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<br />3.4 Seeding~Caused Snowfall Calculations <br /> <br />Laboratory observations have indicated that the NCAR counters used in the Plateau studies were in <br />reasonable agreement with CSU Isothermal Cloud Chamber results. Plateau-top and aircraft observations . <br />of ice particle concentrations have been shown to be in reasonable agreement with NCAR counter <br />estimates of effective ice nucleus concentrations using CSU generator calibrations to extrapolate to <br />temperatures warmer than the -20 oc used by the NCAR counter. Therefore, it is reasonable to use CSU <br />generator calibration data to calculate upper limit snowfall increases possible with the Utah operational <br />seeding. <br /> <br />It will be assumed in the calculations to follow that seeding-caused ice crystals do not participate in any <br />secondary ice "multiplication" process. That is, any single AgI aerosol particle will have the potential to <br />produce only one ice crystal. There is ample justification for this assumption. The conditions necessary <br />for significant ice particle multiplication to occur are reasonably well understood (Mossop 1985). Such <br />conditions are not characteristic of Rocky Mountain orographic clouds, especially at the colder <br />temperatures where AgI can be effective. <br /> <br />~. <br /> <br />The latest CSU calibration of the NA WC generator, for the AgI solution used in Utah's operational <br />seeding, can be used to show that it is highly unlikely that the operational seeding produced the snowfall <br />' . <br />increases suggested by NA WC's published statistical analyses. The normal April 1st snow water <br />equivalent found at snow courses in or near the Plateau's experimental area is about 50 em. The most <br />recent statistical analysis by the seeding operator (Griffith et al. 1997) suggests that about a 15 percent <br />seasonal increase was achieved, equivalent to 7.5 cm for a normal winter. Using the typical AgI generator <br />spacing of 16 km (Griffith 1996) and approximate Plateau top width of 10 km provides an estimated area <br />per generator of 1.6 X 1012 cm2. The water volume provided by a 7.5 cm increase would be 1.2 X 1013 <br />cm3, equivalentto 1.2 X 1013 gofwater mass. <br /> <br />We will make the highly optimistic assumption that all the AgI aerosol reached SLW cloud at a relatively <br />cold temperature of -8 oc. This is contrary to the large NOAAlUtah AMP data set which indicates that <br />only limited Agl reaches cloud temperatures that cold. The CSU calibration indicates the generator output <br />at -8 oc is 1.4 X 1014 crystals g-I for natural draft conditions. We will make the additional highly <br />optimistic assumption that all the available aerosol nucleated ice crystals at that temperature. Then the <br />resulting seasonal output of ice crystals can be calculated as 2.8 X 1017, since generator output is 8 g h-I <br />(Griffith et al. 1992). and the generators are operated for about 250 h per 5 month winter (Super and <br />Huggins 1992<1, table 2). These crude but conservative calculations yield an average mass per ice crystal <br />of 0.04 mg per crystal. But as discussed by Super and Huggins (1992a) and Super (1994), observations <br />from a number of locations show that the mass of a "typical" natural ice crystal is less than half that value. <br />Furthermore, seeded ice crystals are likely to be smaller because of less in-cloud residence time. <br /> <br />'. <br /> <br />These calculated results would require that all AgI reached a significantly colder temperature than <br />supported by the multitude offield observations. In reality, AgI was sometimes trapped in the upwind <br />valley and did not reach sLw cloud. When it was transported over the Plateau, the bulk of the AgI was <br />typically in a thin layer where temperatures were too warm for significant ice crystal ~ormation. Often, <br />this layer contained negligible SL W because it was consumed by natural snowfall processes. In-cloud <br />residence times provided a further limitation to AgI nucleation ability. Finally, natural snowfall could be <br />expected to sweep out some AgI aerosol. For all these reasons, the above calculations of the typical ice <br />crystal mass w:hich AgI would need to produce to achieve a 15 percent seasonal snowpack increase are <br />absurdly low. During the fraction of the time when' storm conditions made it possible for seeding to create <br />ice crystals, they would have to grow to unrealistically huge sizes (masses) to produce the claimed <br /> <br />15 <br /> <br />