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<br />OcrOBER 1988 <br /> <br />ARLlN B. SUPER AND BRUCE A. BOE <br /> <br />1173 <br /> <br />25~ <br />CO? <br />~E <br />(I) ~O I <br /> <br /> <br />---~ <br /> <br />NNE <br /> <br />100 <br /> <br />... <br />:.J <br /> <br />860320 <br />1300-1304 mst <br />ALONG-THE-WIND PASS <br /> <br />z <br />o <br />~ <br />CE: <br />I- <br />Z <br />W <br />U <br />Z <br />o <br />U <br />~50 <br />U <br />j: <br />CE: <br />:. <br />w <br />~ <br /> <br />o <br />o <br /> <br />5 <br /> <br />10 15 <br />DISTANCE FROM SEEDING SITE IN KM <br /> <br />20 <br /> <br />24 <br /> <br />'I <br /> <br />--- -- <br />I I <br /> <br />~ <br /> <br />ssw I <br /> <br />FIG. 7. As in Fig. 4 for a single pass flown along-the-wind during the ground-based seeding experiment of 20 Mar 1986. <br /> <br />zones. The mean IPC in the central seeded zone at <br />3.25 km (below cloud) was 19 L -1, with an estimated <br />precipitation rate of 0.14 mm h -1. The snowfall was <br />again visually observed to be reaching the surface. The <br />crosswind control zones had 1 L -1 IPC and essentially <br />zero snowfall rate. <br />A third shower developed only 8 km south of the <br />seeding generator after the second shower began to dis- <br />sipate in the lee of the mesa. Crosswind sampling in <br />cloud and below base yielded results very similar to <br />the earlier showers: The central seeded zone IPC at <br />3.45 km altitude was 26 L--1 with an associated pre- <br />cipitation rate of 0.11 mm h -1. The mean IPC was <br />well below 1 L -1, and precipitation rates were again <br />virtually zero in the adjoining nonseeded zones. <br />Five final passes at 3.8-3.85 km altitude both cross- <br />wind and parallel with the wind again found SL W con- <br />tents up to 0.1 g m -3 above the windward slope. Peak <br />IPCs observed on these passes, nearer the AgI generator, <br />increased to 40-100 L -I, coincident with pass-total <br />counts up to 414 from the acoustical counter. <br /> <br />c. Summary of ground-based seeding experiments <br /> <br />Silver iodide released from either 285 or 505 m below <br />the top of the Grand Mesa was consistently transported <br /> <br />~:\. <br /> <br />well above the mesa top within several km downwind <br />of the generator. The AgI plume was quite narrow on <br />19 Mar and quite wide on 20 Mar, in agreement with <br />the observed differences in horizontal wind fluctua- <br />tions. Holroyd et al. (1988) discuss in detail the plume, <br />wind and stability characteristics for these and several <br />other experiments with ground-released AgI. <br />Seeding produced dramatic enhancements in IPC, <br />mostly crystals smaller than 0.6 mm. Many of these <br />were embryonic dendrites on 19 Mar when the tem- <br />perature at the sampling level was -140C, but were <br />hexagonal on 20 Mar when the temperature of the <br />shallow stratocumulus deck was between -8.5 and <br />-1l.50C. <br />Estimated precipitation rates within the seeded zones <br />at 3.8 km altitude were near 0.1 mm h -Ion both dates. <br />This was about triple the natural rate on 19 Mar, while <br />natural precipitation was virtually nil on 20 Mar. <br />(Note: these calculated precipitation rates are believed <br />to be underestimates, and caution should be used in <br />quantitative interpretation.) <br /> <br />4. Airborne seeding experiments <br /> <br />Six similar direct detection cloud seeding experi- <br />ments were conducted on 18 and 19 Mar 1986. In each <br />