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Last modified
7/28/2009 2:35:12 PM
Creation date
3/11/2008 11:22:28 AM
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Weather Modification
Title
Applications of the Clark Model to Winter Storms Over the Wasatch Plateau
Prepared For
Utah Department of Natural Resources Division of Water Resources
Prepared By
James A. Heimbach, Jr.
Date
7/1/1993
State
UT
Weather Modification - Doc Type
Report
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<br />1. "Edge" = first second of seven seconds having ~ 3 IN counts. <br />2. "Edge" = first second of three seconds with ~ 3 IN counts. <br />3. "Edge" = first of three seconds which all had IN counts. <br />A single second is the primary time unit because the time resolution of the DAS was 1 s. The first gave <br />the most consistent results and was the most appropriate for the small concentrations typically detected by the <br />aircraft. This is also the method first described by Super et ai. (1988). Figure 4 shows a plot combining the surface <br />arid airborne edge calibrations points. Some of the points of the original data sets have been eliminated. The lowest <br />lag time, 13 s, though extraordinary, could not justifiably be eliminated. The power curve fit is, <br /> <br />lag=.96. 7 (~= counts) 0.228_0.5 (seconds) . <br /> <br />(4) <br /> <br />There is much scatter in Fig. 4 for r. counts less than 100, which unfortunately represents most of the passes <br />over the Plateau. The range for lag times for small IN sums is approximately :I: 20 s, which corresponds to <br />approximately 1.8 km flight distance. Nevertheless, a coarse estimate of plume position can be made by this method, <br />which is of use for the model testing described later. <br />Typical values for lag times to plume edge are 44 s for r. counts = 30, and 33 s for r. counts = 1000. This <br />corresponds to flight distances of approximately 4.0 and 3.0 km respectively. <br />For r. counts < 15, the three counts in 7 s criteria could usually not be met. In this case, a coarse estimate <br />of the mean lag for all IN detected in a given pass was used to approximate the plume position. The time to mean <br />was not found to be a function of count summation~ however, as r. counts decreased, the variance (or scatter) of lag <br />time to mean increased. For this reason, only the ground test data having higher summations were used to derive <br />this parameter, with values ranging from 81.5 to 111.5 s. The average lag to mean was 89 s. <br />5. Modeling the 2 March 1991 Case <br />a. Weather <br />On the moming of 2 March a NW-SE stationary front was positioned NE of Utah and a weak cold front <br />trailed through Colorado and New Mexico from a small closed low to the NE of Utah. The weak front had crossed <br />Utah before daylight. The air mass over Utah was maritime polar. The sounding used to initialize the model is <br />shown in Fig. 5. The atmosphere was conditionally unstable, which produced embedded convection over Utah in <br />the post-frontal environment. Most of the precipitation occurred before noon with rates tapering off as the short <br />wave and associated surface features moved to the east and weakened. At the Plateau-top gauges downwind of the <br />DOT site, average hourly amounts in the AM ranged from 0.2 to 2.3 mm hr"1 and from 1200-1600 (all times MST), <br />less than 0.5 rom hr "I. Snowfall was negligible on the west slope of the Plateau in the afternoon. The DOT wind <br />speed was fairly steady during the experimental period at about 5 m S"l. Wind direction gradually backed from 255 <br /> <br />-10- <br /> <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 />
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