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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 />t <br />I <br /> <br />degrees true prior to 1200, to 245 degrees at 1430, and thereafter was between 210-240 degrees. The wind speed <br />ranged from 10-15 m S-1 at the head of the north fork of Birch Creek Canyon, consistently from the west Winds <br />on the valley floor were between 205-240 degrees near 5 m S-1 until after 1500 when they began to decrease. Winds <br />in the lower portion of Fairview Canyon were consistently up-canyon with speeds of 14 m S-I. <br />Until 1540 the vertically-integrated SLW varied markedly, typical of embedded convection The peak hourly <br />average was 0.3 rom in the hour ending 0700. Between 0200 and 1600, the hourly LW averages were at least 0.12. <br />The SLW after 1600 was negligible as skies cleared with the end of the storm. <br />b. Simulated Flow Patterns for 2 March 1991 <br />Most of the discussion for flow of the 2 March case will center on the 2.5 hr point of the model integration. <br />This is near the center of the maXimum run made and is far enough into the modeling to insure the run had properly <br />stabilized. Figure 6 shows the surface W-E wind component (u) for this time. In general, there is a westerly <br />component, but some valley areas show the reverse. This variability is only for a very shallow surface layer. This <br />trend is also shown for the S-N component (v) of wind (not shown). From this directional shear, one would <br />anticipate some releases of tracer material to show a rapid lateral spread. <br />The vertical velocity above 5 km shows poor organization (see Fig. 6 for W-E vertical cross section). This <br />is due to the lack of a stable layer which is required for the formation of gravity waves. There is some terrain-forced <br />vertical motion in the lower 2 km which gives positive w upwind of the crests and negative w downwind of the <br />crests. Figure 8 shows the surface patterns of w. The band of negative w on the lee of the San Pitch Mountains <br />makes seeding from that crest a poor option for this date. The entire length of the Wasatch Plateau shows positive <br />w at and west of the western flight track, but negative w east of the track, except for the vicinity of the DOT site. <br />This suggests good targeting potential for seeding material released from the windward foothills and canyons. Some <br />of the valley seeding sites lie in an area of weak negative w for this simulation <br />c. Comparing Airborne Plwne Tracing to Model Simulations: Birch Creek Release of2 March 1991. <br />On 2 March SF6 was released from the mouth of the Birch Creek Canyon for two periods, 1000 to 1110 <br />and 1230 to 1410, at the rate of 3.42X1<f gm lIT1. The latitude, longitude and elevation of the release site was 39" <br />35.7' and 1963 m, respectively. The SF6 plume encounters of the morning and afternoon flights are depicted in Figs. <br />9 and 10. Both flights were terminated prematurely due to icing, which restricted flow through the intake manifold <br />for the AM flight and possibly the PM flight. There was only one plume encounter in the AM, probably because <br />of the late start of the SF6 arid early termination of the flight. The peak SF6 concentrations from each pass ranged <br />from 46 to 202 ppt. If a Skyfire-type generator was dispensing 30 gm hr-1 at this release point, then the <br />concentration of AgI inferred by Eq. 3 would be 1.7XlO-4 to 7.0XlO-4 pgm m-3. The largest concentration was from <br />the single morning SF6 penetration. Estimates of maximum effective IN concentrations can be inferred using the <br />Skyfire natural draft calibration described by Super and Heimbach (1983, p 1993). The temperatures recOlded at <br />the times of peak SF6 encounters in each plume ranged from -12.6 to -10.0 oC. The estimated concentrations of IN <br />rarIged from 15 to 153 IN L-1. These concentrations are suitable for cloud seeding operations. <br /> <br />-13- <br /> <br />