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<br />. . <br /> <br />SUMMARY AND RECOMMENDATION <br /> <br />The seeding experiments produced obvious changes in microphysical characteristics and snowfall during <br />periods with very limited natural snowfall. One of these experiments used an AgI release while HAS and <br />Target cloud temperatures were near -9.0 and -10.7 oC. The resulting seeded IPC and snowfall was <br />estimated at 140 L-1 and 1.0 mm h-1, respectively, 4.2 km east-northeast of the HAS at the Target. <br /> <br />The second experiment released liquid propane at HAS cloud and Target temperatures of -3.3 oC and <br />-4.5 oC, respectively. Propane seeding was estimated to have enhanced IPC by about 30 L-1. and snowfall <br />by 0.25 mm h-1. Higher propane release rates may be appropriate for these slightly supercooled <br />temperatures. <br /> <br />A third experiment with AgI seeding conducted after the second did not enhance IPC during temperatures <br />near -2.5 oC at the HAS and -4.0 oC at the Target. These experiments have provided valuable <br />information to partially address seeding effectiveness. Additional seeding experiments should be <br />conducted on the Plateau between the HAS and the Target. These experiments are simple and <br />economical, and targeting of the Target site is routine with southwesterly flow at the HAS. Future <br />experiments of this type should concentrate on periods with no more than very light natural snowfall. <br />Detecting microphysical changes caused by seeding during higher natural snowfall rates is difficult <br />because of the variability of such snowfall. <br /> <br />. <br /> <br />1997 articles and papers: <br /> <br />8.24. Cripps, D., and B. Abbott, 1997: The use of capacitance to detect icing. J. Weather Modification, <br />29, 84-87. <br /> <br />INTRODUCTION <br /> <br />The Utah State Division of Water Resources currently operates several cloud seeding sites in the Utah <br />mountains. The sites use icing probes to detect supercooled liquid water; the presence of which indicates <br />favorable conditions for cloud seeding. Propane is released into the atmosphere when supercooled liquid <br />water is present providing refrigeration which freezes existing water droplets leading eventually to <br />precipitation. Accurate sensing of ice, and thus the presence of supercooled liquid water is critical to <br />efficient use of resource in this process. <br /> <br />Currently, icing probes designed for the aerospace industry are used to detect the presence of supercooled <br />water. The probe consists of a vibrating rod which is extended into the atmosphere and ice build up on the <br />rod causes the frequency of vibration to shift. It is this shift of frequency that is detected and processed to <br />control propane release. When icing is detected, the decision to release propane is made, and then the <br />current build up of ice on the probe is melted off and the detection cycle begins again. Propane is released <br />continuously after icing is detected until an ice free cycle occurs. . ' <br /> <br />The probes in use are expensive, require an AC power source, and have moderately high power <br />requirements. The cost and power requirements of these probes prohibit widespread development of <br />cloud seeding sites. This paper presents research on an alternative icing probe based on a capacitive <br />measure of icing conditions. Research on this icing sensor is being sponsored by the Utah State Division <br />of Water Resources. <br /> <br />76 <br />