The Feasibility of Operational Cloud Seeding in the North Platte River Basin Headwaters to increase Mountain Snowfall

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r� discovered that many small ice particles were found well below cloud top in regions of strong orographic uplift. The latter naturally- formed crystals would have shorter travel and growth times and, therefore, be more similar to seeded crystals. Redder and Fukuta (199 1) presented a companion paper to Redder and Fukuta 1989 which included PP ( ) information on fall velocities. Fall velocities of individual crystals ranged between 30 and 60 cm s' with the maximum at -10 °C (columns and thick plates) and minimum near -5 (needles) and -15 °C (dendrites and stellars). Using a fall velocity of 45 cm s', an average mass of 20 X 10-6 g and crystal concentration of 20 L-' yields a snowfall rate of near 0.028 inch V, a typical snowfall rate in the Intermountain West. It should be noted that snowfall accumulations and rates in this paper refer to melted snow water equivalent, not snow depth. Super et al. (1986) discussed two winters of high resolution gage observations on the Grand Mesa of western Colorado. They showed that the median hourly snow water equivalent accumulation was 0.028 inch for all hours with at least 0.01 inch accumulation. This median is the same value as in the above calculated example. These simple calculations show that significant snowfall can be produced by about 20 L` individual crystals allowed to grow by diffusion for approximately 20 min at temperatures in the dendritic growth zone ( -13 to -17 °C). Lighter but still meaningful snowfall rates would be achieved at warmer temperatures. Of course, greater crystal concentrations and longer growth times would produce greater seeded snowfall rates, as would riming or aggregation which can significantly increase fall velocities and mass growth rates. Ice crystal size (not mass) growth rates were presented by Holroyd (1986) using aircraft observations from simple orographic clouds. He found a growth rate near 0.07 mm min' valid through 13 min at -13 °C and a rate near 0.1 mm min-' valid through 15 min and probably through 30 min at -14 °C. These results from orographic rather than laboratory clouds are noted because they are in reasonable agreement with Ryan et al. (1976) and Redder and Fukuta (1989), adding credibility to the laboratory results. 3c. Artificial Ice Crystal Initiation ' Ice crystals can be artificially formed (caused by seeding) within a population of SLW cloud droplets by either of two forms of nucleation, heterogeneous and homogeneous. Heterogeneous nucleation involves interaction between a SLW droplet and a foreign particle known as an ice nucleus. Most natural atmospheric ice nuclei are tiny insoluble clay particles transported from the ground by the wind (Dennis 1980). It could be questioned whether such ice nuclei should always be considered "natural" because plowed fields are often the source of the clay particles. While some industrial emissions produce artificial ice nuclei they rarely have more than local importance. The homogeneous nucleation process does not require interaction between foreign particles and water droplets, but only that cloud be chilled a little colder than -40 °C. Practical means of seeding by this process include dropping dry ice (solid COO pellets into SLW cloud or the expansion of a (possibly liquified) gas to achieve very local within -cloud cooling colder than -40 °C. Such chilling of cloudy air produces very large supersaturations resulting in the condensation of vast numbers of tiny droplets which immediately freeze, forming embryonic ice crystals. Freezing of preexisting cloud droplets is of limited importance because of their much smaller concentrations. The most frequently used agents for homogeneous nucleation are dry ice and liquid propane, with occasional use of liquid nitrogen in foreign ' countries and compressed air in the laboratory. The use of compressed air released through a supersonic nozzle has been proposed for supercooled fog dispersal by Weinstein and Hicks (1976). However, this I