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Show of Salt Lake City (Figure 128). This trough sharpened during the_ day and from the Mt. Harris 18, 000 ft. MSL wind (plotted in hatched rectangle on the map analysis) in Figure 12 9, was apparently very near the experimental area at 1700 MST. General 700 mb flow veered slightly with time through the day as seen in Figures 130 and 131. Significant departures from geostrophic direction are 1 \ evident in the observed wind at 0500 MST at Grand Junction and Denver and :I.. at Mt. Harris at 0749 MST. The 700 mb flow of moist air across the experimental area can be seen by examining the temperature and dewpoint values at upstream points. There were no frontal boundaries near the experimental area during the day. I 1..- Thus, the large scale pattern was very favorable for orographic snow over I the Park Range, and was not complicated by the passage of any surface frontal boundaries. The slight sharpening of an approaching minor 500 mb trough, probably caused a general weak upward vertical motion field over the area. 1 • 4. 5. 4. 1. 2 I Stability and Moisture Four Mt. Harris soundings were taken during the day. These are shown in 1/ Figures 132-135. All of these soundings indicated slightly unstable strati- : 1 fication through the cloud layer, which would support some convective precipitation component. The most convective sounding was at 07 49 MST (Figure 132) with cumulus tops likely to about 20,000 fL MSL (-35°C). Deep moisture (-35 °C tops) was still evident at 1012 MST (Figure 133) but the air had stabilized slightly. (A wind shift had also occurred, which can be seen in Figure 142.) At 1358 MST (Figure 134) the base of an apparent subsidence layer appeared near 600 mb (14, 000 ft. MSL). This may have represented the first nodal surface at the top of the orographic lifting process. Indicated cloud top temperature was near -22°C. By 1549 MST (Figure 135), moisture again appeared quite deep with a definite stable layer and drying indicated above about 19, 000 ft. MSL (480 mb and -31 °C). The aircraft, seeding at 1600 MST, reported tops around 18, 700 ft. MSL. There was definitely no inversion or stable layer between the Mt. Harris Ag! / release site and _the target area elevatio_n during the ground- based seeding. 1 Thus, the material should have freely diffused vertically. From the point of view of moisture depth and stability, the heaviest natural precipitation should have occurred with the 0749 MST sounding when moisture depth was great and convective instability the strongest. The least precipi· tation should have been centered around the 1358 MST sounding. 212 I 1 |
