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<br />INTRODUCTION <br /> <br />Background <br /> <br />The SRP (Salt River Project) has participated with the ADWR (Arizona Department of Water <br />Resources) and Reclamation in sponsoring cloud research and weather modification studies of <br />winter storms occurring over the Mogollon Rim. The barrier created by the rim leads to <br />frequent development of orographic clouds during winter that contribute most of the annual <br />runoff from the Salt-Verde watersheds. The SRP is interested in processes that may Elugment <br />the water resources from those watersheds. <br /> <br />Reclamation conducted experiments on winter clouds over the Mogollon Rim dwing the <br />January-March periods of 1987 and 1988 (Super et al., 1989). The overall objective of the <br />experiments was to assess the augmentation potential of winter storms with more detailed <br />information. Equipment employed included an instrumented aircraft and ground probes. The <br />latter consisted of a microwave radiometer for remotely measuring CLW (cloud liquid water) <br />and water vapor, and a rawinsonde system for vertically profiling the atmospheric <br />temperature, humidity, and winds. An acoustic sounder was employed for continuously <br />profiling winds up to 600 m above ground level. High-sensitivity gauges determined the <br />precipitation. Cloud and storm studies performed included those of atmospheric moisture, <br />wind, temperature and atmospheric stability, CLW, and precipitation character and amount <br />both aloft and on the ground. <br /> <br />In recent studies, Reclamation scientists have adapted, for the ADWR, a local-scale <br />precipitation model and an airflow model for application to Mogollon Rim storms (rl:!port in <br />preparation). The precipitation model has indicated useful skill in estimating daily <br />precipitation over the Salt- Verde watersheds. Estimates are produced for each grid point of a <br />470- by 500-km grid of lO-km point spacing and by interpolation at other specified locations. <br />The model has demonstrated some applicability for winter storm studies. The airflow model is <br />useful in determining cloud seeding agent release points and targeting of treatment effects <br />under various weather conditions. <br /> <br />Previous cloud and precipitation modeling for Arizona has not confronted the feasibility of <br />employing cloud seeding to augment water supplies from the Salt- Verde watElrsheds. <br />Determination of the feasibility requires knowledge of the amount and frequency of generation <br />of CLW (Super et al., 1989), the basic fuel for enhancement of precipitation by cloud seeding. <br />The presence of unconsumed CLW signifies a less than optimum cloud PE (precipitation <br />efficiency) and perhaps conditions that may be amenable to seeding. The PE refers to the <br />percentage of the precipitation measured on the ground to the potential precipitation. Potential <br />precipitation is defined here as precipitation that would occur when all CLW and ice water in a <br />cloud is deposited as precipitation in a desired area, generally on the windward side of a <br />barrier. <br /> <br />Developing a productive, simple process for forecasting CLW in storms over the Mogollon Rim, <br />using readily available information such as archive soundings, would help determine the <br />frequency of favorable storm conditions for cloud seeding. Operationally, CLW estimates could <br />enhance the conduct of cloud seeding. Additionally, the determination of the conditions that <br />lead to the development of CLW is of great interest to the aviation industry and is being <br />pursued in some long-term cloud research and three-dimensional modeling studies. <br /> <br />1 <br />