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. . . . A second case study from 26 January showed the capability of the mobile radiometer to characterize the SL W conditions within ground seeding plumes, when used in combination with aerosol detection instruments. These mobile systems are advantageous in ground-based seeding experiments, where plume dispersion and SL W development are often highly dependent on local topography, and difficult to monitor solely with aircraft. The case in this study showed that the cloud seeding plume was frequently coincident with a SL W maximum generated by a local ridge. However, at 5 km from the seeding generator there was no consistent observation of SL W depletion within the seeding plumes, possibly because the temperature was relatively warm for the AgI compound in use, and without rapid nucleation near the generator, the ice concentration from contact nucleation alone would have been relatively low. The third study described a case where seeding plume locations were well documented at the surface and aloft. The cloud was confined to a shallow layer within about 600 m of the mountain barrier, and the seeded plume was shown to have penetrated vertically through this region. The temperatures were optimum for ice formation by AgI, and the relatively slow wind speeds allowed ample growth time for ice initiated near the upwind edge of the Wasatch Plateau. The case was marked by the appearance of a _plume-shaped radar echo whose position and period of existence matched that ofthe seeding aerosol plume. Radar and precipitation intensities were enhanced within the aerosol/radar plume, in contract to regions both north and south ofthe plume. One precipitation measurement within the plum, about 12 km downwind of the generator, exceeded 3 mm over a two hour period. This compared to a measurement of 0.6 mm outside the plume. Ice crystal microphysics measurements are expected to help determine whether these results represent a well defined seeding signatures. 8.23. Super, A. B., 1996: Two case studies showing physical effects of both AgI and liquid propane seeding on Utah's Wasatch Plateau. 13th Conference on Planned and Inadvertent Weather Modification, Atlanta, GA, American Meteorological Society, 156-163. INTRODUCTION Cloud seeding experiments conducted during the 1994-95 winter on the Wasatch Plateau (Plateau) of Utah investigated the ability of silver iodide (AgI) to create a significant ice particle concentration within orographic (mountain-induced) cloud. AgI ice nucleation has been investigated in laboratories, but orographic cloud characteristics raise questions about laboratory results. Laboratory clouds have higher droplet concentrations and liquid water contents, are more homogeneous, and have less turbulence. The experiments also documented microphysical effects of high altitude liquid propane seeding. Published results of propane effectiveness (Hicks and Vali 1973; Kumai 1982) show enough variability to justify further field testing, especially at temperatures above -5 oC where AgI is expected to be ineffective. . AgI or liquid propane was released in I-h "pulses" from the High Altitude Site (HAS) at 2540 m elevation on the west-facing Plateau slope. The site is exposed to winds from south through west as elevations decrease rapidly with distance. Microphysical effects of propane seeding were obserVed at the Target on the Plateau top's west edge at 2855 m, 4.2 km northeast of the HAS. Instruments verified that seeded cloudy air passed by the Target and monitored ice particle characteristics before, during, and after each passage of seeded air ( each pulse). Instruments at both sites provided measurement of wind, air temperature, and presence of supercooled liquid water. , . 75