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WINTER OROGRAPHIC STORM plume position changed with time. Its center was 6.6 km south of the DOT at 1319 but almost over the DOT by 1512. Aircraft Sampling over the Wasatch Plateau: All aircraft sampling was at an altitude of 3750 m or 900 m above the average upwind highway elevation. Turrets were seen rising about 500 m above the aircraft and no clouds existed at higher altitudes. Aircraft-mllasured winds ranged between 275-310 degrees with speeds between 11-19 m S-I. Pass average liquid water contents were between 0.2-0.3 g m-3. With a single exception, both AgI and SF6 were found on each pass edge. Table 2 summarizes the measurements of the N-S aircraft passes. Plume width is the N-S extent of each plume. The 1334 and 1342 passes were over the east edge of the Plateau, all others were over the west edge. Time (MST) 1302 1316 1334 1342 1357 1411 1418 Table 2. Summary of aircraft measurements of AgI and SF6. mean SF6 JJ?mL 20 60 45 20 45 10 o max. SF6 J1m!L 60 185 90 55 125 45 o SF6 plume width (km) 7.7 5.5 4.1 7.2 4.8 2.0 0.0 Plume dist. south of DOT 10.7 6.6 11.4 13.9 6.1 6.1 N.A. Total IN ~ 61 40 11 32 24 11 15 AvL z..8 The SF6 plumes were centered from about 6 to 13 km south of the DOT on all passes. Widths over the windward Plateau edge were between 4.8 and 7.7 km except for the last 2 passes. Widths over the lee edge were similar suggesting little additional cross-wind dispersion during passage over the Plateau. It is possible that the plume diminished during the final passes because convective intensity was decreasing as the ~ storrn ended. 1,1 l(\O-IG ~ t-l An effective IN concentration is estimated assuming an operational seeding .;; generator ~ad been run ~t,~e SF6_ release site. .Table 2 shows a typical SF6 value. of ~ l!A, 40 ppt which represents 1.8 X 10 10 g 1"1 of arr. The eqUIvalent IN concentratIon ~ ~ at -120C and 640 mb, the temperature and pressure at the aircraft sampling level, \.-r "" would be ab04tf'!N r1. This estimate assumes equal dilutions of SF6 and All plumes ~ ~ and ignores i~cloud scavenging losses of the AgI. A concentration of! IN 1"1 is "'- considered marginal for effective cloud seeding.' ij Temperatures at turret tops are estimated as -160C where IN concentrations, estimated from the SF6 data, would be about 30 IN r1. This should be high enough for effective cloud seeding. However, Table 2 shows the total IN counts per pass were very limited, especially when one considers that 8 AgI generators were releasing AgI. Possibly the valley floor releases had less vertical transport than the canyon mouth release of SF6. A fraction of the sampled AgI should have nucleated ice, but most of the total AgI population should have been sampled by the acoustical counter, operated goC colder than the cloud outside the aircraft. The main AgI "sink" was probably scavenging by cloud droplets and ice crystals. Scavenging could significantly reduce 557 ~ . ~ "- \.\ l ~ "--.J --r- t +- ,. Q' /I'LMl .Iff.. Ii; lID S S 8 S- 17' ~ q) 1 ~ ~ .,., ,.. ... I ~ - x Jlo I't\dII ~ TI