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APRIL 1990 DESHLER, REYNOLDS AND HUGGINS 321 i I; image concentration, with no artifact rejection. Total image concentrations were used since, in the orographic clouds investigated during this SCPP experiment, the dominant source of artifacts was "zero area" images. Since seeding initially produces many tiny ice crystals, particularly dry ice seeding, seeding signatures are likely to be seen first in the uncorrected 2D-C 2D-P data. Averages of liquid water content (LWC) were calcu- lated from the liquid water probe available for the day, with preference given to the CSIRO probe. For the radar data the maximum effective reflectivity factor (dBZ) detected in a box centered on the mid- point of the initial seedline position was averaged. The analysis box was 3 km on a side, with a height of I km. For the averages, data were taken from the volume scan prior to seeding, all scans during seeding, and the scan following the end of seeding. The radiometer liq- uid water measurements represent a 3-hour average closest in time to the midpoint of each seeding exper- iment. Note that the radiometer was located at KGV and not at the seeding location. Precipitation echo types (PETS) are also included in Table 4. These are specific radar echo patterns that existed over the American River basin as seeding oc- curred. Heggli et al. (1983) describe PETS in detail, but basically major bands (MB) and embedded bands (EB) are various types of rainbands, area-wide (A W) represent full scope echo coverage and stable condi- tions, orographic (0) are echoes confined to the mountain barrier, C1 and C2 represent convective echoes with scattered to broken echo coverage, and CT represents nearly stationary lines of convective cells. It is obvious from Table 4 that the experiments were conducted in a variety of conditions; although seeding material was always released at temperatures ~ -150e. The most common range was from -12 to -60e. Liq- uid water at aircraft altitude was generally quite low, but the average of the maximum values indicates SLW was present, if only sporadically, in amounts well above instrument threshold (0.05 g m -3) during nearly all experiments. Table 5 compares the initial conditions from Table 4 for experiments with observed seeding effects and those with no effects (ignoring placebo cases). Average liquid-water content was identical in the two samples; however, radar reflectivity, 2D-P concentration, and 2D-C concentration were, respectively, 2, 2, and 3 times greater in cases with no detectable seeding effects com- pared to cases with some type of observed seeding effect. This comparison indicates that the capability to mea- sure seeding effects depended on initial cloud condi- tions. The instrumentation used could best separate I' seeding signal from background noise when clouds had low natural particle concentrations and relatively few precipitation-sized particles (low radar reflectivity). It is likely that in other cases effects were produced but not observed. Results from other projects have shown similar tendencies. Observations of seeding effects in the Cascades were almost entirely from nonprecipitat- ing clouds with simple structure (Hobbs et al. 1981; Hobbs 1975b). Likewise Super and Heimbach (1988) were able to delineate effects from ground generators in clouds with very low background ICe. a. Seeding effects observed by the research aircraft Results from seed curtain penetrations by the re- search aircraft are compiled in Table 6. The first five columns document for each day the seeding material, the number of curtains released by the seeding aircraft, the number penetrated by the research aircraft, and the percent observed to have seeding signatures (as de- fined in the case studies). The remaining columns summarize measurements from within individual cur- tains. These columns do not contain data from all the curtains, but give a representative sample across a rel- atively wide range of times after seeding and may con- tain measurements from two or more curtains. Particle growth rates and curtain spread rates were determined as in the case studies. Altogether 238 seedlines were initiated during this experiment. Forty-two of these seedlines were placebo and the research aircraft pen- etrated 34 of these or 81 percent. Of the remaining 196 treated seedlines, the research aircraft penetrated 123 of them or 63 percent. Seeding signatures were detected in 43 of these curtains or 35 percent. I) CO2 PELLETS There were 172 seedlines initiated using CO2 pellets and 100 of these were penetrated by the research air- craft. Seeding signatures were observed in 33 of these seedlines. This is not a high success rate considering the high concentrations of ice expected to result from seeding with CO2. There may be several reasons for these poor results. Insufficient liquid water along the seedline would limit the growth of ice crystals to less than a detectable size. High natural ICC would mask the high concentrations expected from seeding and also would compete with the artificial ice embryos for the available water. Problems with dry-ice delivery would cause the distribution of CO2 along the seedline to be sporadic, and sometimes nonexistent. The CO2 delivery system was a problem in the early years of the exper- iment, particularly 1983/1984. The dispensing system FIG. 22. Measurements collected at KGV of cloud liquid water, atmospheric water vapor, precipitation rate, and surface snow crystal concentrations, size, habit, riming, and aggregation as in Fig. 15. The PPE for SI-SIO are indicated at the bottom along with the arrival of NI and the extrapolated arrival of N2 and N3. At the top, the solid line is liquid water and the dashed line water vapor, as measured by the radiometer. The missing 2D-C data at 2355 is due to a tape change, while from 0025 to 0035 there was no precipitation.