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<br />Hygroscopic flares are not recommended at this time, despite their very promising results, <br />because: a) their true effectiveness cannot be estimated until data on the nature and <br />variability of the natural nuclei spectrum in the target area are obtained, and b) the <br />hygroscopic flare seeding operation at very low altitudes in the sub cloud layer probably <br />cannot be accomplished in the mountainous terrain of the target area without considerable <br />practice, if at all. Nevertheless, future implementation of this approach should be actively <br />explored because it is potentially more efficient and much less expensive. Therefore, the <br />procurement and installation of a ground-based CCN counter at the AARRP radar site is <br />highly recommended so continuous measurements of CCN can be made. <br /> <br />3.5.2 Seeding strategy <br /> <br />Seeding should be conducted as soon as the qualification criteria in the target areas are met <br />. without regard to or fear that the clouds will grow above the freezing level. Seeding should <br />be conducted as low in the cloud as possible consistent with good flight safety practices. <br />Seeding should be conducted by two CASA aircraft with appropriate time and/or spa.ce <br />separation to maximize the seeding concentration. Considering that the maximum payload <br />of each aircraft is 1200 kilograms, that the aircraft customarily fly at 120 knots, and that the <br />seeding material cannot be dispensed in less than 15 minutes, those factors will result in a <br />seeding concentration of 0.0685 gram per cubic meter. <br /> <br />3.5.3 Additional Studies <br /> <br />The warm cloud seeding findings are based almost exclusively on numerical model <br />experiments which, although consistent with other model studies and field experiments <br />conducted elsewhere, do not yet have the benefit of field verification in the proposed target <br />area. Preliminary confirmation of expected physical effects was planned during the 1993 field <br />season; but could not be pursued because of equipment failure. Therefore, it is recommended <br />that the demonstration project include physical studies and additional model studies to <br />confirm the nature and magnitude of predicted effects on individual warm and cold clouds. <br /> <br />3.6 The Conceptual Model for the Warm Cloud Seeding Demonstration Project <br /> <br />The results of the numerical model seeding experiments support the scientific feasibility of <br />all three approaches that are designed to produce microphysical effects that will improve the <br />efficiency of the rain evolution mechanisms and decrease the time required to initiate the <br />precipitation process. Considering the fact that economic and logistic factors require that the <br />seeding be done with relatively large polydisperse CaCl2 particles, the conceptual model for <br />the warm cloud seeding demonstration project is based on accelerating the coalescence <br />process by bypassing the auto conversion process and initiating the collision-coalescence <br />process earlier in the life ofthe cloud. It is postulated that by reducing the time required fiJr <br />the precipitation process to evolve with respect to the time available, rain efficiency wlill <br />increase such that clouds that would not naturally rain will rain, and clouds that would <br />naturally rain will rain more. <br /> <br />3. 7 Warm Cloud Seeding Hypotheses <br /> <br />The following four null hypotheses will be tested in the warm cloud seeding demonstration <br />project: <br /> <br />26 <br />