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<br />growth by accretion and aggregation, and settling of the snowflakes to the surface. All of these <br />processes are time dependent, several are dependent upon temperature and/or moisture content, <br />and all occur in a complex three-dimensional airflow regime. Added to these complexities are the <br />difficulties of direct sampling in the lowest kilometer above mountainous terrain where recent <br />investigations indicate most of the SL W is concentrated (Hill, 1986). It is, therefore, not surprising <br />that many physical experiments over mountain barriers have failed to demonstrate that seeding <br />leads to enhanced snowfall. A review of the complex processes involved, what has been learned <br />about them in recent years, and what remains uncertain is given by Reynolds (1988). <br /> <br />1.2.3 Great Lakes Experiments. - A series of notable physical experiments was carried out <br />from 1968-1972 as part of the Great Lakes overseeding project summarized by Weickmann (1974). <br />Holroyd and Jiusto (1971) reported on one case in which an aircraft-released line of AgI resulted <br />in marked changes in IPC (ice particle concentration) at a surface site 28 km downwind. The basic <br />approach was to examine the time history of IPC. This alone would not be sufficient to <br />demonstrate a seeding effect, but the increase in IPC was in agreement with the arrival time <br />expected from the prevailing windspeed, and the enhanced IPC was associated with above <br />background levels of iodide. A slight but not statistically significant increase in precipitation rate <br />corresponded with the passage of the seeded cloud. Other similar experiments were reported by <br />Jiusto and Holroyd (1970). <br /> <br />Eadie's (1970) discussion of the Great Lakes experiments pointed out that snowfall distributions <br />were highly variable, making it very difficult to isolate seeding effects from natural variations <br />occurring on the same time and space scales. He recommended aircraft sampling of the clouds and <br />a number of mobile ground observation stations. <br /> <br />Weickmann (1973) presented a comprehensive report on the Great Lakes experiments. He <br />concluded that, "This research constitutes one of the few available studies that have shown <br />unambiguously without postexperimental data manipulation that artificial precipitation can be <br />generated, and that cases in which 'nature misses her chance' do occur often enough to warrant <br />continued exploration," Numerical modeling of the clouds and their response to seeding was an <br />important component of this project. However, the real strength was that many of the case study <br />experiments were able to track the seeded cloud volume. This was done by using visual <br />observations from aircraft, by monitoring the AgI, and by radar measurements, so that it was known <br />when the seeded cloud passed over the ground observing sites. Surface measurements of ice crystal <br />characteristics, especially of IPC, revealed temporal changes that were associated with the seeded <br />cloud passage. In addition, AgI particles were identified in the seeded snow. However, in spite of <br />successfully following physical processes from release of seeding material to snow on the ground, <br />quantitative measurements of snowfall rate were not achieved. <br /> <br />1.2.4 Bridger Range Experiments. - Super et al. (1972) described several attempts to directly <br />detect the effects of seeding on the Bridger Range of Montana. Both airborne and ground seeding <br />were done, and radar and various surface measurements were made in the target area. The radar <br />was very useful in providing wind information for targeting, by tracking reflectors on balloons, <br />However, changes in radar reflectivity due to seeding were inconclusive except in nonprecipitating <br />situations. Changes in IPC with time appeared to be the strongest evidence of seeding effects, but <br />natural temporal changes often made interpretation difficult. It was recommended that additional <br />surface stations be operated crosswind of the target to help monitor natural changes with time. No <br />instrumented aircraft was available to track seeded cloud volumes during these experiments, which <br />proved to be a serious shortcoming. <br /> <br />3 <br />