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Last modified
7/28/2009 2:33:58 PM
Creation date
3/5/2008 10:53:13 AM
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Weather Modification
Title
Summary of the NOAA/Utah Atmospheric Modification Program: 1990-1996
Date
9/1/1998
State
UT
Weather Modification - Doc Type
Report
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<br />2.2 Field Observations of SL W <br /> <br />Huggins (1995) presented seasonal average portrayals of the cross-barrier SL W distribution over the <br />Plateau. That work and other articles herein demonstrate the expected formation of SL W by forced <br />orographic uplift over the west (windward) slope ofthe Plateau. Embedded convection, usually weak, <br />enhances SL W production during some storm phases, and maximum SL W amounts are found above the <br />windward slope. The gravity wave mechanism can also enhance SLW production as discussed in <br />section 5. Snowfall often begins to reduce SL W amounts by the time cloudy air reaches the Plateau top's <br />west edge, and the reduction continues as air flows eastward across the top (Huggins 1995). Both <br />snowfall and subsiding airflow during passage across the Plateau lead to substantial SL W reduction. The <br />SL W is largely depleted by the Plateau top's east (downwind) edge, approximately 10 km from the west <br />edge. <br /> <br />Sassen and Zhao (1993) used lidar, microwave radiometer, and other observations to investigate SL W <br />.over the Tushar Mountain.s, about 175 km southwest of the Plateau experimental area. They demonstrated <br />that SLW was usually found at barrier levels in the 0 to ..:10 oC range. Sassen and Zhao (1993) concluded <br />that SL W cloud thickness was often only 500 to 800 m, with some SL W clouds nearly 1,000 m thick. <br />That finding, along with layer temperature observations, led to their important conclusion that there was <br />only a limited "window" for AgI seeding success. This "window" involved the upper portions of <br />relatively warm SL W clouds with base temperatures warmer than -7 oc. Clouds with tops colder than <br />-12 oC appeared to efficientiy convert SLW to ice particles. Silver iodide is not effective for ice particle <br />formation for temperatures warmer than -6 oC even when high AgI concentrations are present. With the <br />limited in-cloud AgI concentrations resulting from operational seeding, there is often little opportunity for <br />the seeding to be effective. <br /> <br />Super (1994) discussed the implications of almost continuous observations during the early 1991 two <br />month field program. Measurements included SL W by microwave radiometer~ gage precipitation, AgI by <br />acoustical ice nucleus counter, and supporting observations. Super (1994) concluded that the SLW flux <br />over the Plateau top's windward edge well exceeded the average snowfall over the Plateau top, suggesting <br />that seeding might have the potential to convert some of the "excess" flux to additional snowfall. <br />Moreover, many of the wetter hours had no detectable snowfall. These findings indicated significant <br />seeding potential ifa large fraction of the excess SLW flux could be converted to snowfall. <br /> <br />To put microwave radiometer vertically-integrated SLW amounts into perspective, Super (1994) <br />compared them with observed hourly snow water equivalents. Accumulations for all 179 h with at least <br />0.01 inch observed by one or more of the three Plateau top gages provided the mediap-hourly <br />accumulation value of 0.015 inch. Half the total snowfaUfell during the 83 percent of the precipitating <br />hours with hourly accumulations of 0.05 inch or less. These data illustrate that mountain snowfall usually <br />occurs at light rates over numerous hours as has been shown at a number of Rocky Mountain locations. <br />Super (1994) calculated the equivalent snowfall rate if all SL W flux was converted to snowfall which fell <br />uniformly across the 10 km wide Plateau top with a typical cross-barrier wind speed of 10m s.\. He <br />showe~ that a vertically-integrated SL W amount in excess of 0.1 0 mm was needed to produce the median <br />hourly snowfall accumulation of 0.015. Sixty-five percent of all hours with detectable SL W over the <br />Plateau top's west edge were less than 0.10 mm. A significant portion of the SLW will be naturally <br />converted to snow. Therefore, it follows that even when AgI seeding is successful; typical hourly <br />accumulations would be limited. Moreover, hours with SL W amOl.mts less than about 0.05 mm have very <br />limited potential for snowfall production by seeding. <br /> <br />Super (1995a) presented the results from all six early 1991 experiments during which valley-released AgI <br />was detected at lowest aircraft sampling levels over the Plateau. Observations of SL W were presented <br /> <br />8 <br /> <br />I <br />I <br />! <br />I <br />i <br />l <br />I <br />~ <br /> <br /> <br />I <br /> <br />l <br /> <br />i <br />~ <br />! <br />i <br /> <br />~ <br /> <br />1 -- <br /> <br />~ <br />I <br /> <br />1 <br />I <br />5 <br />~ <br />U <br />~ <br />I <br />~ <br />If. <br /> <br />i <br /> <br />i <br />I <br /> <br />i <br />i <br />! <br />I <br />! <br />r~ <br />~ <br />l <br />r. <br />., <br />g <br />~ <br />j <br />t <br />I <br />i <br /> <br /> <br />i <br />
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