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It is difficult to estimate IN concentrations from airborne sampling because of the mixing within the IN detector's cloud chamber, flush time on the order of 5 min, and the speed of the aircraft (approx. 90 m S-I). A better estimate can be inferred from SF6 sampling. Nevertheless, the airborne IN count rate was far less than that of ground sampling, indicating low concentrations at flight level, consistent with other winter orographic programs. The valley sites were input to the model, each releasing 8 gm AgI hr-1. The results show a pooling of the AgI in the Sanpete Valley (Figs. 15 through 17). After two hours of release, the surface plume is shown to be just reaching the HAS. The variability of the surface wind direction in the valley is shown by the westward transport of the AgI plume from the southernmost release site in Fig. 15. There was a very small contoured area reaching the 2.9 km level by the end of the 2 hr release time (Fig. 16) which was approximately 5 km south of the HAS. This day had imbedded convection; however, the model did not adequately predict this. There may be some convection produced by the model as evidenced in the sequences of vertical cross sections of Fig. 17. The last of this series shows a mass of AgI moving upward more than what topography alone would suggest. The minimum contour of this analysis was 6.1XlO-s J.lgm m-3, corresponding to 6.1XlO-14 gm L-1. Converting this to IN effective at -20 using the natural draft calibration of the NA WC generator (lXI016 IN gm-t, Griffith, 1992), gives an IN concentration of 610 IN L-1. This is at least two orders of magnitude larger than what was detected by the aircraft, implying that if the model was faithful in its predictions, it could not contour the low concentrations. The contour interval can be specified in the analysis; however, a contour interval this small would have provided messy, unusable graphics products at, or below, 2.9 km. Presently the contour interval is set at default for the tracer output. e. Simulated AgI Release from Crest of San Pitch Mountains, 2 March 1991. An accessible point on the crest of the southern San Pitch Mountains was selected for a modeling test. This site had a radio tower implying available power should the site be used operationally. Its position and elevation are 39" 28.0' 1at., -111 0 44.5' long., and 2.5 km. A Skyfire-type AgI generator was modeled. Figure 18 shows the surface concentrations of AgI after 4 hrs release. The plume appears to be spreading to the east; however, Fig. 19 shows that the spread is limited to the surface. Figure 19 is a horizontal cross section at 2.2 km which is below the release level and, therefore, represents a slice through the descending plume. The AgI was released in a negative w zone and has therefore little or no vertical dispersion as it is transported into the Sanpete Valley. There was a period from about 1 hr to 2.5 hr after the release started when the simulated plume had contours at the 2.9 km level (not shown). These were 10 to 15 km east of the release point. Associated with this were some small areas of low level positive w. Later, there were no contours above the 2.2 km level and an organized N-S band of negative w formed east of the San Pitch Mountain crest. It can be concluded that this crest site would be an unfavorable location for seeding for this case. In spite of its good exposure, the vertical velocity field in its vicinity had a deleterious effect. -24- I I I I I I I I I I I I I I I I I I I