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<br />near Allen Lake Tank (hereafter called Allen Lake), located midway between the town of Mormon <br />Lake and Happy Jack at an elevation of 2270 m, longitude of 1110 26' 24", latitude of 340 49' 27". <br />The site is far enough from Hutch Mountain to permit overhead aircraft sampling down to 2800 m, <br />or 530 m a.g.l. It is near a powerline and the highway, and is large enough to permit tipping curve <br />calibrations of a microwave radiometer and operation of a Doppler acoustic sounder. The clearing <br />is in the Verde River Drainage, within a kilometer of the crestline of the Mogollon Rim. The most <br />serious drawback is the lack of nearby dense forest for ice particle and precipitation measurements. <br />Snowfences may be required in addition to the existing tree cover to minimize wind effects on these <br />observations. <br /> <br />1.5 Observing Systems for Physical Experiments <br /> <br />1.5.1 Scientific Uncertainties. - In the case of Arizona winter clouds, scientific uncertainties <br />were substantially reduced during the early 1987 and early 1988 field programs as discussed by <br />Super et al. (1989). Most important, it was shown that abundant excess SLW existed during <br />portions of most synoptic-scale winter storms, particularly during the beginning and ending phases <br />and at other times when clouds were shallow so the IPC was limited, Flow perpendicular to the <br />barrier, usually south-southwesterly but sometimes the reciprocal, was responsible for the uplift that <br />produced most of the SL W. The bulk of the SL W appeared to be near the surface, where <br />maximum uplift and water vapor content should combine into greatest condensate. The SL W at <br />aircraft levels was usually limited. <br /> <br />Ground releases of tracer gas showed that it usually dispersed through the lowest several hundred <br />meters of the atmosphere with typical neutral stabilities, but that this layer would often be too warm <br />for significant ice formation by conventional types of AgI. <br /> <br />Ice multiplication was not frequent in Arizona winter clouds, at least at aircraft sampling levels, <br />contrary to findings over the Sierra Nevada where cloud seeding potential was markedly reduced <br />by the naturally abundant IPC High IPC frequently existed in the Arizona clouds, but usually as <br />a result of high, cold clouds and presumably primary ice nucleation. <br /> <br />In spite of the progress made in the first two field programs, a number of uncertainties remain. <br />Key questions to be addressed include: <br /> <br />1. What is the SL W distribution upwind of the barrier? How frequently does SL W extend <br />far enough upwind, and at a cold enough temperature, for ice particles created by seeding to <br />have the opportunity to grow and fall on the upwind side of the barrier? <br /> <br />2. How rapidly will seeding-produced crystals grow and fall to the surface? Not only the <br />SLW distribution and its temperature influence crystal growth and fallout. Ice particle fall <br />speeds are reasonably well known in still air, but vertical motions in excess of crystal fall speeds <br />are common near mountain barriers. For stable and neutral conditions these may be <br />approximated by the slope of the barrier multiplied by the horizontal wind component normal <br />to the barrier. However, embedded convection can significantly enhance vertical speeds. Even <br />more important are the ice crystal growth mechanisms involved. Diffusional growth is <br />reasonably well documented, at least for several minutes following nucleation. Accretional <br />growth is less understood and aggregational growth is poorly understood. Yet the latter is <br />likely when large concentrations of dendritic crystals or needles are formed by seeding. <br /> <br />1 <br />1 <br /> <br />12 <br />