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290 JOURNAL OF APPLIED METEOROLOGY VOLUME 29 \ I I and fallout of crystals in a sheared environment, re- sulting in effective diffusion rates of 1-2 m S-I (Hill 1980a; Stewart and Marwitz 1982; Super and Boe 1988). These rates can exceed the spreading due to turbulence (W eickman 1974). Rangno ( 1986) and Hill (1980b) provide generalized descriptions of these cloud types based on other field studies. b. Conceptualized treatment effects The conceptual model (Bureau of Reclamation 1985) suggests that under conditions appropriate for seeding there is an excess of condensed cloud water that passes over the crest. By introducing additional ice embryos a portion of the excess liquid may be con- verted to precipitation. The increase in ice crystal con- centration (ICC) should be managed so that precipi- tation is not diminished by overseeding. Seeding shallow orographic clouds via aircraft with either vertical curtains or horizontal lines of glaciogenic seeding material should result in the following sequence of events (Ludlam 1955; Hobbs 1975b). 1) 0-10 minutes-An initial region of high ICC de- velops that increases in width at a rate of 1-2 m S-I. For droppable seeding material this region extends 1 km below the seeding altitude. Accretional growth may begin approximately 5 minutes after nucleation. Ag- gregation of particles larger than 300-500 JLm may oc- cur when the liquid water is insufficient to support fur- ther growth by diffusion or accretion (Holroyd and Jiusto 1971; Huggins and Rodi 1985; Silverman 1986). When clouds have a maritime droplet distribution rapid glaciation of the clouds by secondary ice crystal production is often observed and may obscure seeding effects (Marwitz 1987). 2) 10-30 minutes-Differences in ice crystal fall speeds and vertical wind shear become significant in tilting and spreading the seeded ice crystals; however, higher ICC should still be evident compared to un- seeded cases. The dominant growth processes are dif- fusion and accretion with aggregation a secondary pro- cess (Heymsfield 1986). Detectable increases in radar reflectivity are not anticipated in regions with natural echoes; however, an increase in echo coverage may be expected if seeded precipitation fills in breaks in the natural radar echo pattern. Depletion of SL W may be noted during this period. 3) 30-60 minutes-A majority of the seeded snow crystals reach the surface leading to increases in radar echo coverage and precipitation rates. At the surface, increases in precipitation will be accompanied by in- 'creases in ICC and a decrease in SL W passing overhead. c. Instrumentation A summary of all the instrumentation employed by the SCPP from 1976 to 1985 has been given by Reyn- olds and Dennis ( 1986). The project-area instrument locations and an elevation transect through the Sierra Nevada are shown in Fig. 1. The following instrumen- tation was available during the fixed target experiments: a 5-cm weather radar at Sheridan (SHR); rawinsonde stations at SHR and KGV; telemetered precipitation gauges throughout the fixed target area with a resolution of 0.1 mm in 5 min (Price and Rilling 1987); and several telemetered mountaintop stations that mea- sured wind, temperature, humidity, and icing rate. In- tensive surface and remote sensing observations were collected at the fixed target, KGV, with a dual-channel microwave radiometer, a vertically pointing Ka band radar, an aspirated 2D-C particle sensor, and a high- resolution precipitation gauge. Habit, rime, and aggre- gation characteristics of the snowfall at KGV were cap- tured by photomicrography, and snow samples were collected for chemical analysis. The seeder aircraft was a Cessna Aero Commander capable of dispensing dry ice pellets from a chopper on the aircraft, and 20 g AgI, or indium pyrotechnic flares. In 1986/87 the aircraft was equipped to seed with a generator burning an AgI NH41 NH4Cl04 mix- ture in an acetone solution. The University of Wyo- ming operated the cloud physics aircraft, a Beechcraft Super King Air 200T, which carried a full complement of instrumentation for cloud physics research (Cooper et al. 1984). Operations were coordinated from the project field office in Auburn, California, where a com- plete forecast facility was maintained. In addition to the standard forecast charts and satellite imagery, tele- metered data from the microwave radiometer, moun- taintop stations, and 5-cm radar at SHR were available in near real titp.e. '~ I I d. Fixed target procedure During storm episodes the aircraft were dispatched when the greatest amount of SL W was expected. Ini- tially the cloud physics aircraft made an ascent sound- ing to 6 km, and then descended along the barrier at Minimum Obstruction Clearance Altitude (MOCA) to characterize the cloud kinematically and micro- physically. To target seeded ice crystals to KGV; the fixed target, a parameterized operational targeting model (OTM) was used (Rauber et al. 1988). The OTM constructed a two-dimensional wind field in a vertical cross section normal to the mountain barrier based on soundings at SHR and KGV, and the ascent sounding of the research aircraft. Ice crystal growth rates as a function of temperature (Ryan et al. 1976) and fall velocities as a function of size (Locatelli and Hobbs 1974) were used to calculate ice crystal trajec- tories. An iterative procedure was then used to select the location and orientation of a seedline so that seeded precipitation was predicted to fall at KGV. The research aircraft then proceeded to that point to measure the concentration of SL W. An experiment was initiated if SLW> 0.05 g m-3 was found along at least one-third