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<br />-. <br /> <br />concentration and precipitation rate for the natural data set. That indicates that the mass and terminal <br />velocity relationships for seeded crystals were about the same as for natural particles. <br /> <br />The strongest increases were for the propane cases, which had HAS release temperatures ranging only <br />between -0.4 and -3.4 oC. The use of one nozzle produced concentration increases for small particles of <br />about ten L-I and two nozzles about twenty L-1. Precipitation rate increases of 0.17 mm h-I for 2 nozzles <br />are admittedly small, but they are a 37% increase over that in the natural data set for the same sizes. <br />Additional growth times for a traverse greater than the 4.2 km should further boost the precipitation rate, <br />until the crystals hit the ground somewhere downwind. <br /> <br />No increases were found in the gage measurements for the AgI and propane cases, though the nozzle <br />number analysis showed a strong difference when the second nozzle was added. <br /> <br />The seeded volume of cloud is not known and therefore the total production rate of ice particles cannot be <br />.accurately compared to the rate of release of the seeding agents. However, an estimate can be made to <br />examine the ordet: of magnitude. Holroyd et al. (1988) found a plume dispersion angle of about <br />15 degrees at distances close to the surface release site of AgI generators on the Grand Mesa of Colorado. <br />Using a half-angle from the center line of7.5 degrees in both the horizontal and vertical directions, the <br />plume cross section at the TAR, 4.2 km downwind from the HAS, can be approximated by a half circle of <br />radius 550 m. The average transit time for the propane experiments was 17.0 min, yielding an average _ <br />wind speed between the HAS and TAR of 4.1 m S-I. Two nozzles released liquid propane at about 6 to <br />8 gal h.1 or about 3.7 g S-I , producing about 21 ice particles L-1. Combining the flux of particles through <br />the plume cross section at the TAR and the release rate yields a production rate,for liquid propane of <br />about 1.1 x 1010particles g-I between -0.4 and -3.4 oC. Super and Holroyd (1997) quoted other <br />investigators of liquid propane production rates of 1010 to 3 x 1011 , 108 to 1011, and 1012 particles g-l at <br />similar temperatures. Finding 1010 particles g.l remaining 4.2 km downwind after losses to coagulation <br />and evaporation and to the trees and ground, appears to be of comparable magnitude to those previous <br />estimates. <br /> <br />~ <br /> <br />Operationa,luse of liquid propane, costing $0.80 gal-I, using two nozzles at7 gal h-.I, indicates a materials <br />cost of $5 .60 h-I for a seeding unit that produces a plume about a kilometer wide at 4.2 kIn downwind with <br />an increase of about twenty particles L-I at only slightly supercooled conditions. The precipitation rate <br />increase according to the 2D-C was 0.17 mm hr'l. According to Table 1, the gage equivalent rate <br />(dividing by 0.56 to convert from 2D-C rates to gage rates) is 0.30 mm hr-I for the small particles. <br />Additional growth time could result in larger amounts of precipitation. <br /> <br />For comparison, the materials cost for operating aNA WC AgI generator is 8 g h.j is about $20 h-I. The <br />production from such a generator is highly dependent on the nucleation temperature. <br /> <br />These experiments show that AgI appears to have no effect when released at similar temperatures and <br />comparable or greater effect at TAR temperatures colder than about -6 oC_, though the data ~re noisy. <br />Therefore the use of liquid propane as a seeding agent at only slightly supercooled temperatures is an <br />effective and inexpensive means to boost ice particle concentrations downwind in orographic clouds. <br /> <br />. ., <br /> <br />A future experiment should try to document the fallout pattern with downwind distance of precipitation <br />generated by the use of liquid propane. That could be accomplished in a straight canyon with road from <br />the generator to the crest and beyond. The best experiments will be with only trace natural snowfall, <br />permitting the use of numerous snowboards scattered downwind and crosswind to supplement recording <br />precipitation gages and a mobile 2D-C probe. <br /> <br />J <br /> <br />82 <br />