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Weather Damage Modification Program 24 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Meteorological Society (AMS) recently published a Scientific Background Statement (AMS 1998b) to accompany its most recent Policy Statement on Weather Modification (AMS 1998a). It was emphasized that, "Successful treatment of any suitable cloud requires that sufficient quantities of appropriate seeding material must enter the cloud in a timely, well-targeted fashion. As the need for stringent spatial and temporal targeting becomes more established, it is apparent that inadequate delivery of seeding agents may in part account for the failure of some earlier cloud seeding programs to produce significant results." Silver-in-snow sampling and AgI plume observations from a number of seeding projects, (to be discussed later) suggest that failure to adequately seed the SL W cloud volume has been common even with some long-term projects. Obviously, failure to properly target winter orographic clouds with seeding agent will result in failure to increase target area precipitation. Warburton and Young (1968) of the Desert Research Institute (DR!) of the University of Nevada were among those who pioneered analysis of silver in precipitation. They demonstrated that seeding with AgI could lead to silver in snow or rain that was significantly above instrumental threshold levels which were then about 10-11 g mr1 (10 parts per trillion, ppt, by weight). Natural precipitation was shown to contain lower silver concentrations. Finding more silver in snow does not prove that AgI seeding increased the snowfall, or even that AgI created new ice crystals, because scavenging by ice crystals and snowflakes or direct deposition might explain increased silver levels. However, failure to find enhanced silver levels in an expected target area strongly suggests that proper AgI targeting was not achieved. The only other potential explanation of observed positive effects would have to be dynamic seeding impacts that persist beyond the microphysical effects. Silver analysis will be of interest to this program but will be a part of phase II research proposed when funding becomes available. Most operational seeding programs have not even addressed the fundamental transport and dispersion issue, and the Santa Barbara Project heretofore has been no exception. Projects have simply assumed that this critical link in the physical chain of events leading to successful seeding takes place, as discussed by Super and Heimbach (1983). The first recommendation of the AMS (1998a) states, "Whereas a statistical evaluation is required to establish that a significant change resulted from a given seeding activity, it must be accompanied by a physical evaluation to confirm that the statistically observed change was due to the seeding." This proposal is specifically designed to provide such physical documentation. Verification of targeting was attempted by Reynolds et ai. (1989), who reported on some other ground-based seeding experiments in which the AgI generator elevations ranged from the foothills to well up the west slope of the Sierra Nevada of California. The network of 24 generators consisted of 3 long-term operational project networks of remote-controlled generators and an additional network of manually operated generators. All generators were coordinated during a 2- month period. A targeting model (Rauber et ai. 1988) was used to compute ice nucleation and snow fallout locations for each generator on each operational day. The model appeared to provide reasonable estimates of AgI plume transport and dispersion when compared with aircraft observations. It was subsequently discovered however, that in some cases, the AgI plumes released from foothill generators had trajectories parallel to the mountain barrier rather than over it. In this program, downwind targeting was more successful from the higher elevation generators.