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003183 crystals will grow to snowflakes or snow pellets if they remain for sufficient time in SLW cloud. This process has been demonstrated in cloud simulation chambers and by a limited number of physical experiments in real clouds. These processes have also been simulated by numerical cloud models. Several recent studies have documented the existence of SLW during portions of many winter storms using microwave radiometers. A number of these investigations have shown the cross-barrier flow (flux) of SLW to be a large fraction ofthe streamflow or precipitation from the same regions (Sassen, 1985; Rauber et al., 1986; Boe and Super, 1986; Rauber and Grant, 1987; Thompson and Super, 1987; Heggli and Rauber, 1988; Super and Boe, 1988a; Super and Holroyd, 1989; Huggins et al., 1992; Long and Huggins, 1992; Sassen and Zhao, 1992). Thus, SLW frequently exists as the needed "raw material" which seeding can convert to ice crystals that grow and begin to settle toward the ground. What has not been satisfactorily demonstrated is that significant quantities of the SLW can be converted to additional snowfall on mountain ranges over the course of a winter season. Several statistical experiments have been conducted with winter orographic storms in the West, but most produced inconclusive results. These experiments had a number of common characteristics. Hypotheses were stated, with varying degrees of detail, which noted the chain of physical events expected to follow seeding. In broad terms, the hypothesis would note that SLW in the form of tiny droplets would have to exist within the cloud, and seeding would have to convert some of these droplets to ice crystals capable of growing and settling to the surface as snow or melting and falling as rain. When a presumably suitable storm appeared or was forecast, a random decision was made to seed or to reserve the event as a nonseeded control case. In either event, the same observations were taken. After a number of field seasons, cases were statistically tested for treatment effects on precipitation. The entire data set might have been partitioned into meteorologically similar categories; for example, by estimated cloud top temperature. Each category (partition) was statistically tested for differences between seeded and nonseeded precipitation. Observations usually were limited to target area precipitation and some general indications of the storm structure, the latter used for partitioning. Such efforts have been referred to as 'black box" experiments because, if the statistical testing did not indicate significant differences between populations of seeded and nonseeded storms, insufficient physical observations existed to determine any point(s) of failure in the hypothesized chain of physical processes. Even when statistical testing suggested differences between seeded and nonseeded populations, the exploratory nature of most analyses left doubts about probability estimates and questions about the physical mechanisms involved. Without an adequate understanding of the latter, transferability of results to other locations was questionable. I ~ ~ The need to document the key physical processes involved with cloud seeding has become increasingly apparent. Vincent Schaefer, the discoverer of modern weather modification, recently made a plea for a return to physical evaluation of seeding effects (Schaefer, 1990). Brabam (1981) made a strong case for improving our physical understanding of how cloud processes react to seeding before embarking on any more large 'black box" experiments. He stated that 'We need to design experiments that can follow seeding effects as perturbations moving through the varying cloud systems." Fortunately, such observations have become increasing practical with new instrumentation developments. A limited number of physical experiments of this type have been conducted as discussed later. 6