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<br />SEPTEMBER 1996 <br /> <br /> 0.12 <br />IL- <br />(J,c 0.01 - <br />ILE <br />OE 0.04 - <br />N- <br /> 0.00 . <br /> -2' <br />aU -4- <br />IL T <br />:Ii ~ <br />W <br />I- <br /> -8- Td <br /> .10. <br /> 0.4 - <br />~ 0.3. <br />S <br />(J 0.2 . <br />!i <br /> 0.1 . <br /> 0.0 . <br /> 2700 . <br />! 2500 <br />~ <br />oC <br /> 2300 ' <br /> 2100 . <br /> 80 ' <br />i=' 60. <br />IL <br />!!:. <br />CD 40. <br />... <br />In <br /> 20 <br /> 0 <br /> 18.40 <br /> <br />REYNOLDS <br /> <br />1451 <br /> <br />->>N- ~~JW <br /> <br /> <br />.~~ <br /> <br /> <br />18.46 <br /> <br />~ <br />10 I <br />~ <br />~ <br />8 ~ <br />o <br />N <br /> <br />11.80 <br />TIME (UTe) <br /> <br />18.66 <br /> <br />18.60 <br /> <br />FIG. 21. Same as Fig. 18 but for pass 15 along the valley. <br /> <br />17 FEB 93 P15 <br /> <br />the most distinct seeding signature observed (see Fig. <br />18). It shows a well-defined SF6 plume (vertical lines <br />denote the true position of the SF6 plume) centered on <br />a region of enhanced, but rather small, ice crystals <br />(dashed line in the 20 panel are crystals smaller than <br />250 /.lm). A distinct habit change from hexagonal to <br />needle crystals occurred at this location. The liquid wa- <br />ter content measured at this location is also the highest <br />observed on this flight. This is contradictory to the gla- <br />ciogenic seeding hypothesis in that seeding should re- <br />duce cloud water through the growth of the enhanced <br />ice crystals. An explanation is that seeding rates with <br />propane are much higher than those normally used for <br />silver iodide, 6000 versus 30 g h-1, respectively. Be- <br />cause of homogenous nucleation, propane produces <br />very rapid condensation and subsequent freezing ofbil- <br />lions of ice crystals, releasing latent heat, which can <br /> <br />increase the clouds buoyancy by several tenths of a <br />degree centigrade. This additional buoyancy is real- <br />ized as increased updraft in the seeded parcel of air, <br />leading to the production of additional liquid water. <br />These results are consistent with numerical model re- <br />sults simulating seeding of orographic clouds with <br />glacio genic seeding material (Orville et al. 1984). <br />Similar, though less impressive, signatures were seen <br />on three other passes (Figs. 19-21). It may be that <br />this enhanced updraft has brought seeded crystals up <br />to the aircraft sampling level, in contrast to site 7. <br />Ridge pass 4 on 9 February (see Table 5) indicated <br />results very similar to the signatures seen on 17 Feb- <br />ruary (Fig. 22). Although the SF6 plume is weak, the <br />location of this seeding signature is within the pro- <br />jected plume, as predicted by the targeting model <br />(Fig. 23). <br />