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<br />oonH <br /> <br />and improve the model. The use of state-of-the-art models with comprehensive observations <br />from direct detection seeding experiments is expected to provide the most rapid progress <br />possible toward demonstrating a scientifically sound weather modification technology. <br /> <br />3. GENERAL APPROACH TO THE PROPOSED PROGRAM <br /> <br />This section presents cloud seeding fundamentals and the general hypothesis by which seeding <br />is expected to enhance snowfall_ An overview of the proposed experimental program is <br />presented, and the results of the experimental site selection process are described (and <br />presented in detail in app. A). <br /> <br />3.1 Fundamentals of Winter Orographic Storms and Cloud Seeding <br /> <br />Winter storms over the Basin's mountains vary from simple orographic to Drographically <br />enhanced. As described by RangnD (986), orographic storms are "cloud systems that form <br />solely as the result of air rising over terrain, which are seen as quasi-stationary clouds of <br />variable coverage on satellite imagery." RangnD refers to DrDgraphically enhanced storms as <br />"long-lived cloud systems associated with fronts and troughs that are trackable on satellite <br />imagery prior to impinging on mountain barriers." Most significant precipitation-producing <br />storms are orographically enhanced where the uplift caused by the mountains produces <br />condensate in the lowest kilometer or so above the terrain during passage of synoptic <br />(large-scale) storm systems. <br /> <br />The degree to which the barrier-enhanced SLW is converted to snowfall partially depends on <br />the natural ice particle concentration, which, in turn, is related to cloud depth. Shallow clouds <br />with warm top temperatures (above about -20 OC) generally appear to be the most suitable for <br />seeding (e.g., Grant and Elliott, 1974; Dennis, 1980) because they tend to have limited ice <br />crystal concentrations. Thick clouds with cold tops produce abundant ice crystals that may <br />settle through and largely convert the DrDgraphically enhanced SLW to snowfall. However, wide <br />variability in ice crystal concentration exists at a given temperature in some clouds because of <br />variations in available ice nuclei and secondary crystal production processes (Hallett and <br />MOSSDp, 1974). Sometimes, even a high concentration of natural ice particles cannot convert all <br />of the SLW to snowfall because of strong cross-barrier winds which produce abundant liquid <br />water. <br /> <br />1".' <br /> <br />The existence of SLW in excess of that converted naturally to ice particles and snowfall is a <br />necessary but not sufficient condition for seeding to have potential. (Diffusional ice crystal <br />growth can occur when the atmosphere is saturated with respect to ice, but the presence of <br />some SLW will be considered necessary for significant seeding potential.) Abundant SLW is <br />formed in the lowest several hundred meters above windward slopes when moist air is forced to <br />ascend mountain barriers. ThiS process produces liquid water droplets at rates determined by <br />the temperature, humidity, and upward motion of the air; these tiny droplets tend to evaporate <br />rapidly in the descending air in the lee of the mountain barriers. In addition, the forced uplift <br />will sometimes release latent instability in the atmosphere, resulting in (generally weak) <br />convection with additional SLW production. <br /> <br />10 <br />