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<br />atmosphere was moderately stable in five cases and plume widths ranged from 2 to 8 kIn Olver the target <br />about 17 kIn downwind of the generator. Associated plume tops were about 250 to 400 m above the <br />highest peak in the target. The sixth case was very stable with a resulting meandering plume that was <br />quite shallow. Since convective mixing was absent in all six experiments. mechanical mixing must have <br />created the observed plume dispersion. In the three cases with SL W present the AgI clearly created large <br />enhancements in the ice particle concentration and an estimated doubling or tripling of the calculated <br />snowfall rates at the lowest aircraft sampling level. Admittedly. estimated snowfall rates within the <br />seeding plume were very low. ranging from 0.03-0.09 mm iiI. Ground observations were not available <br />so one can only speculate about seeding effects at the surface about 600 m below the sampling aircraft. <br /> <br />Further evidence that high altitude ground releases of AgI can consistently result in seeding plumes <br />crossing the mountain barrier was presented by Holroyd et al. (1988). Instantaneous plume widths had <br />a median spreading angle of 150 and meandered with a median angle of 380. The median height above <br />the crest exceeded 500 m. Even so, the wanner one-third to one-half of the winter stonos over the Grand <br />Mesa of Colorado were not cold enough for adequate nucleation with the type of AgI used within the <br />seeded zone. Perhaps convective mixing would transport the seeding material to higher. colder levels <br />during some stonos, but convective episodes were not sampled. <br /> <br />Preliminary indications from the early 1991 observations over the Wasatch Plateau are that AgI particles <br />from the high altitude site were always transported over the plateau if the wind had a westerly component. <br />The high altitude plume usually was detected at the lowest aircraft sampling level (near 12.000 feet m.s.l.) <br />but not much higher. TIlis suggests results were similar to studies in other mountain ranges where high <br />altitude seeding was investigated. High altitude generator sites will succeed in targeting winter orographic <br />clouds almost all the time. Their disadvantages include cost of installation and operation (usually remote- <br />controlled units are required). Furthennore, it is usually not possible to target the upwind side of a <br />mountain barrier with a high altitude generator on the same barrier because of limited time and distance <br />for ice crystal nucleation. growth and fallout. <br /> <br />3.2.3 Storm structure and SLW availability. . Studies of seeded winter clouds over the San Juan <br />Mountains of Colorado were reported by Cooper and Marwitz (1980). Based on a composite of several <br />stonos they showed that the earliest stono stage was characterized by a stable atmosphere in the lower <br />cloud layer and low level winds that were not toward the east-west oriented barrier, Ground-based seeding <br />was unlikely to be effective during this stage. The next stage had modest SL W with neutral stability and <br />wind trajectories that might be favorable for seeding. The following unstable stage had a convective <br />region developing ahead of the mountains in association with horizontal convergence. This provided <br />abundant liquid water and a mechanism for transport of ground-released AgI into the SL W zOne so this <br />stage was judged most seedable. The final stono stage was one of dissipation and little seeding potential. <br /> <br />While there are similarities, the above conceptual model does not totally agree with the Utah winter stonn <br />reported by Long et al. (1990), believed representative of a significant fraction of winter stonos in the <br />intenoountain West. The Utah stono also had four stages: Stage I had altostratus clouds with significant <br />blocking of the wind by the Tushar Mountains and no appreciable SL W. Hence. seeding potential was <br />very limited. Stage II was dominated by passage of a short wave aloft. reasonably abundant SL W and <br />eventually a veering of the wind to approximately perpendicular to the mountains. The latter portions of <br />this stage might have been seedable with valley generators as the low level winds should transJPOrt the AgI <br />up the windward slopes. Stage III was influenced by a passing cold front It had strong. d(:ep updrafts <br />extending from near the ground and a maxima in water condensation. Winds below the frontal surface <br />veered from southwesterly to northerly; that is from approximately perpendicular to parallel to the <br /> <br />19 <br />