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00319!: Much ofthe SLW produced by forced ascent over mountain barriers is not naturally converted to ice particles during portions of some winter storms. The SL W can exist at temperatures well below 0 oC because of the scarcity of ice-forming nuclei capable of converting the supercooled cloud droplets to ice crystals. Ice nuclei are highly temperature dependent and the concentration of effective ice nuclei increases by about a factor of 10 with a 4 OC decrease in cloud temperature. Once ice particles are formed in SLW cloud, they initially grow at the expense of the surrounding supercooled droplets because of the difference in saturation vapor pressure over liquid and ice (diffusional growth). Later growth may be accelerated by accretion, if many cloud droplets collide with and freeze on falling ice particles, and/or by aggregation, if many ice crystals chain together into large snowflakes. Larger snowflakes and snow pellets formed by accretion are more likely to reach the surface as snow because of their higher terminal fall velocities. Natural ice particle production is often too low for efficient conversion of the SL W to snowfall. In some inefficient clouds, ground-based seeding from high altitude AgI generators or propane dispensers can create a significant increase in the ice particle concentration in the lowest several hundred meters above the windward slopes; that is, within the primary SLW zone. The SLW and ground-based seeding zones are shown conceptually on figure 1. Cloud above the primary SLW zone consists mostly of ice crystals with perhaps limited liquid in some portions. Ice crystals initiated by seeding will grow as the crystals are transported along quasi-horizontal trajectories through the SLW zone. Growth will be especially rapid when SLW is abundant across the entire barrier, but will continue when at least ice saturation exists. Cloud temperature also influences ice particle growth rates (Redder and Fukuta, 1989). r ~ When the environment is suitable for adequate growth of some ice particles to permit their fallout to the target surface, the seeding will cause additional snowfall. Depending upon the growth environment and three-dimensional wind field, additional snowfall may occur for some distance beyond the mountain barriers, although evidence is weak for far downwind effects (e.g., Grant et al., 1992) On a long-term basis, the great majority of seeded snowfall is expected to settle on the higher elevations of the target barrier. ~ Conditions which favor seeded crystal growth and fallout include limited natural ice crystals, abundant and nearly continuous SLW from the generators to the target, and moderate winds. Exceptionally strong winds may produce abundant SLW, but transport time across the barrier may be too short for many snowflakes to settle to the surface before being carried to the lee evaporation/subsidence zone. As shown by Rauber and Grant (1986), the 'best" ice particle concentration for conversion of SLW to ice is a complex function of several variables. For practical purposes, the desired concentration in the crystal growth zone should be at least 10 ice crystals VI and may be over 100 VI in some situations (this point will be further investigated during the direct detection experiments). Attaining such concentrations requires that at least part of the SLW zone through which the ground-released AgI is mixed by mechanical turbulence (and sometimes weak convection) be colder than about -8 ac. This temperature might be increased by about 2 to 3 oC with newer, more effective AgI solutions and high output generators. For ground-based propane seeding, significant ice particle concentrations may be formed in supercooled cloud at 11