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1158 JOURNAL OF APPLIED METEOROLOGY VOLUME 27 al. 1976; Holroyd 1986). Though these crystals were most probably nucleated while passing over the wind- ward (west) slope of the Bangtail Ridge, they may also have formed over the Main Ridge, ceased growing or even partially sublimated in the lee subsidence, and resumed growth in the ascending SLW-rich air ap- proaching the BRTA. While seeding clearly enhanced the IPC, such en- hancement will not necessarily result in greater precip- itation. If, for example, smaller seeded ice particles were created at the expense of some natural particles rather than in addition to them, seeding might result in less snowfall. Even if small seeded crystals were present in addition to the natural background level of ice particles, their contribution to precipitation might be insignifi- cant. Most snowfall mass tends to be concentrated in the largest particles, which often comprise a small frac- tion of the total population. North-south gradients in SL Wand natural snowfall existed, with both being greater to the south. While the estimated snowfall rate in Zone N-S was about twice that of the adjoining Zone N-C, it was less than in Zone S-c. However, Zone C-S had a mean snowfall rate nearly twice that of Zone S-C and several times that of Zone N-C. Zone S-S had even a higher rate, near 0.17 mm h -I (all snowfall rates are for melt water equivalents). Particles greater than 1.0 mm, generally classified as aggregates, accounted for most of the snowfall in Zone S-C. In contrast, though some aggre- gates and graupel-like particles were observed in the seeded zones, much of the snowfall resulted from par- ticles smaller than 1.0 mm classified as hexagonal. The mean precipitation rate of 0.09 mm h-I in the seeded zones was three times the mean rate for the two control zones, so it appears that the AgI seeding mark- edly increased the snowfall. However, the precipitation rates calculated from 2D-C probe images are believed to be underestimates (Part I). Consequently, it is likely that actual precipitation rates were greater at the 2.7 km level than is indicated in Fig. 3. Recent observational evidence from a number of mountain ranges, some supported by numerical mod- eling, indicates that SL W is often concentrated above the windward slope and crest of the barriers in stable winter storms (e.g., Boe and Super 1986; Rauber and Grant 1986). These regions should contain greater vertical velocities and water vapor contents than colder regions further aloft. Though it was not possible to monitor SL W between the lowest flight level and the surface over the Bridger Range, it seems likely that some SL W existed below the 2.7 km level. Even if only ice saturation was exceeded, ice crystal growth could continue, possibly resulting in greater snowfall at the surface than at 2.7 km. However, no surface observa- tions were made during the January 1985 experiments to test this conjecture, although it was practical to ob- tain such measurements over the Grand Mesa, Colo- rado, as reported in Part III (Super and Boe 1988). b. 15 Jan, a.m. The climbout sounding over the Bozeman Airport revealed very light amounts ofSLW from 2.6-2.8 km, and concentrations of 0.05-0.15 g m -3 in the 3.5-3.9 km layer. A zone ofSLW was encountered near cloud top while flying at 4.5 km toward the Main Ridge. The icing rate detector at the Crest Observatory measured hourly mean values of 0.03-0.04 g m -3 throughout the mission. Single N-S passes were made at 3.9 and 3.3 km over the BR T A. Supercooled liquid water amounts were near 0.1 g m-3, and only 1-2 L -I IPC were found, with the exception of a 1.5 km wide region at 3.3 km centered about 9 km south of the SSL, which peaked at 20 L -I. It coincided with a weak (6 counts) AgI plume. Pairs of passes were made at 3.0, 2.85 and 2.7 km. The SL W content increased with altitude, with a two- pass mean of 0.06 g m-3 at 2.7 km and 0.17 g m-3 at 3.0 km. The threshold diameter, Dr. averaged 15 tLm for the wettest nonseeded 1 km at the 2.7 km level, and the mean droplet concentration was 130 cm - 3. A zone of very marked increase in IPC was evident on each of the six passes in the 2.7-3.0 km layer, with peak concentrations from 28-48 L -I. These enhanced IPC zones were in spatial agreement with the AgI plumes. Total counts per plume transit from the acoustical counter ranged from 31-83. Figures (not shown) similar to Fig. 2 were examined for evidence of decreased SL W in the seeded zone. Any such decrease was not obvious with the N-S gradient and the considerable spatial and temporal variability that were present. The seeded zone for the mean of the two 2.7 km sampling passes was again subdivided into three equal segments and compared with the nearby 2.5 km wide control zones. The mean IPC in the seeded segments ranged from 6 L -I in Zone N-S to 12 L -I in Zone C- S, while that in the controls was under 1 L -I . Most of the increase was in the smaller size ranges, generally less than 0.6 mm. As during the afternoon mission on this date, most of the enhanced ice particle population was classified as either hexagonal or spherical. Since the temperature over the BRTA ranged from -10.50C at 2.7 km to -12.50C at 3.0 km, production of hex- agonal plates would be expected from AgI nucleation. The estimated natural precipitation rate showed about a threefold increase from Zone N-C to S-C, as- sociated with a N-S gradient in SL W as during the afternoon mission. A similar N-S gradient existed in the seeded zones, with Zone N-S greater than the north control but less than the south control and the other two seeded zones above both controls. The south con- trol had a low concentration oflarge (> 1.6 mm) ag- gregates that produced most of its snowfall. The natural N-S gradient complicates interpretation of any precip- itation change due to seeding. However, the mean of i ~1 ,I i