usually too limited for significant snowfall augmentation. However, marked enhancement of ice crystal concentrations and snowfall rates resulted from a number of high altitude releases of both AgI and liquid propane. Propane seeding was effective within even slightly supercooled cloud. Several recommendations were given for improving the operational seeding program's effectiveness.