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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />4.6.5 Transport of seeding-created ice particles toward the target <br /> <br />During daytime experiments, ice particles created by seeding will be monitored by aircraft <br />2D-C (two-dimensional cloud) and 2D-P (precipitation) imaging probes (Knoll enberg, 1976). <br />The computer software of Holroyd (1987) will calculate IPC (ice particle concentration), size <br />spectra, and general shapes. These aircraft measurements will "follow" seeding-caused ice <br />particle plumes, provided the "signal" is well above the natural background "noise." The most <br />obvious signal-in past experiments has been a large increase in the IPC, well above the <br />concentration of natural ice crystals. But because of the large variability in natural IPC, it is <br />prudent to "tag" the seeded zone by an additional means. For example, convincing evidence of <br />seeding-caused ice particles might be a large IPC increase in the same cloud region containing <br />tracer gas co-released with the seeding agent. Conversely, a large IPC increase well separated <br />from the tracer gas would be interpreted as a natural phenomena. <br /> <br />An airborne acoustical ice nucleus counter and the method of Super et al. (1988) will be used to <br />define approximate AgI plume positions. More accurate positioning of the instantaneous plume <br />position will be obtained from measurements of SF 6 tracer gas co-released with the AgI or <br />propane, and from radar chaff tracking. The specific boundaries of the seeded zone will be <br />based on 2D-C imaging probe measurements showing well above background IPC's co-located <br />with the AgI plume as illustrated by Super and Heimbach (1988). One of their figures is <br />reproduced in section 4.7 as figure 5. <br /> <br />Previous experience over a number of mountain ranges indicates that aircraft detection of <br />either the AgI seeding plumes or the tracer gas will not be difficult in most experiments. <br />Associated changes in IPC and size spectra should be evident when seeding markedly enhances <br />the IPC. Changes in crystal type will be more elusive because definite habit identiJrrcation with <br />2D probe data is difficult. However, larger graupel and aggregates will be evident. Precipitation <br />rates will be estimated from the 2D probe measurements, and these estimates from the seeded <br />zone and crosswind control zones should strongly suggest whether seeding increased snowfall <br />rates at flight levels. Evidence of seeding is expected to be discernible at aircraft levels from <br />most direct detection experiments. <br /> <br />It would be desirable to sample seeded crystals over the windward barrier slopes where SLW <br />production is expected to be concentrated because of strongest forced uplift of relatively warm, <br />moist air. Much of the artificial ice nucleation and crystal growth is expected to occur in this <br />region. However, aircraft sampling cannot be done low enough because of higher terrain on top <br />the barriers. Operation of 2D-C probes, acoustical ice nucleus counters, and tracer gas detectors <br />is practical on the surlace. These probes will be transported by four-wheel drive vehicle to <br />provide observations through plumes along the "upwind highway," between thE: junction of <br />highways 31 and 264 and Huntington Reservoir on the Wasatch Plateau (fig. 3). <br /> <br />No similar highway exists over the target area portion of the Grand Mesa, but ~m oversnow <br />vehicle will be instrumented to sample seeding plumes and tracer gas. Oversnow vehicles large <br />enough to contain the needed instruments are slow, so the number of sampling passes made <br />during each experiment will be limited. A fixed oversnow vehicle trail will be established, as <br />close to the crestline of the Mesa as practical. <br /> <br />Operating fixed sites over the windward slopes of either barrier is not considered a practical <br />option. Such sites require electrical power and equipment operators, and can bE~ difficult to <br /> <br />27 <br />