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

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I Some of the droplets may be "nucleated" and become embryonic ice crystals. This ice nucleation process requires the presence of very tiny particles known as ice nuclei, concentrations of which vary widely in time and space. Natural ice nuclei, in the form of clay particles and leaf litter, have concentrations much less than those of cloud droplets. There are some ice multiplication processes, including crystal fragmentation, that also provide "nuclei" for additional ice crystal growth. After embryonic ice crystals form, they grow by the process of diffusion involving the vapor transfer of water molecules from droplets to crystals. This transfer occurs because the saturation vapor pressure is greater over water than ice. C. Many droplets may be captured by, and freeze onto, faster falling individual ice crystals or snowflakes, a process known as accretional growth or "riming." In some clouds this can lead to the growth of graupel (snow pellets) and hail. ' d. Aggregation is a process whereby smaller ice crystals collide and "chain together," becoming larger snowflakes with faster fall velocities. Sometimes riming will help this process by acting as a "glue." Stellar and dendritic crystal habits are especially prone to aggregation but other habits, such as needles, also aggregate. Heavy snowfalls usually involve aggregation into large, fluffy snowflakes. ' Snow crystals have a variety of habits, or shapes, and growth rates, mostly based on the temperature of the cloud environment and to a lesser extent the moisture content of the air. Laboratory growth rates between -3 and -21 °C were reported by Ryan et al. (1976) for times up to about 3 min. Dendritic crystals grow the fastest at about -15 °C, which is at a peak in the vapor pressure difference between SLW and ice. A secondary growth peak may exist for needles and columns at about -7 °C. Thick plates, at about -10 °C and -20 °C have the slowest growth rates. Growth rates are slow between -3 to -4 °C. P 1 1 n Redder and Fukuta (1989) presented equations for ice crystal mass and dimensional growth. They used six experimental data sets from cloud chambers or supercooled cloud tunnels for growth periods up to 30 min. Molecular diffusion was the predominant growth mechanism in all reported experiments. The authors indicated that after 20 min growth time, the mass of individual ice crystals would be in the range of 5 to 10 ug for temperatures between -5 and -12 °C and for colder than -17 °C. Crystal masses greater than 10 ug were found between -13 and -17 °C in the dendritic growth zone, with the maximum of 30 gg at -15 °C. The minimum mass after 20 min growth was 2 gg at -4 °C, the warmest temperature considered. This same information is presented in table 1 in section l lb. While their results agreed with the -15 °C maximum found by the short duration studies of Ryan et al.. (1976), a secondary maximum at -7 °C was not indicated after 20 min growth. Super (1994) referred to earlier field observations which suggested that a typical natural ice crystal from an orographic cloud had a mass of approximately 20 µg. Of course, considerable variation existed among individual crystals and the "typical" value is roughly a median of the various field observations which were examined. The typical value is similar to the fastest diffusional growth results at 20 min but less than the masses expected for most of the supercooled temperature range of interest. Natural crystals often have growth times well in excess of 20 min because they form high in the cloud and have long growth and fallout trajectories. Rauber (1987) found that the primary source region for ice particles observed near the Park Range was near cloud top. Most of the resulting snowflakes on the mountain surface likely originated tens of kilometers upstream from the Park Range, with travel times on the order of 30 to 60 min. He also