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<br />aircmll and. "- suggested that in other conditions persons who rele-.c;e silver iodide from the <br />ground would be wise 10 find out where it goes. "(Emphasis added). Unfortunately. their set.-mingly <br />obvious advice ha'i seldom t:N.-cn followed. in part lx.'Causc tracking seeding plumcs over <br />mountainous terrain is difficult and expensi'\'c. ~10st past v.inter orographic projects havc nOl <br />documented where their AgI went. much Icss in what con(.'Cntrdtions as effectivc ice nuclci. In <br />other words. most pa.<;t and current ground-release seeding projccts cannot physically demonstrate <br />that their target clouds were routinely seedcd or evcn seldom scedt.-d. Re~nolds (1988) and Super <br />(1990) c..'Choed earlier concerns that the transport and dispcnsion of ground-rek.a::;cd Agl remained <br />the most serious problem for v.intcr orographic eloud sc..-cding. The difficulties of targeting SLW <br />regions in v.inter orographic clouds were most recently noted in the review article of Bruintjcs <br />(1999). -Ibe approach recommended in this report essentially eliminates the rargl..'ting problem by <br />releasing seeding material directly \\;thin orogr.tphic SL W cloud upv.ind from the target area. <br /> <br />Rhea et a!. (1969) reportt.-d on the results of a 5-ycar prugidffi of :-'1atistical and physical <br />cloud seeding investigations on the Park Range. A large number of t~'ts were conducted to track <br />the transport and dispersion of b'TOund-~ and aircraft rele;)S(.'S of both Agl and tracer material. <br />The authors made the important point that valley-based invcrsions were present during at least half <br />of the hours ",ith sno",fall. The inversions totally lr8ppcd any seeding material released below <br />them. which greatly limited the efTcc.:tiveness of the valley-based seeding generators. The trapping <br />inversions were found to typically occur up to a hcight of at least half of the ml..'aI1 terrain height of <br />th~ main barrier. but \\ith many variations. Trapping inversions often existed between 8.000 and <br />Q.OOO ft_ For referem."e the Yampa Valley to the Wt.-st has elevations below 7.000 ft and the Park <br />Range cn.-st line cxceeds 10.000 ft. Consequently. ground-based generators should be located more <br />than half.v.-ay from the valley floor elcvation to the crest line elevation. H <br /> <br />The largt.'S1 and most-fundt.-d v.inter orographic cloud seeding experiment ct."cr conducted in <br />Colorado. and likely anyv.here. 'Was the Colorado River Basin Pilot Project. It was carried out in <br />the San Juan Mountain.<; ofsouthwcstem Colorado from 1970 to 1975 as summarized by Elliott et <br />al. (197&). This experiment produo.-d inconc1u.~ive result,> in that no signiticant difference in <br />precipitation was found from seeded and nonseedt.-d days. At least rv...o serious design problems <br />were discovered invoh;ng inadequate transport and dispersion of the seedinj! uf{ent and inability to <br />forecast cloud top tcmpenHun.'S and winds. Further discussion of problems \\ith dispersion of the <br />Agl and other issUt."'S were discussed by Rangno and Hobbs (1980). In many C3Si..'S lhc Agl v.as <br />transported at relatively low le..-els around thc San Juan Mountains rather lhan ovcr them. <br /> <br />In view of these earlier Colorado studies. and more recenl investigations conducted at <br />Colorado State University. which are not cited here. one might expect that valley Agl releast.'S will <br />often not be trans~ned into mountain-induced clouds. Holroyd ct al. (1988) tracked Agl releases <br />over the Grand Mesa of v.estem Colorado. Their observations Sugg01ed that ground generators <br />should not be placed lower than about 2300 ft belo\\ the barrier cn.-st for plumes to pass over the <br />mesa_ But the Grand ~Iesa presented a relatively simple target. v.ith \ery broad valleys in up\\ind <br />directions making uplift more likely. The routine targeting achieved in the Bridgcr Range <br />Experiment. again on a simple barrier. used generators only 1500 ft below the crest. line. No doubt <br />the lowest elevation ITom \\hieh set.-ding plwnes \\;11 be tmnsported over higher tcrrain varies from <br />storm to stonn and c\en during the pha-.es of indi\idual stonns. and is strongly influenced by the <br /> <br />,. <br />-, <br />