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<br />1. INTRODUCTION AND GOALS <br /> <br />The cooperative Federal-State Atmospheric Modification Program (AMP) includes weather modification <br />investigations involving the National Oceanic and Atmospheric Administration (NOAA) and the States of <br />AriZona, Illinois, Nevada, North Dakota, Texas and Utah. This program was developed in response to a <br />1978 recommendation of the Weather Modification Advisory Board that the Federal government provide <br />support for research to better understand, evaluate and improve some well-designed and controlled State <br />cloud seeding projects. It was recognized that more scientific progress could be realized from this approach <br />at limited cost to both parties, and with significant benefit to the specific State programs and the general <br />field of weather modification. Utah's operational seeding program, like that of most other State programs, <br />attempts to enhance water supplies. Specifically, Utah's program is intended to increase the high mountain <br />snowpack leading to subsequent streamflow enhancement. This goal will have added importance if climatic <br />change results in decreases in rainfall and snowfall in the intermountain West as indicated by some <br />numerical models. <br /> <br />The overall goals of the Utah/NOAA AMP are, frrst, to evaluate the effectiveness of the Utah State <br />operational winter cloud seeding program and, second, to recommend specific improvements in that <br />program. This is intended to be an ongoing process in which the research efforts, overlain on the <br />operational program, "feed back" results which lead to improved effectiveness in the operational seeding. <br /> <br />In the context of winter orographic clouds, weather modification, commonly called "cloud seeding", refers to <br />the introduction of man-made ice crystals into supercooled clouds. Under appropriate conditions these <br />crystals can grow to snowflake sizes and settle to the surface. Ice crystals can be made by chilling small <br />regions of supercooled liquid water (SLW) cloud to temperatures below -40 oC (homogeneous nucleation), or <br />by producing large numbers of very small particles with the ability to form ice crystals in SLW cloud <br />(heterogeneous nucleation). Such particles are called ice nuclei (IN). Silver iodide is the most Common IN <br />used in cloud seeding, and has been the agent used in the Utah operational seeding program. Silver iodide <br />IN can be effective at temperatures colder than about -6 to -9 oC, depending upon the specific chemical <br />formulation, type of AgI generator, atmospheric conditions and other factors. <br /> <br />The scientific objectives of the Utah/NOAA AMP have remained the same over the course of the <br />cooperative program. They are: <br /> <br />a. To describe the spatial and temporal distributions of SLW in winter orographic (mountain- <br />induced) clouds over Utah's mountains and the factors controlling the SLW. <br /> <br />b. To understand the atmospheric processes permitting, or preventing, the transport and dispersion <br />(T&D) of significant concentrations of ground-released silver iodide (AgI) IN to SLW regions in orographic <br />clouds. <br /> <br />c. To improve understanding of the physical processes responsible for winter precipitation <br />development in natural and seeded orographic clouds. <br /> <br />d. To document and explain the trajectories of natural and seeded precipitation particles in winter <br />orographic clouds. <br /> <br />1.1 SLW Studies <br /> <br />Research to date has mainly tested scientific objective a. because SL W presence is a necessary (but not <br />sufficient) condition for practical snowfall enhancement through cloud seeding. If SLW was not found in <br />reasonable abundance, there would be little point in conducting cloud seeding. In fact, a number of recent <br /> <br />1 <br />