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<br />b) Seeding Technologies and Effectiveness <br /> <br />The principal seeding methods have used AgI, hygroscopic particles, dry ice, and LP. These first <br />three can bc disperscd from ground genemtors or aircratl. Hygroscopic particles and dry ice <br />havc not been used much in California, and the former is mainly employed in warm-season <br />secding of convective clouds. Thcreforc the focus \\'i11 be on Agl. LP, and their methods of <br />delivery. The effects of these two methods on cloud microphysics were discussed in scction CI, <br />so the following will address equipmcnt and related logistics. <br /> <br />Aircrall seeding with AgI can be done through combustion in place or via droppabh:: tlarcs. <br />Becausc of aircraft movcment, thc result is a "line source" of AgI, which combined with <br />atmospheric motions and ice particle fall speeds, produces a "curtain" of seeding efTects, The <br />shapc and extcnt of this curtain arc highly determined by T&D that is in lurn controlled by <br />atmospheric winds, turbulence, and cloud microphysics. Whether this curtain routinely cnvelops <br />the target area to produce desired precipitation amounts has not becn conclusively showns8, so <br />more research is necded in this arca. Furthermore, since the bulk of SL W is frequently confincd <br />to the lowest 3000 fcet above the surface, it presents a problcm for aircraft sccding. Safety <br />rcstrictions frcquently prohibit aircratl operation that close to mountainous terrain. The FAA <br />mandates a minimum flight level ovcr mountains of 2,000 feet above the highest terrain within a <br />horizontal distance of four nautical miles. although in special circumstances a waiver permits a <br />clcarance or 1,000 feet. Icing conditions, especially during darkness, add to the concern. There <br />may be some mountain barricrs where aircrafi seeding might be worth consideration, particularly <br />where ground seeding is not feasible (e.g., in wilderncss areas). Such mountains should be <br />relatively isolated so aircrafi could safely descend below the freezing level when airframe icing <br />bccomcs cxcessive, or where aircraft could remain wcll upwind where exposure to icing would <br />be limited, Aircratl sceding is substantially more expcnsivc than ground sceding, so the valuc of <br />water augmcntation would nccd to be high cnough to justify such an option. <br /> <br />Seeding from the ground has been accomplished in the Sierra Nevada principally through Agl <br />generators, At least one project in the California coastal mountains seeds from the ground using a <br />rack with end-burning Agl flares. 111ere have been experiments using LP dispcnsers (Section <br />C2a). All three dcvices may be rcmotely controlled, allov,'ing them to be locatcd at high altitudes <br />that arc not routinely accessible by operators and technicians. Jligh altitude, remotely controllcd <br />devices allow fast response to changing stornl conditions, and increase the chance of seeded <br />plumes reaching the proper temperaturc and SL W regions of orographic clouds, These devices <br />arc less common than their manually operated counterparts. because of increased cost and <br />opcrational complexity: hO\vever, some California and Ncvada projects have used thcm <br />exclusively. Remote controlled AgI gcnerators are more costly and complex than remote <br />controlled LP dispensers, <br /> <br />Whatever types of secding devices arc used. it is critical thai tlu.'y be sited so they provide <br />adequale and routine coveraKe of Ihe rarKel art'a_ This is no simple task. as it must take into <br />account highly variable meteorological conditions during stomlS - see the T &D discussion in <br />Section C2a, Nevertheless. earlier studies indicate that secdcd plume widths arc less than 30 <br /> <br />21 <br />