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<br />altitude sites assumes adequate routine transport from them into 8L W cloud zones as well as adequate <br />source strengths. ' <br /> <br />. . <br /> <br />It is recommended that analysis of the early 1994 microphysical observations, and additional future <br />observations, be combined with numerical modeling to address the' above concerns. A sophisticated <br />microphysical model, combined with a two or three dimensional airflow model, should be adequate to <br />address ice particle growth and fallout trajectories. <br /> <br />It may be that only relatively wide mountain barriers offer sufficient time and distance for propane <br />seeding to routinely affect snowfall on that barrier. In that case, the best use of high altitude propane <br />seeding may be in seeding a secondary downwind barrier similar to the Bridger Range Experiment target <br />(Super and Heimbach 1983). In any event, the frequency of orographic SL W cloud at temperatures too <br />warm for AgI nucleation is significant over mountains of the western U.S. That fact alone argues for <br />continued experimentation with alternative seeding methods such as release of liquid propane which <br />might augment AgI seeding when slightly supercooled clouds are present. <br /> <br />1996 articles and papers: <br /> <br />8.19. Griffith,D. A., 1996: Potential application of results from the NOAA atmospheric modification <br />program to the conduct of a Utah winter orographic cloud seeding program. 13th Conference on <br />Planned and Inadvertent Weather Modification, Atlanta, GA, American Meteorological Society, 118- <br />120. <br /> <br />INTRODUCTION <br /> <br />An operational winter orographic cloud seeding program has been conducted in Utah in most winter <br />seasons since 1973-74 (Griffith et aI., 1991). This program utilizes'networks of valley based silver iodide <br />generators that are selectively turned on and off during the passage of winter storms through the State of <br />Utah. Generators are sited upwind of target mountain barriers and are operated upon instruction from <br />North American Weather Consultants' (NA We) meteorologists, located in Salt Lake City. The goal of <br />the program is to augment mountainous snowfall for subsequent use for irrigation and municipal water <br />supplies. The program is funded by counties, water conservancy districts and the State of Utah, Division <br />of Water Resources.. . <br /> <br />BACKGROUND <br /> <br />The design of the Utah operational cloud program is based upon the results of earlier research programs <br />(i.e., Climax, Colorado River Basin Pilot Project). Three general hypothesis cOl1stitute the conceptual <br />model for the program. ( <br /> <br />, . <br />". Sufficient supercooled liquid water is present in winter storms over Utah to permit seeding <br />material to nucleate and grow additional precipitation sized hydrometeors. <br /> <br />. . <br /> <br />· Si,lver iodide seeding material released from ground generators can reach seedable locations in <br />sufficiently dilute but wide-spread concentrations to affect a significant portion of a storm. <br /> <br />68 <br />