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1164 JOURNAL OF APPLIED METEOROLOGY VOLUME 27 However, a dry arctic air mass existed in the lower levels above the BR T A where the AgI was transported. A shallow liquid cloud with limited SLW content was found over the BRTA on 10 Jan, when a liquid cap cloud was seen on the Main Ridge as well. The missions of 28 Jan were flown in a deep storm with cold top temperatures. Small, transient amounts of SL W were observed that were quickly depleted in the high con- centrations of natural ice particles. The three missions of 19 and 28 Jan, with negligible SL W at AgI plume levels over the BR T A, produced no detectable changes in IPC, ice particle sizes, or habits in the seeded zone. Conversely, the response of the clouds to the AgI seeding was very noticeable on the three flights of 10 and 15 Jan. During these three mis- sions, the AgI clearly encountered SL W at temperatures of -1OoC or lower, resulting in severalfold increases in IPC, typically from < 1 L -1 natural background lev- els to mean values near 10 L -( . Values of the threshold diameter, D/, were near the low value of of 15 Jlm at the 2.7 km level on each of these flights. Therefore, indications of ice multiplication by the Hallett-Mossop process were neither expected nor observed. Mean widths of the enhanced IPC zone on these three flights were 5.2-8.4 km about 17 km downwind of the seeding site. The ice particles caused by seeding on 10 and 15 Jan were generally hexagonal plates mostly less than 0.6 mm across, although some grew to the 1.0-1.6 mm range. The estimated seeded zone precipitation rates were approximately twice that of the mean of the cross- wind control zones. Because the small sizes of the ice particles imply limited growth times, it is probable that most of the observed seeded crystals nucleated over the windward slopes of the BR T A. It is also likely that they grew near aircraft sampling levels due to their small settling ve- locities. Ice crystals that formed further upwind over the windward slope and crest of the Main Ridge, which is known to have frequent SLW, most likely settled to the surface upwind of, or below, the aircraft sampling line at 2.7 km. It is speculated that much ofthe effect of AgI seeding was nearer the Main Ridge and at lower levels, where aircraft sampling was not practical. The physical observations of January 1985 lend considerable support to the previously reported statis- tical results. Clouds over the target area routinely con- tained an AgI plume several kilometers wide within 300 m of the highest terrain. Supercooled liquid water was observed on the Main Ridge crest from 7% to 9% of all hours from January through March 1985, an unusually dry period. During three of six in-cloud mis- sions, the. AgI intercepted SL W over the target. area, and consequently, severalfold increases in IPC were found in the seeded zone. The crystal habits were ap- propriate for growth at the temperature of the sampling level. Most crystals in the seeded zone were small, im- plying recent nucleation. Estimated precipitation rates were greater in the seeded zones than in the crosswind control zones. While the limited physical sampling of January 1985 obviously does not completely verify the earlier statistical findings, which were based on a large number of cases, there is no obvious disagreement be- tween the two approaches. It is recommended that a confirmatory statistical seeding experiment be designed and conducted in the Bridger Range. A strong physical component should be part of the design to test that the key steps in the hypothesis are routinely met. The ad- vantages of this particular mountain range include 1) a relatively simple and isolated N-S Main Ridge lying across the prevailing westerly flow with a broad sec- ondary ridge as the downwind target; 2) a reasonably high frequency of stable orographic clouds containing SL W cold enough to be nucleated with AgI near crest- line levels; 3) a demonstrated ability to routinely target the clouds with ground-based generators; 4) the exis- tence of an apparently successful exploratory statistical experiment; and 5) the existence of the physical evi- dence contained in this paper. The authors are not aware of any mountain barrier that would appear to offer more promise for a successful confirmatory seed- ing experiment with winter clouds. Acknowledgments. The authors wish to acknowledge the participation of the National Center for Atmo- spheric Research-Research Aviation Facility, without which the January experiment could not have been conducted. Key to the NCAR-RAF involvement were William Cooper and Erick Miller. In addition, Gilbert Summers and Donald Darnell skillfully piloted the air- craft, and Mr. Summers took the initiative to obtain a special FAA waiver to fly at low levels, which was very important for the results obtained. Gerhard Langer, the inventor of the acoustical ice nucleus counter, con- tributed his time and talents to updating and testing the counter used on the King Air aircraft. The Uni- versity of Wyoming, through John Marwitz, generously loaned the acoustical counter. The software developed by Ed Holroyd to process 2D-C images and estimate precipitation rates was very beneficial to the analysis. Jack McPartland and Bruce Boe made several sugges- tions to improve the manuscript, and Boe drafted the figures. The many helpful comments of the reviewers also contributed significantly to this paper. The field portions of this research were mainly sup- ported by the National Science Foundation, contract ATM 8414143. The Division of Atmospheric Re- sources Research, Bureau of Reclamation, also pro- vided considerable support, especially in the analysis phase. The National Center for Atmospheric Research is supported by the National Science Foundation. I /' REFERENCES Boe, B. A, and A. B. Super, 1986: Wintertime characteristics of supercooled liquid water over the Grand Mesa of western Col- orado. J. Wea. Mod., 18, 102-107.