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<br />The policy statements are markedly more positive than previous AMS and WMO statements, <br />largely because of recent direct physical evidence from a variety of new observing systems. This <br />evidence comes from a limited number of brief (1 to 3 h) experiments from a few locations as <br />discussed later. The general seeding hypothesis for winter orographic clouds apparently applies <br />to some cloud conditions, which is certainly encouraging. However, as suggested by the policy <br />statements, convincing physical evidence is needed to demonstrate that area-wide precipitation <br />can be increased in an economically viable manner over the course of several winter seasons. A <br />need to provide such convincing scientific evidence clearly exists before the emerging technology <br />will gain wide acceptance as a practical option for water resources management. <br /> <br />2.2 Overview of Past Investigations <br /> <br />Scientists have been aware of the key processes in the general winter orographic cloud seeding <br />hypothesis for decades (e.g., Ludlam, 1955). These processes are known to operate as expected <br />when meteorological conditions are suitable. Silver iodide ice nuclei will definitely create high <br />concentrations of ice crystals if introduced to sufficiently cold SLW clouds. Some of these ice <br />crystals will grow to snowflakes or snow pellets if they remain for sufficient time in SLW cloud. <br />This process has been demonstrated in cloud simulation chambers and by a limited number of <br />physical experiments in real clouds. These processes have also been simulated by numerical <br />cloud models. <br /> <br />Several recent studies have documented the existence of SL W during portions of many winter <br />storms using microwave radiometers. A number of these investigations have shown the <br />cross-barrier flow (flux) ofSLW to be a large fraction ofthe streamflow or precipitation from the <br />same regions (Sassen, 1985; Rauber et al., 1986; Boe and Super, 1986; Rauber and Grant, 1987; <br />Thompson and Super, 1987; Heggli and Rauber, 1988; Super and Boe, 1988a; Super and <br />Holroyd, 1989; Huggins et al., 1992; Long and Huggins, 1992; Sassen and Zhao, 1992). Thus, <br />SLW frequently exists as the needed "raw material" which seeding can convert to ice crystals <br />that grow and begin to settle toward the ground. What has not been satisfactorily demonstrated <br />is that significant quantities of the SLW can be converted to additional snowfall on mountain <br />ranges over the course of a winter season. <br /> <br />Several statistical experiments have been conducted with winter orographic storms in the <br />West, but most produced inconclusive results. These experiments had a number of common <br />characteristics. Hypotheses were stated, with varying degrees of detail, which noted the chain <br />of physical events expected to follow seeding. In broad terms, the hypothesis would note that <br />SLW in the form of tiny droplets would have to exist within the cloud, and seeding would have <br />to convert some of these droplets to ice crystals capable of growing and settling to the surface as <br />snow or melting and falling as rain. When a presumably suitable storm appeared or was <br />forecast, a random decision was made to seed or to reserve the event as a nonseeded control <br />case. In either event, the same observations were taken. After a number of field seasons, cases <br />were statistically tested for treatment effects on precipitation. The entire data set might have <br />been partitioned into meteorologically similar categories; for example, by estimated cloud top <br />temperature. Each category (partition) was statistically tested for differences between seeded <br />and nonseeded precipitation. Observations usually were limited to target area precipitation and <br />some general indications of the storm structure, the latter used for partitioning. Such efforts <br />have been referred to as "black box" experiments because, if the statistical testing did not <br />indicate significant differences between populations of seeded and nonseeded storms, <br />insufficient physical observations existed to determine any point(s) of failure in the <br /> <br />6 <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I, <br />I <br />I, <br />I <br />I <br />I <br />