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Weather Damage Modification Program 30 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . downwind distance will be increased. Experience from mountainous plume tracing has shown plumes to typically be about 15 degrees in horizontal width, including the meandering component, and that most plumes are within a factor of two of that angle (Super 1974; Holroyd et al. 1988). Plume spreads within convective bands may be greater due to the interaction of convective motions upon the plumes. Aircraft passes will be no lower than 1,000 feet above the highest nearby terrain, but as the plume moves out over the valleys to the east and north, flight at crest elevation will be possible in many cases. The aircraft will initially be flown crosswind to sample through the plume from edge-to-edge. This will determine the plume width. Because the SF6 release rate will be metered and constant throughout tracing effort, the detected SF 6 concentration will provide an indication of the dispersion rate, which can be applied to the ice nuclei plume as well. As the seeding plume is ingested by the convective elements embedded within the convective bands passing through the area and transported vertically, the aircraft will climb above the highest terrain and begin flying in-cloud along the axis of the convective band. When SF6 is detected at one altitude, the subsequent pass will be made -500 feet higher. The aircraft cloud physics instrumentation suite will be used to measure the natural and seeded cloud characteristics. The presence of SF 6 concomitant with ice concentrations as measured by the Particle Measuring Systems OAP-2D-C probe will confirm the location of the seeding plume, as well as document the initial microphysical effect of seeding. After nucleation, the AgI plume will be converted to ice particles having appreciable terminal velocities, and will thus settle (separate) from the SF6 plume, which will remain gaseous. Nevertheless, the tracking of the plume from source to nucleation should unambiguously confirm the position of the plume up to and immediately after nucleation in super-cooled cloud. In the event that sampling at and above the crest line altitude fails to detect SF6, passes will be made at lower altitudes further upwind (near the windward barrier slope) if conditions allow. This will be done to determine if the SF 6 and AgI are "pooling" at lower levels, or are being transported around the barrier rather than over it. (This is thought unlikely because of the relatively high locations of the project seeding sites, but this too is important.) Because of the distance from source to project target, it is anticipated that SF6 (and AgI) concentrations will be near the threshold of detection as the plumes approach the most distant downwind target boundaries. The observed positions of the SF6 plume (and therefore the AgI plume) will be compared to the positions predicted by the transport and dispersion model. Mapping the plume dispersion and confirming the nucleation of supercooled cloud will reaffirm the critical first step in the conceptualized physical chain-of-events in the Santa Barbara precipitation augmentation program. Failure to track the plumes from source to cloud will identify a serious shortcoming in the program methodology, and will provide information helpful in the remediation of the problem. Tracking will be aided by the targeting and dispersion model, which may itself benefit from the lessons learned. The aircraft data system will provide real-time wind measurements during each pass, which will guide the aircraft scientist in continuously revising estim~tes of where the plume