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1178 JOURNAL OF APPLIED METEOROLOGY VOLUME 27 ~ -100 cj 75 Z 8 50 W 25 Q 0 W 2.0 !c- &%1: e: E 1.0 frlE a: a. 0.0 t ;n 1MI , ~ I '\ . I ,.\..;\.! \ , \,' \I 1 2 , I 1200 1300 'j/'r, ~ I . '1 ....,l1 ~ ..<..<~;;.,." Bar indicates seeding plume over Snow Lab _____ 3 1400 1500 1600 TIME ImstJ 4 - 1700 FIG. 11. Precipitation rates and IPCs recorded by photography at the Snow Lab during the four airborne seeding experiments of 18 Mar 1986 compared with a time bar indicating the passage of the seeded zone aloft. deck extended between 10 and 15 km upwind of the Snow Lab, with scattered cloud over the valley to the north. The cloud deck over the mesa contained SL W up to 0.2 g m -3 but little ice, and the coldest cloud tops were warmer than -160C. Total AgI counts per pass ranged from 97 to 358 on the six postseeding passes. A maximum IPC between 40 and 100 L -I was encountered coincident with the AgI plume on each pass after the AgI entered the oro- graphic cloud. Nonseeded cloud south of the Snow Lab, however, still contained very little ice. Only limited SL W was found in the core of the seeded cloud on either of the final two passes. The mean IPCs in the 5 km upwind of the Snow Lab in the seeded and non- seeded clouds were 21 L -I and 0.2 L -I, respectively. The enhancement was again identified by the process- ing software as predominately consisting of small hex- agonal plates. Calculated precipitation rates in the 5 km-wide zone upwind of the Snow Lab averaged 0.40 mm h-I when seeded and <0.01 mm h-I when non- seeded. The seeded zone passed over the Snow Lab between 1319 and 1337, while precipitation began there at 1327 and lasted until 1341 (Fig. 11). As observed in airborne experiment 1, the higher surface precipitation rates re- sulted largely from aggregates and dendrites. The GMO West high resolution precipitation gage recorded 0.025 mm, the minimum resolvable, between 1330 and 1345. The other gages detected no precipitation during this experiment. c. Airborne seeding experiment 3 After landing and refueling, the aircraft returned to the Grand Mesa at 1454. The stratocumulus cloud tops had risen and cooled slightly, to 4.6 km and -180C, and the orographic cloud deck extended 15 km upwind of the Snow Lab. The cloud base was still near the mesa top, as indicated by the GMO icing rate detector. The ascent sounding is shown in Fig. 2b. Peak SL W contents of 0.2 g m -3 were observed a few kilometers north of the Snow Lab. The third seeding arc was completed at 1509, 28 km upwind of the Snow Lab. Strong AgI signatures were detected in clear air on each of the first four sampling passes and in concert with high IPCs on each of the last six passes, after the AgI entered the cloud. Within the 5 km zone upwind of the Snow Lab, mean IPCs were 13 L -I when seeded, compared to 2 L -I when nonseeded. Calculated precipitation rates were 0.34 mm h-I when seeded and 0.08 mm h-I when non- seeded. Differences in crystal habits in the natural and seeded clouds were similar to those observed in exper- iments 1 and 2 in terms of both IPC and precipitation rates. Light precipitation was observed briefly at the Snow Lab prior to 1540, but it did not exceed 0.2 mm h-I. This shower preceded the arrival of the seeded zone and is thought to have been of natural origin. Heavier precipitation began at 1601, diminished after 1616, and essentially ceased after 1633. The heavier shower co- incided with the passage of the seeded zone, which was over the Snow Lab from 1559 to 1629 (Fig. II). The network precipitation amounts coincident with the passage of the third seeding line ranged from 0.025 mm at the West Arm site (near the west end of the seeding arc) to 0.20 and 0.35 mm at the GMO East and West sites, respectively. The upwind Mesa Lakes gage caught 0.125 mm, and the downwind Ward Creek gage caught 0.10 mm, both less than the sites nearer the target. By 1645, precipitation was no longer being recorded by the two gages closest to the Snow Lab. \ /1 ~