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<br />~ . <br /> <br />. . <br /> <br />The evidence available suggests it is reasonable to assume that ground-released AgI is seldom transported <br />as high as 1 kIn above the Plateau. This is in agreement with earlier observations over other mountain <br />barriers. Because cloud temperatures are usually only slightly supercooled in this shallow seeded layer, <br />the challenge is to produce enough AgI IN, effective at slightly supercooled temperatures. An abundance <br />of observations exists in the intermountain West to demonstrate that the lowest kilometer over mountain <br />barriers is often too warm for significant nucleation with conventional types of AgI and generators, and <br />typical release rates. The problem is even more serious in the Sierra Nevada, which led the State of <br />California to investigate propane seeding (Reynolds 1991). <br /> <br />Aircraft 2D-C probe measurements oflPC were examined for possible microphysical changes associated <br />with the AgI seeding. NCAR counter observations were used to estimate the average positions of the AgI <br />plume (or, more likely, intermingled plumes), and in-plume IPCs were compared with natural IPCs <br />crosswind from the AgI. <br /> <br />The main findings of the six previously described experiments are summarized in Tables I and 2. NCAR <br />-counter measurements and a recent cloud simulation laboratory generator calibration were used to <br />estimate AgI IN concentrations effective at in-plume aircraft sampling temperatures in Table 1. The most <br />important and reliable values in Table 1 are the IPC increases based on observed differences in 2D-C <br />particle imaging probe measurements within and crosswind from seeded zones. <br /> <br />Table 1 shows increases for both the estimated AgI IN concentrations and the observed IPC enhancements <br />as the seeded zone temperatures decreases. No discernable increase occurred in seeded zone IPC in the <br />first three experiments where temperatures were no colder than -9 oC. Valley seeding during the first <br />experiment of2 March may have increased the IPC. The second experiment of2 March and the single <br />experiment of 6 March both strongly suggest IPC enhancements within the seeded zone. The 6 March <br />case showed IPC increases from both terrain-following and constant altitude passes. The two experiments <br />with apparent IPC increases had the coldest clouds, below -12 oC at aircraft sampling altitudes. <br /> <br />Table 1.-Summary of results for six valley seeding experiments. All values are averages for the <br />seeded zone. The IPC calculations ignored particles smaller than 100 flm size. <br /> <br /> 2D-C IPC <br /> Temp. range Ave. Plume width Est. AgllN conc. increase <br />MMDD/Exp. (oC) (km) (L-') (L-') <br />0228/1 * -5 to -9 20.6 0.05 0 <br />0301/1 -6 to -9 30.8 0.3 <0.2 <br />0301/2 -6 to -9 25.2 3 <0.5 <br />0302/1 -9 to -12.5 36.2 15 <7 <br />0302/2 -12.5 4.7 70 >14 <br />0306/1 -15- to -18.5 25.6 70 10 <br />0306/1 -19 10.2 60 27 <br /> <br />. . <br /> <br />* Most aircraft AgllN observations were made below cloud base. <br /> <br />. . <br /> <br />The AgI IN estimates in Table 1 and the estimates of seeding-caused IPC are in reasonable agreement <br />when it is recognized that not all available AgI IN will nucleate ice crystals, especially when cloud L WC <br />is limited. The type of AgI used is expected to result in contact nucleation, a slow process. The degree of <br />agreement between AgI IN and IPC in Table 1 may be fortuitous. Instrumentation limitations and the <br />uncertainties in the representativeness of cloud chamber generator calibrations result in lack of precision <br /> <br />63 <br /> <br /> <br />L <br />