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<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 /> <br />The study also showed the importance of synoptic/mesoscale triggers to precipi- <br />tation production. The satellite imagery showed cold fronts and well defined <br />comma features, which indicate strong vorticity centers aloft, to contribute 60 <br />percent to the precipitation. Upper l~vel cutoff lows and overrunning prefron- <br />tal periods contributed another 20 percent. The other 20 percent of the preci- <br />pitation could not be associated with any identifiable feature in the imagery. <br />Unfortunately, time did not allow detailed analysis of synoptic charts to better <br />define the daily synoptic/mesoscale controls. Such an analysis would help <br />substantiate the results inferred from the satellite imagery alone. <br /> <br />3.3 Radar Echo Characteristics <br /> <br />Hourly manually digitized radar data for March, April, and May from the Bristol <br />WSR-57 radar for the 6 years 1982-1987 were analyzed. The radar results are <br />consistent with the satellite studies in showing a transition from a generally <br />stratiform precipitation regime to a more convective regime from March to May. <br />The transition is accompanied by an increase in the number of hours of echo, <br />with almost a third of all hours in May having echo within range of the Bristol <br />radar. Intensity levels (precipitation rates) are significantly lower for the <br />stratiform cases than for the convective cases1, as one would expect. Echoes <br />occur on about half of all days between March and May. On days with radar <br />echoes, the echoes are present for almost 12 hours per day on average. Echo <br />motion is remarkably consistent (from 2500), probably due to the local <br />topography in the vicinity of the radar. <br /> <br />3.4 Rawinsonde Studies <br /> <br />Rawinsonde data from four sites close to the eastern TVA region were analyzed. <br />The analysis of rawinsonde data provided information on the occurrence of clouds <br />as defined by layers of water saturation, ice supersaturation, or ice satura- <br />tion. As artificially induced precipitation particles will only evolve in water <br />saturated or ice supersaturated cloud, only these events were examined. For the <br />3 months, March through May, the soundings showed that the clouds extend above <br />the freezing level for about 90 percent of the time that clouds are present. <br />This result would imply a contribution from the ice ~hase process for most pre- <br />cipitation events. Therefore, a purely warm-rain process can be eliminated as a <br />significant contributor to precipitation over the eastern TVA region. <br /> <br />Using the Huntington, West Virginia sounding as typifying the eastern TVA <br />region, it is observed that almost 50 percent of the soundings indicate some <br />water saturated or ice supersaturated cloud layers. These layers occur <br />generally below 6000 m AGL and at temperatures colder than -5 oC. Supercooled <br />water-saturated layers generally occur below 3000 m AGL at temperatures warmer <br />than -15 oC. When all temperatures are considered, about 50 percent of the <br />soundings show water saturated layers warmer than 0 oC and 15 percent show water <br />saturated layers warmer than 10 oC. These last cases would imply a mixed preci- <br />pitation process, with large drops forming by coalescence and then freezing to <br />continue growth as ice particles as they are carried upward in the clouds. <br /> <br />- <br /> <br />xvii <br />