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Bulletin American Meteorological Society ect-aircraft positions. A C-Band Doppler radar has been made available by the National Center for Atmospheric Re- search about every other year to measure both the intensity and motions of precipitation particles for the purpose of estimating local precipitation and wind variations. The K-band radar/ first operated at Kingvale in 1983/84 by the Desert Research Institute, is used mainly to determine the vertical extent of clouds passing over the crest. Precipitation measurements are made by a network of re- cording precipitation gauges. The number of gauges has var- ied from 20 to 60. Some of the gauges are in remote areas and require servicing by helicopter. The snowfall atthe ground is studied also by the Ground Microphysics Laboratory (GML) (Humphries, 1985). The GML consists of a camper truck equipped with instruments for counting and measuring snow crystals and for studying their shapes. In addition, snow samples sometimes are collected for analysis of trace impuri- ties, including silver, to note if changes in precipitation rate can be related to changes in silver content in the snow. A new addition to the SCPP instrumentation list is a dual- chaimel microwave radiometer. This instrument, which has been operated at high elevations in the ARB for the past two seasons, will be discussed in the following section. 5. Physical observations within Sierra Nevada storms a. Supercooled liquid water and its relation to Sierra Nevada meteorology SL W is necessary if glaciogenic seeding is to affect precipita- tion. Therefore, SCPP scientists have given close attention to the problem of finding and measuring SL Wand relating its occurrence to the meteorological setting. The principal tools for finding SL W in the early years of SCPP were the optical probes mounted on the cloud-physics airplane. These probes recorded sizes and concentrations of cloud and precipitation particles. This information com- bined with radar echo patterns could then be used to relate the presence of SL W to stages in a storm's life cycle (Heggli et aI., 1983). The first few winters of exploration inside deep storm clouds over the ARB showed SL W to be less abundant than had been expected. The highest concentrations of SL W mea- sured by aircraft were found in cumulus clouds forming about 30 to 65 km upwind of the crest, i.e., over the foothills, behind eastward-moving storms (Rangno et aI., 1977). How- ever, frequent observations at the surface of rimed snow- flakes falling from the shallow orographic cloud remaining on the barrier after the deep orographically enhanced storm clouds moved through provided an indication that a seeding potential might exist in the shallow orographic clouds as well. The instrument that appears to provide the most useful measurement of liquid water is a dual-channel microwave radiometer (Hogg et aI., 1983). Such an instrument was op- erated on SCPP in the 1979/80 winter program and again in J K-band denotes 9 mm wavelength radar. 517 I:l ~ 0 w o / / ~ "~w~. {J--=-;:::'~)IIIIII,I "'~' CJ (1r.~r::"~Ir:.:..\:q:,,~~<:"IJIJ 1"'1'1"1111 DRY (~J::/<: II .> >:l~;:.', ...:......:\ 111/ ';111 ~11't I ~ Shallow moist zone +- Time v Seeding Opportunity FIG. 4. Cross section through a split front. Stippled areas denote relative humidities above 90 percent. Most seedable regions deter- mined from radiometric and aircraft observations within the storm are annotated. (After Browning and Monk, 1982). 1980/81 (Snider and Rottner, 1982). In both seasons, the in- strument was located below the snowline so it was not clear how much, if any, of the observed liquid water was super- cooled. During March 1983, a dual-wavelength scanning ra- diometer was operated at Blue Canyon (1585 m). Consider- able information was collected on the presence and amount ofSL Wwithin the larger storm systems. During the 1983/84 season the radiometer was operated for three months at King- vale (1980 m) (Figure 3, bottom), which is above the snow line in nearly all storms, so that the liquid water detected had to be supercooled. With the radiometer at Kingvale, the monitoring of SL W can be continuous throughout a storm instead of being confined to a single aircraft flight, which provides only limited, discrete samples within a restricted vertical air space. In combination, the radiometer and K-band radar data from Kingvale, the cloud-physics aircraft data, and the vertical temperature and humidity measurements (from the rawinsondes at Kingvale, Blue Canyon, and Sheri- dan) have begun to clarify when a potential exists for positive seeding effects. The cross sections of some storms passing the Sierra Nev- ada (Fig. 4) have been found to resemble those of certain storms over the Pacific Northwest (Hobbs, 1978) and the British Isles (Browning and Monk, 1982). SL W often devel- ops behind the upper cold front (prefrontal cold surge) as the cloud top lowers. (Heggli and Reynolds, 1985). The residual clouds, which are generated by moist air flowing over the mountains, are purely orographic. Preliminary studies of rawinsonde data over the barrier show that the water satu- rated layers in these clouds, and thus the bulk of the SL W, generally lie below the -lOoC level. The liquid water ob- served in these clouds sometimes causes extreme airframe icing and constitutes a hazard to flight operations (Sand et aI., 1984). Once the surface cold front passes the barrier, the clouds become more convective. Convective cells form near the foot- hills and move upslope. These newly formed cells often con- tain little ice and account for the relative abundance of SL W over the foothills. Presently, it is believed that the shallow orographic clouds provide the best potential for precipitation increases through cloud seeding. This is because the SL Win them is long lasting