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<br />major canyon was found over the Plateau during the two aircraft missions at temperatures ne.ar -12 to <br />-17 oC, respectively. Ice particle concentrations, calculated from SF6 measurements, were sufficient for <br />seeding to have been effective. However, it is suspected that natural cloud processes were quite efficient <br />during the unusually cold cloud temperatures of one (March 6, 1991) of the two missions, as very little <br />SLW existed above the Plateau top's west edge or at aircraft levels. Moreover, Super (1995a) reported <br />that the maximum snowfall accumulation for the three Plateau top gages was only 0.01 inch during the <br />entire 3 h experiment. Therefore, even if seeding increased the snowfall from this cold, weak storm, any <br />enhancement was at the trace level. <br /> <br />r <br />, <br /> <br />The Plateau top snowfall during the second mission of March 2, 1991, was also quite limited, even though <br />all eight valley AgI generators had been on for many hours, and AgI was abundant at relatively cold <br />temperatures where SL W was plentiful. Super (1995a) reported only two of the three Plateau top gages <br />detected snowfall during the experiment, and only at trace levels, in spite of a noticeable increase in the <br />aircraft observed IPC (ice particle concentration) within the AgI plume(s). This finding is not <br />encouraging for the valley seeding mode of operation. Even though the March 2 mission was flown under <br />apparently nearly ideal conditions of seeding potential, and the valley-released AgI was transported to <br />sufficiently cold SL W levels, any resulting snowfall was insignificant. It is speculated that AgI nucleation <br />occurred too late for snowflake growth and fallout to occur on the Pl~teau. <br /> <br />Super (1994) examined SLW, snowfall, and AgI during 12 experimental days ofthe early 1991 season <br />when valley seeding was being conducted. He concluded that the AgI concentrations measured on top the <br />Plateau were insufficient for effective seeding unless the AgI reached SLW colder than about -12 oC. <br />However, based on a later AgI generator calibration reported by DeMott et al. (1995), the estimated <br />concentrations were too low as discussed by Super (1995a). . <br /> <br />3.3 Analyses based on Recent Laboratory Studies <br /> <br />A subset of the early 1991 data reported by Super (1994) has been reexamined for this report using the <br />latest calibration data for the NA WC AgI generator from DeMott et al. (1995). Values were used for <br />"natural draft" conditions typical of light valley winds during seeding. An even 100 h from seven <br />different relatively wet storm episodes met the criteria that the eight valley AgI generators had been on for <br />at least 2 h to allow for transport time, microwave radiometer SLW equaled or exceeded 0.05 mm above <br />the Plateau top's west edge, and the co-located NCAR acoustical ice nucleus counter operated properly <br />with its cloud chamber temperature near -20 oC. <br /> <br />The NCAR counter mean INC (ice nucleus concentration) observations for the 100 h were multiplied by <br />3.0 to bring them in line with the CSU (Colorado State University) calibration of this device based on the <br />isothermal cloud chamber results, also at -20 oC. A correction was applied ~o the cloud chamber values to <br />compensate for dilution airflow (DeMott et al. 1995). For the uncorrected values usually provided by <br />CSU generator calibrations, the two NCAR counters tested were found to indicate about two-thirds of the <br />/ ( <br />INC observed in the Isothermal Cloud Chamber, and there was excellent cross consistency between the <br />two 'counters tested atCSU. A third NCAR counter, not tested at CSU, had been used during the Plateau <br />experiments: Perio'dic side-by~side comparisons showed consistent results a.n1ong all three units in the <br />field. <br /> <br />An adjust~d INC exceeding 10 L-1 at -20 oC can be considered clear evidence of AgI presence because the <br />natural background observed during many hours without seeding was rarely that great, usually being in the <br />o to 5 L-1 (adjusted) range. Ninety percent ofthe 100 sampled hours meeting the stated criteria showed <br />evidence of AgI, with the adjusted INC exceeding 10 L-1. Half the hours exceeded 500 L-1, effective at <br /> <br />12 <br />