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<br />Measurements in actual orographic clouds are needed to test the validity of cloud chamber results. While it <br />seems unlikely that AgI will produce significant IPCs when Plateau top temperatures are warmer than -4 oC, <br />some AgI seeding will be attempted at warmer temperatures to test this expectation. However, in most <br />warm temperature cases, propane seeding will be done as long as the seeding sites are colder than 0 oC and <br />in cloud, or at least near cloud base so that ice saturation is likely at the release sites (i.e., ice crystals <br />formed by the cooling associated with liquid propane expansion can be expected to survive transport into <br />cloud). <br /> <br />The SF 6 tracer gas will always be co-released with propane or AgI from the high altitude sites. This will <br />allow precise position monitoring of the seeding plumes and resulting ice crystal plumes using rapid response <br />gas detectors on the aircraft and van. Post-experiment analysis of syringe samples from the RRS and TAR <br />will indicate whether or not the SF 6 was present at these sites. <br /> <br />While the rapid response gas detectors can determine SF6 plume boundaries to within 1-2 s, the NCAR IN <br />counter response time can have uncertainties of up to about 20 s. Resulting plume edge errors with the fast- <br />moving aircraft platform are only 150 m for the SF6 but up to 2 km with the AgI. However, AgI plume edge <br />errors from the van's NCAR IN counter should be only about 250 m because of the van's relatively slow <br />speed. Consequently, nighttime van sampling of AgI plumes will provide accurate plume positioning. <br /> <br />Both AgI and propane direct detection experiments are expected to last about 4 h. Normally, seeding and <br />SF6 release will commence when the aircraft crew is alerted to fly a mission, about 1 h before takeoff. This <br />should establish plumes across the Plateau top by the time the aircraft begins sampling. Seeding will <br />continue until the aircraft leaves the Plateau. . However, SF6 releases will end about 0.5 h earlier to conserve <br />gas. During experiments where a microphysical seeding signature is obvious to the aircraft scientist, seeding <br />maybe halted earlier so that microphysical characteristics can be monitored while plumes diminish. Such <br />temporal changes can provide convincing evidence of seeding effectiveness. <br /> <br />The 1994 direct detection experiments will have high-altitude seeding sites about 14 and 17 km from the <br />target (TAR), respectively. Aircraft sampling will be made upwind of, and over the target, with ice particle, <br />AgI and SF6 detectors sampling within the plumes and crosswind of them. Crosswind passes through the <br />seeded cloud will be made with the instrumented van to document plume position on the Plateau top's <br />upwind edge, and any associated microphysical effects as measured with a vane-mounted 2D-C particle <br />imaging probe. Fixed mountain observatories on the windward (RRS) and lee (TAR) edges of the Plateau <br />top will monitor for AgI and SF6. In addition, a vane-mounted 2D-C probe on a tower, and photography <br />will be used to monitor ice particles at the TAR site. , <br /> <br />Microwave radiometer measurements of SLW will be made by driving back and forth along the upwind <br />highway, under natural and seeded cloud. A Ka-band radar, positioned to be able to scan downwind toward <br />TAR, will observe cloud thickness and structure, and should detect seeding-related changes in reflectivity <br />factor when natural snowfall is nil. <br /> <br />Three precipitation gauges with a resolution of O.13mm water equivalent and time resolution of 5 min will <br />be placed along the target ridgeline crosswind to the prevailing southwest flow (fig. 1). For typical wind <br />speeds, ice particles will be transported above the Plateau top for about 15 minutes before reaching the <br />gauge line. Several minutes additional growth time will be provided when nucleation occurs near the seeding <br />sites. <br /> <br />Two similar gauges will be operated upwind of the target ridge, along the typical transport direction, and one <br />will be maintained downwind and downslope of the target ridge. These gauges may be especially useful in <br />documenting the movement of both natural and seeded snowfall pulses. <br /> <br />11 <br />