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<br />However, temperatures in the layer that can be targeted from the ground sometimes will be too wann for <br />significant nucleation by commonly used types of AgI. A review of T&D investigations showed that <br />releases of AgI from high altitude sites along windward slopes would routinely result in the seeding agent <br />reaching the SL W zone. Recent work on the Wasatch Plateau suggests that valley-released AgI will be <br />transported well above the mountains during convective periods and perhaps at other times as well. <br />However, further investigation of valley seeding is needed, especially during stable stonns. <br /> <br />Aircraft seeding could, of course, be used but was not considered because of high costs and logistic <br />difficulties, Sustained missions can be impractical during the heavy icing episodes that characterize <br />. periods with large SL W flux. Moreover, during wanner stonns, aircraft AgI seeding may have the same <br />limitations as ground seeding in attempting to provide high enough concentrations of effective ice nuclei <br />for the wann SL W zone near the mountains. Ground-based propane seeding should be explored as an <br />alternative to AgI seeding during wanner stonn periods. <br /> <br />There seems to be little reason to doubt that properly located AgI generators with sufficiently high outputs <br />can create significant ice crystal concentrations in the SL W zone within 0.5-1.0 km of Sevier Basin <br />mountain ranges when the atmosphere is cold enough. The question is will such seeding result in <br />significant increases in snowfall While a number of statistical evaluations indicate a positive answ~r, <br />other studies have been inconclusive. Statistical analyses should not be considered conclusive without <br />supporting physical observations. The latter generally have been lacking in past programs. <br /> <br />Simple calculations of ice crystal growth rates and fallout velocities for typical conditions indicate that <br />some seeding-created ice crystals should reach the mountain surface before passing into the downwind <br />subsidence region. For example, on the Wasatch Plateau, characteristic distances between high altitude <br />seeding sites and the lee edge would be about 15 lan, With a 10 m S-1 windspeed, an air parcel would <br />take 1500 s (25 min) to traverse this distance. If nucleation occurred near the seeding site and the <br />resulting ice particle grew at 0.5 J.UIl S-I, it would reach 0.5 mm size and have about 0.4 m S-1 tenninal <br />velocity after 1000 s (Rauber et al., 1988). Ignoring the vertical motion field over the plateau, the particle <br />could fall 200 m in the remaining 500 s before reaching the lee slopes. <br /> <br />Only the lowest 200 m above the plateau could be effectively seeded in the simple example just given. <br />But nature is much more complicated than the example suggests. A sophisticated numerical model (e,g., <br />Cotton et al. 1986), capable of calculating the motion field and microphysical processes, should be used <br />to provide realistic estimates. For example, nucleation of ice crystals is time dependent, especially when <br />contact nucleation is dominant (DeMott et al., 1983). Crystal growth rates vary significantly with <br />temperature and other factors (Holroyd, 1986, estimated rates over 1 J.UIl S-1 in clouds at -13 to -14 OC). <br />Tenninal velocities also vary markedly with crystal size, type, and degree of riming (Rauber et al" 1988). <br />Overall, ice crystal nucleation and growth can be considered a stochastic process in which a small fraction <br />of the particles will become much larger than the general population. These "fortunate" particles will <br />contain much of the total snowfall mass and will have a greater change of reaching the surface before <br />passing beyond the target area. <br /> <br />In general, stronger winds produce greater condensate (more SL W) which will enhance ice crystal <br />nucleation and growth, but allow less time for air parcels to traverse the mountain barrier. Factors that <br />enhance ice particle growth and fallout to the surface include temperatures ranges conducive to rapid <br />crystal growth, abundant SL W leading to growth by riming, barriers with large along-the-wind distances, <br />and moderate windspeeds. It is usually prudent to release AgI from as far upwind of the target as <br />practical, perhaps even from the next mountain range upstream. This enhances volume filling of the SL W <br />zone and allows nucleation to occur sooner. <br /> <br />31 <br />