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1154 JOURNAL OF APPLIED METEOROLOGY VOLUME 27 seeded ice particles should tend to be smaller than older natural crystals, and a decrease in the SL W content of the seeded volume might be anticipated. For these rea- sons, measurements of ice particles and SL W were critical to the conduct of the experiment. Observations of ice nuclei (effectively of AgI; see Part I) were also necessary to verify the targeting and approximate extent of the seeded cloud volume. The lowest IFR sampling over the BR T A was at 2.7 km, 300 m above the highest terrain and approximately the same level as the crest of the Main Ridge. Results of previous VFR plume tracing indicated that this low flight level was necessary for the successful conduct of the experiments. It should be noted that IFR sampling this near to the barrier, made possible by a fortuitous combination of light air traffic, a nearby navigational station, and low terrain crosswind of the target, is not practical in most mountain locations. As will be shown, monitoring at normal terrain clearance altitudes of 600 m agl usually detected little of the seeding material that was consistently present nearer the BRTA surface. Prior to 0900 (all times MST) of each operational day, a forecast was prepared using National Weather Service products as well as pibal and surface observa- tions from the seeding site. If a westerly wind com- ponent and substantial clouds existed or were forecast, a technician was instructed to start the AgI generator approximately an hour prior to aircraft takeoff. At the start of most research missions, the aircraft entered a holding pattern over the Bozeman Airport (BZN), 13 km west of the seeding site, while a sounding was made to about 5.2 km, or to above the cloud deck, whichever was lowest. Following the sounding, a west- to-east pass was made over the seeding site to the Shields Valley (Fig. 1). The aircraft then returned to the BRTA and conducted a series of N-S passes de- scending from 3.9 to 2.7 km in 300 m intervals to 3.0 km, and 150 m steps below. The flight to the Shields Valley and the descending profile were to test for un- manageable turbulence. Following the BR T A traverses, a sounding was often made over the Shields Valley, after which the aircraft returned to BZN along the SSL (see Fig. 1). The flight plan was generally adhered to, excepting variations imposed by the flight controller due to other aircraft traffic. 3. Determination of seeded and control zones Most results presented in this paper are from the analysis ofN-S passes made over the BRTA. The seg- ment of each pass with an enhancement in ice particle concentration (IPC), presumably due to AgI seeding (hereafter enhanced IPC zone), was determined in the following manner. The natural background IPC was established for the sampling level by examining the regions well north and south of any obvious enhanced IPC zone. The upper values of the natural background IPC were used to determine the edges of the high IPC zone by examining the buffer-by-buffer listing of IPC from the 2D-C probe. The north (south) edge of the enhanced IPC zone was considered to be that distance from the SSL where travel further north (south) would encounter no greater than natural maximum IPC levels for several kilometers. Once the enhanced IPC zone was defined for each pass, attention was given to determining the entire seeded zone and the neighboring nonseeded cloud re- gions that would serve as control zones. Each side of the seeded zone was considered to be bounded by the enhanced IPC zone or by the estimated AgI plume edge position nearest in time for that side, whichever resulted in greater crosswind extent. Most commonly, the enhanced IPC zone boundaries determined both edges of the seeded zone, probably due to the errors in estimation of the AgI plume edge position discussed in Part I. However, on a few passes, AgI was detected up to a few kilometers further crosswind, which may be because the AgI did not encounter SL W in these limited regions. Control zones of 2.5 km width were finally desig- nated both north and south of the seeded zone using 1.0 km "buffer zones" between the seeded zone and each control zone. The purpose ofthe buffer zones was to minimize the effects of any underestimates of AgI plume width and the "tails" of the seeding-caused ice particle plume that might have IPC values just below the background selected. The above procedures might seem unduly compli- cated, since in practice, defining the seeded zone was usually straightforward, because of abrupt increases in IPC and because the enhanced ice particle region was wider than the estimated AgI plume width. However, on a limited number of passes the seeded zone was not obvious and an objective means of handling these "problem cases" was considered desirable. The seeded zones were subdivided into thirds for more detailed examination. If, for example, the IPC had a Gaussian distribution, the center of the seeded zone would exhibit the highest concentration. If ice particles settled to the sampling level in the presence of vertical wind directional shear, the seeded zone could have a skewed distribution, with the highest IPC on the side overlain by the falling particles. North-south gradients in SL W could also cause variations across the seeded zones. -\ . I ~ 4. Results of six in-cloud sampling missions January 1985 was colder and much drier than usual; the average monthly temperature at the Bozeman Air- port was 2.70C below the normal. The 1 Feb 1985 snowpack water equivalent at the highest altitude Bridger Range snow course was only 63% of the long- term average. The colder periods of the month were usually dry.