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<br />EVALUATION OF CLOUD SEEDING EXPERIMENTS: SOME LESSONS TO BE LEARNED <br />FROM THE CASCADE AND SAN JUAN PROJECTS <br /> <br />Peter V. Hobbs <br /> <br />Cloud Physics Group, Atmospheric Sciences Dept., <br />University of Washington, Seattle, Wa. <br /> <br />1. INTRODUCTION <br /> <br />The three essential components for the scientific evaluation of a cloud <br />seeding project are theoretical, physical and statistical analysis. A wide <br />range of techniques for the physical evaluation of the effects of artificial <br />seeding are now available. We illustrate this here by reference to the <br />Cascade Project. The statistical design of a cloud seeding experiment should <br />be based on prior physical measurements. Moreover, physical evaluation should <br />continue through the statistical experiment. The neglect of adequate physical <br />measurements will generally result in considerable weakening of the conclusions <br />which can be drawn from a cloud seeding experiment. This is demonstrated by <br />the recently completed Colorado River Basin Pilot Project (called hereafter the <br />San Juan Project). <br /> <br />2. CASCADE PROJECT <br /> <br />The Cascade Project was carried out by the Cloud Physics Group at the <br />University of Washington from 1969 through 1974. The principal objectives were <br />to determine through field measurements the physical processes responsible for <br />the formation of snowfall in winter clouds over the Cascade Mountains, to deduce <br />from these observations possible w'ays in which the snowfall might be modified, and <br />then to investigate through a series of detailed physical case studies whether <br />any significant artificial modification of the snowfall could, in fact, be <br />produced. <br /> <br />Field studies, involving airborne, ground and radar observations, revealed <br />that clouds over the Cascades are generally unglaciated and that snow particles <br />which reach the ground, particular>ly on the western slopes, are generally <br />heavily rimed (Hobbs, 1975a). A numerical model of this situation (Hobbs et aI, <br />1973) indicated that if the clouds over the western slopes were glaciated by <br />artificial seeding, the region of maximum snowfall should be shifted from the <br />western to the (otherwise drier) E!astern slopes of the Cascades with a concomi- <br />tant increase in the snowfall ratE!. <br /> <br />This possibility for beneficial artificial modification of snowfall was <br />investigated through a series of case studies in which the effects of heavy <br />cloud seeding with silver iodide cmd Dry Ice were evaluated through detailed <br />physical measurements (Hobbs, 197:ib; Hobbs and Radke, 1975). The general <br />procedures which were followed in these case studies were as follows. On <br />specified experimental days manned and automatic ground stations distributed <br />across the Cascades were activated; at these sites continuous measurements and <br />observations of snow crystal types, masses and concentrations and snowfall rates <br />were obtained throughout the day. Rawinsondes were launched at regular intervals <br />from a site well upwind of the Cascade crest. The University of Washington's <br />B-23 cloud physics research aircra.ft would obtain an initial set of in-cloud <br /> <br />2'1 <br />