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<br />Figure 5. Thirty second "snapshots" of2D-C ice crystal irnages during times of peak ice particle concentration for 3 adjoining
<br />EUs used in the case study of21 December 2003. The strip widths are 800 rnicrons, the field of the 2D-C probe. Measurements
<br />along the wind are unlimited. The upper and lower panels show typical 2D images ofnaiural snowfall while the central panel
<br />shows seeded ice crystals. Mean ice particle concentrations for the 3 periods from top to bottom are 67, 148 and 28 per liter.
<br />
<br />While such rates are light snowfall, about half of all
<br />hours with detectable natural snowfall fall at the rate
<br />of 0.0 1 0 inch per hour or less in the experimental
<br />area. Frequent hours with light snowfall are common
<br />at higher elevations in the Intermountain West. The
<br />seasonal contribution of light snowfall is important
<br />because of its high frequency of occurrence.
<br />Increasing the snowfall rate by 0.010 inch per hour
<br />for 200 hours would result in a seasonal increase of
<br />10% for a normal November through March winter,
<br />based on long-term snow pillow observations in the
<br />experimental area. Examination of past icing rate
<br />sensor and other SL W data from the Wasatch Plateau
<br />revealed that approximately 200 hours with available
<br />SLW could be expected during a typical November
<br />through March winter. Consequently, there is both
<br />statistical and physical support for the expectation
<br />
<br />that propane seeding might produce an all winter
<br />SWE increase in the vicinity of 10% for the limited
<br />target area examIned.
<br />
<br />Consideration of Ice Particle Concentrations
<br />
<br />Ice particle concentrations (IPCs) of smaller ice
<br />crystals were monitored at the TAR with a 2D-C
<br />particle imaging probe. Only 21 seeded and 22
<br />nonseeded EUs had available 2D-C measurements
<br />and HAS wind directions suitable for targeting
<br />because of electrical generator and other problems.
<br />The IPCs were compared for these seeded and
<br />nonseeded sample populations which may not be
<br />totally representative of all 69 EUs meeting the HAS
<br />wind direction criterion. The comparison ofthe
<br />seeded and nonseeded samples revealed about a 1/3
<br />
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