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<br />003196 <br /> <br />The seeding hypothesis for ground-based AgI and propane cloud seeding is expected to be <br />similar to, but more specific than, the following statements: <br /> <br />. When the prevailing wind is approximately normal to a mountain barrier, forced uplift of <br />moist air sometimes produces SLW in excess of that naturally converted to snowfall. The <br />SLW zone is concentrated in the lowest kilometer over the windward slopes and barrier <br />top. <br />. Routine and reliable cloud seeding requires production of AgI ice nuclei, or release of <br />propane gas seeding agent, well up the windward slope of the target barrier (or from an <br />upwind barrier). <br />. Mechanical turbulence, sometimes aided by convection, results in the T&D of the AgI ice <br />nuclei or propane-created crystals throughout a substantial fraction of the SLW zone. <br />When the AgI-seeded portion of the SLW zone is cold enough, significant ice crystal <br />formation results. Propane expansion within liquid cloud below 0 oC will cause similar ice <br />crystal formation. <br />. When the atmospheric temperature, moisture, and wind environment are suitable, a <br />fraction of the seeded ice crystals grow to snowflake or snow pellet (graupel) sizes while <br />being transported toward the target area. <br />. Some seeded snowflakes and pellets fall to the target surface before being carried into the <br />lee subsidence (and evaporation/sublimation) zone. When suitable conditions exist for <br />prolonged periods (hours), the accumulation of seeded snowfall can be significant. <br /> <br />l' <br /> <br />t <br /> <br />3.3 Overview of Experimental Approaches <br /> <br />The approach at each experimental area will involve conducting a series of direct detection <br />seeding experiments. Past experience has shown that three winters will be required to obtain a <br />reasonably large population of these experiments over the range of atmospheric conditions <br />which typically affect a mountain region. Analysis of the direct detection experiments will <br />indicate the range of cloud conditions and specific seeding techniques most likely to lead to <br />enhanced snowfalL This knowledge will significantly sharpen the design of the statistical <br />experiment to follow. <br /> <br />l <br /> <br />A four-winter randomized statistical experiment will incorporate considerable physical <br />monitoring. Four winters are considered necessary to build up a sufficiently large population of <br />experimental units for adequate statistical power. The combination of statistical and physical <br />experimentation can provide a powerlu1 approach to furthering knowledge of cloud responses to <br />seeding as demonstrated in summer convective clouds during the HIPLEX-l (High Plains <br />Cooperative Program) Experiment (Cooper and Lawson, 1984; Mielke et al., 1984). <br /> <br />Direct detection experiments, which monitor the important links in the chain of events <br />following seeding, have been made increasingly practical by recently deveioped <br />instrumentation. Reclamation scientists have participated in such experiments starting with <br />the HIPLEX summer program in the late 1970s and, more recently, in winter projects in <br />California, Colorado, Montana, and Utah. Although some direct detection experiments strongly <br />indicated increased precipitation, others failed to do so, at least at the intended point target. In <br />the latter cases, identification of plausible reasons for the apparent failure often was possible <br />because of the comprehensive nature of the observations. These probable explanations led to <br />improved understanding and better experimental design. The sophistication of these <br />experiments has increased dramatically in recent years. <br /> <br />13 <br />