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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 />