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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 />are present about 15 percent of the time in March and April and often are asso- <br />ciated with rime icing above 4500 ft. May provides relatively few stratiform <br />clouds. <br /> <br />As for treatment, about 60-70 percent of the cases, amounting to about 70 or 80 <br />hours per month in March and April appear treatable from ground generators. <br />Given predominant southwest to westerly flow during these events andwindspeeds <br />usually less than 15 m s-l (30 kt), suitable generator locations could be found <br />upwind of target areas Nos. 1, 2, and 4 as defined by TVA members. <br /> <br />3.6 Opportunities in Convective Clouds <br /> <br />A literature survey suggests that some convective clouds in the TVA region would <br />respond to glaciogenic seeding by increases in cloud height and, presumably, in <br />rainfall. An increase in cloud height is one type of dynamic response to cloud <br />seeding. Orville (1986) recently reviewed the subject of seeding of convective <br />clouds for dynamic effects. He discussed the pioneering studies of Simpson <br />(1967), who demonstrated unequivocally that treating growing cumulus towers with <br />large dosages of AgI sometimes leads to increased vertical growth. <br /> <br />The techniques used in single cloud experiments have not led to success when <br />applied over large areas for precipitation increases. The two most relevant <br />area experiments which might apply to the TVA region were Project Whitetop in <br />Missouri and FACE (Florida Area Cumulus Experiment) in southern Florida. Since <br />Project Whitetop was closer to and simulates the continental environment of the <br />eastern TVA region better than does FACE, results from Whitetop would appear <br />more applicable, although the seeding method was different from that used in <br />most dynamic seeding trials. <br /> <br />Project Whitetop was a randomiied experiment utilizing AgI generators on <br />aircraft to treat convective clouds for precipitation increase. Braham (1964) <br />reviewed the precipitation mechanisms observed in the types of clouds treated <br />and found coalescence to be important. He reported that first radar echoes <br />developed below the 0 oc level in clouds having summit temperatures no colder <br />than -5 oC. The studies by Braham seem to be confirmed by subsequent <br />microphysical studies performed in central Illinois (Ackerman et al., 1979; <br />Czys, 1987). However, results for the eastern TVA region show almost no <br />precipitation on days in which convection did not reach -32 oC (0.02 inch <br />average over basin above Chattanooga). Therefore, radar echoes may develop <br />but precipitation to the ground in the TVA region is minor for a purely <br />warm rain process. <br /> <br />Statistical results for Project Whitetop originally reported by Flueck (1971) <br />indicated a decrease in rainfall and radar echo co~erage, with maximum decreases <br />occurring 5 hours after seeding. Statistical support for the apparent rain <br />decrease was quite strong and for the radar echo coverage moderate to strong. <br />Subsequent stratification of the results, however, indicated increase of 68 to <br />100 percent in rainfall on days with maximum radar echo tops between -10 oC <br />(18,000 ft) and -40 oC (33,000 ft), and that the overall rainfall decreases were <br /> <br />- <br /> <br />xix <br />