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<br />tt) enhance target area silver levels can reasonably be interpreted. as lailurc to sig.nificantly alter <br />target sno.....fall amounts. <br /> <br />2.0 DWB SEEDI:\G PROGRAM F.VALUATlO/i BY SILVF.R.I:\-S:\OW SA\lPl.I:\G <br /> <br />2.1 General <br /> <br />It has Ix~n recognizt.>d for many years that achieving adequatc T &D of the commonly-used <br />AgI seeding agent is the majlJr prob/l1m in S<<ding winter orogmphic clouds (Rangno 1986: <br />Reynolds 1988: Super 1990). Achieving adequate T &D for effective seeding is especially dillicult <br />when Agl gt.'1lcrators are located at relatively low elevations in mountain valleys. <br /> <br />Providing a first tcst of Agl targcting is thc focus of this as."~.'SSmcnt of the OWB Seeding <br />Program. -Ibe applied approach was previously u.sed v..ith success at scveral other mountain <br />locations. It involvcs thc l.:hemical assessment of trace amounts ofsilvcr. from silver iodidc. in tht..' <br />target area snowpack <br /> <br />2.2 Tnce Chemistf')' E,,'idence of Targeting Problems in Past Programs <br /> <br />Trace analysis of silvcr (Ag) ha'i been used to test Agl targeting success or failure for Q\'cr <br />four decades. Warburton and Young (1968) of the Desen Research Institute (OR!) pioneered the <br />use of neutron activation for analysi~ of sih;cr in precipitation. They demonstnned that seeding <br />\\ith Agl could lead to silver concentrations significantly above instrumental threshold levcls <br />whilc natural precipitation was found to contain lowcr silver concentrations. <br /> <br />A brief review is prescntcd of some e.x~rimental and operational Agl seeding: progr.uns. <br />which have been tested with silver.in-snow analyses. This review illustratcs that cvcn some <br />operational programs run for decades have had serious targeting problems because of improper <br />r&D of the seeding agent. It should be understood that most operational programs over the years <br />have not attcmptcd to documcnt whethcr targeting was successful but haVI;' simply assumcd that <br />to be the ca...c. '(be widespread practicc of assuming acceptable targeting. without any physical <br />evidence continues to this day. In view of kno\\-n high frequencies of valle~-ba~-d inversions. <br />and complcxities of airflow over rugged mountainous terrain. it would seem prudent to test that <br />assumption. Re~nolds (1996). in summarizing a reccnt Sierra ~evada seeding experiment. <br />emphasized the neccssity of confinning successful targeting for a majority of storm periods <br />before proceeding v.ith a longAenn program. <br /> <br />Reynolds cl al. (1989) reported upon ground.based seeding experiments with AgI gencrator <br />sitcs ranging from the foothills to far up thc \\l."St slope of the Sierra ~evada of California A <br />network of 24 generators consisted of three long.tcnn opennional networks of l'l'1llotc-.controlled <br />generators and one specially installed netv.ork of manually-operated generators. All generntors <br />were operated in a coordinalt.-d fashion during a t\\o-rrnmth period. A targeting model was USt.-d to <br />compute nucleation and fallout locations for each generator on a gi \en da;. The model appeared to <br />pro\;de reasonable estimates of Agl plume T&D when compared to aircraft observations_ <br /> <br />Q <br />