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<br />mountains. Stage m should have been seedable until low level winds became parallel to the barrier. <br />Stage IV was one of stonn dissipation and little apparent seeding opportunity. <br /> <br />Further analyses of the stonn discussed by Long et al. (1990) was given by Sassen et al. (1990). They <br />also presented a conceptual stonn model based on numerous Utah winter stonns. This model makes a <br />clear distinction between orographic stages with shallow, moist clouds over the barrier and periods when <br />propagating cloud systems dominated local conditions and SL W generation. Orographic stonn stages are <br />typically inefficient in precipitation production with primarily liquid water cloud present, closely related <br />to topography, However, when mesoscale precipitation bands periodically sweep over the mountains, the <br />SL W produced by low-level flow across the barrier is effectively converted to snowfall by ice crystals <br />falling from the bands. The periodic mesoscale bands are a key feature that produce a large portion of <br />the total snowfall. However, liquid-dominated clouds exist between passage of the precipitation bands <br />which likely have seeding potential. <br /> <br />Three northern Arizona winter stonns were presented by Super and Holroyd (1989) that are similar to the <br />Sassen et al. (1990) conceptual model. Vertically integrated SLW amounts observed by a microwave <br />radiometer were inversely correlated with the height of the cloud tops. When bands of high clouds with <br />abundant natural ice crystals passed overhead, the SL W produced at low levels by uplift over the <br />Mogollon Rim was largely converted to snowfall. However, periods with shallow clouds often had <br />abundant SL W and limited snowfall, suggesting they were seedable. Exceptions to this general portrayal <br />occurred when strong cross-barrier winds produced more SL W than the natural ice crystals settling from <br />the deep clouds could totally convert to precipitation. <br /> <br />Reynolds and Kuciauskas (1988) discuss the structure and organization of winter stonns over the central <br />Sierra Nevada. Even though differences might be expected between the Sierra Nevada and more inland <br />mountains, they also found highest amounts of SL W existed in wann-topped shallow clouds with <br />embedded convection and low precipitation rates. Heggli and Rauber (1988) showed that most SLW was <br />within about 1 km of the local terrain over the Sierra Nevada, <br /> <br />Based on the above and other investigations of winter stonns in the West, it is expected that SL W will <br />be concentrated in the lowest 0.5 to 1.0 km over Utah mountains, and will be most abundant when clouds <br />are wann and shallow so that natural ice crystal and snowfall production are limited. Periods with strong <br />cross-barrier winds or convection will have the greatest amounts of SL W while winds parallel to the <br />plateau will produce little SL W. The passage of cloud bands with high, cold tops and abundant ice <br />particles will reduce or eliminate the SL W produced by low-level uplift over the barrier. Considerable <br />liquid water should be produced near times of cold front passage and whenever convection is present. <br />The amount of the SL W available for seeding will depend upon the efficiency of natural processes in <br />converting the water to ice. The natural efficiency can vary considerably and rapidly as mesoscale bands <br />periodically move across the mountains. <br /> <br />The T&D of AgI released from low-level generators will be most effective when near-surface winds have <br />a significant component toward the mountain barrier or when convective cells pass over the generators. <br />From the conceptual models reviewed, it would be expected that the most favorable low-level flow would <br />occur during the middle stonn stages. The earliest stonn stage likely will have blocked flow near the <br />valley floor while the final dissipating stonn stage can be expected to have low-level flow parallel to the <br />mountain barrier. But neither of these stages appear very seedable anyway because of lack of excess <br />SLW. High level releases of AgI can be expected to provide seeding plumes over the plateau whenever <br />an upslope wind component exists, even when the atmosphere has a stable lapse rate. ~ <br /> <br />20 <br />