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Cloud simulation laboratory observations indicate a typical generator effectiveness of 1013 crystals per gram of AgI at -10 oC. The DRI remote-controlled AgI generators to be used release 20 g h-1. With dispersion and wind speeds typically observed in 1991, it is anticipated that IPC's caused by seeding will be no more than a few per liter at temperatures warmer than -10 oC, a typicai plume top temperature. If the response of orographic clouds to AgI is similar to laboratory clouds, only the colder SLW clouds may have a significant response to AgI seeding. But the point of this objective is to' document the response of "real" clouds to AgI. Recent publications suggest that "forced condensation.:freezing" may result when AgI is released in-cloud or near cloud base. This is because high supersaturatiqns can result near generators which burn acetone and propane, producing large amounts of water vapor as a by-product. Forced condensation-freezing should result in significantly higher IPes than predicted by cloud simulation laboratory data. Van observations of seeding-caused crystals should be representative of temperatures between the high- altitude seeding sites and the upwind highway. Differences between the seeding site elevations and those along the upwind highway range from 200 to 600 m. Settling of seeding-caused crystals from above should be limited because the horizontal distances between the seeding sites and the upwind highway range between 4.0 to .65 lan, depending upon wind direction. Unless nucleation occurs very near the AgI generators where supercooling is usually slight, ice crystal growth times and settling distances will be restricted. The small seeded crystals will grow along trajectories that are essentially parallel to the mountain slopes in the forced uplift zone. Because of the relatively warm temperatures along the upwind highway during most winter storms, AgI-caused IPC's are expected to be undetectable by the van during the warmer storm phases. Propane-caused IPC's should be quite detectable unless natural crystal concentrations are high. Again, it is important to test these expectations with field observations. It is planned to release AgI or propane from the high-altitude sites, and make van observations along the upwind highway, during periods when the aircraft is not available (e.g., after dark). Some non-aircraft experiments will be conducted with the van parked near the RRS with seeding conducted at the AHS. Seeding plumes from the AHS, and any resulting ice crystals, are expected to be funneled up Birch Creek Canyon toward the RRS. These types of experiments can be conducted with limited personnel. They should add important information concerning the ability of each seeding agent to create significant IPC's along the Plateau's windward edge. 4.5 Spatial and Temporal Distributions of SLW Another goal of the 1994 field program will be to further understanding of SLW tempt>ral and spatial distributions and associated production and depletion processes over the Plateau. The fixed microwave radiometer will be operated at the RRS site day and night throughout each storm sequence to provide time histories of vertically-integrated water vapor and liquid water. These temporal variations will be related both to production and depletion mechanisms. In the absence of convection, SLW production is primarily due to the vertical component of the wind velocity near the barrier slope and moisture in the atmosphere (temperature/dewpoint and radiometer vapor observations plus rawinsondes during aircraft missions). Production is enhanced when convective instability is released. The presence of convective cells can be apparent in radar observations of cloud-top structure and in radiometer SL W histories which show rapid variations. Depletion of SL W is due to conversion to ice as indicated by high radar echo tops and reflectivities and by surface precipitation. These local production/depletion processes will be related to the larger scale setting of storm stage as determined from synoptic weather maps, satellite photos and the temporal distributions of project measurements. 13