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OCTOBER 1988 ARLIN B. SUPER AND BRUCE A. BOE 1181 Experiments 5 and 6 on 19 Mar probably did not produce precipitation at the Snow Lab, but continuous cloud deck did not extend as far upwind as on 18 Mar. Further, the mesa top near the Snow Lab was visible from the aircraft on some 19 Mar passes but was not seen on 18 Mar. The more broken cloud deck with somewhat higher bases on 19 Mar apparently provided less in-cloud time for ice crystal growth. This may have been a limiting factor in producing snowfall at the sur- face. The drying with time of the atmosphere over the mesa below 60 kPa can be seen by contrasting Figs. 2a and 2c. While on some passes SL W appeared to be inversely correlated with the enhanced IPC resulting from AgI seeding, the natural temporal and spatial variability of SL W over the mesa masked any obvious drying of the cloud due to seeding in most cases. Liquid water in the seeded 5 km zone immediately upwind of the Snow Lab never exceeded that found in the analogous non- seeded zone and overall was about one-third less than the mean in the nonseeded zone (Table 2). This sug- gests that the AgI seeding reduced, but did not elimi- nate, the SL W content of the orographic clouds. 5. Conclusions ." Eight physical cloud seeding experiments were con- ducted in northerly flow and shallow orographic cloud over the Grand Mesa of Colorado on 18-20 Mar 1986. Two experiments employed ground-released Agl smoke, while the six others used airborne AgI seeding, also with an acetone generator. Natural snowfall was very limited during the experimental periods. In all eight experiments, the AgI was consistently detected in cloud by the airborne acoustical ice nucleus counter at the 3.8 km sampling altitude. For the ground seeding experiments this required vertical transport of the AgI of at least 1100 m, which confirms the feasibility of targeting some orographic clouds with either air- borne or properly sited ground-based generators. In all eight experiments, marked enhancements in the observed ice particle concentrations were detected coincident with the AgI plumes. Thus, the efficacy of AgI as a nucleating agent was verified in natural clouds at the temperatures observed (-8.5 to -140C). Large numbers of ice crystals smaller than 0.6 mm, either hexagonal plates (at -9 to -12oC) or minute dendrites (at -13 to -14 OC), were recorded within seeded zones at aircraft sampling altitudes in all eight experiments. A fraction of the seeded crystals grew to larger sizes, and the sizes and habits of the these crystals were con- sistent with the prevailing temperature regimes and es- timated growth times. Precipitation rates estimated from aircraft 2D-C probe images recorded in the seeded zones exceeded those estimated for the nonseeded zones in seven of the eight experiments. Significant precipitation coincident with the passage of the seeded zone aloft in the first three (of six) air- borne experiments was recorded at the Snow Lab, while virtually no precipitation was recorded at other times during that experimental period. Probable reasons for the lack of precipitation at the surface in the final three airborne experiments, as previously discussed, relate to the insufficient residence time of the AgI and re- sulting crystals in SL W cloud. Yery light snow reached the mesa from the thin seeded clouds in one ground-based experiment, while no other clouds precipitated, No ground level obser- vations were practical during the other ground-based seeding experiment, but it appeared probable that sur- face snowfall was increased. All eight physical seeding experiments produced marked microphysical changes due to AgI seeding in shallow orographic cloud. The resulting IPC enhance- ments in turn produced an increase in estimated pre- cipitation rates at the aircraft sampling level about 500 m above the Grand Mesa in all but one case. Finally, light but significant precipitation was produced at ground level by at least half of the experiments. While the physical chain of events from seeding to surface snowfall has often been hypothesized, it has seldom been demonstrated. The ground-based seeding experiments provide direct physical evidence that re- leasing AgI well up the windward slope of a mountain barrier can alter cloud microphysics in a direction that should lead to increased snowfall. Taken together with the results shown in Part II (Super and Heimbach 1988) and in Holroyd et al. (1988) there is reason to be optimistic concerning the potential of seeding some winter orographic clouds with high-altitude ground- based generators. The airborne experiments demonstrated that aerial seeding can also result in increased snowfall at the sur- face under some conditions. Further research of this type will be required to better define those conditions that are suitable for seeding. The approach used in these experiments is believed to have considerable value for future work. Simple orographic clouds are excellent candidates for testing various seeding methods, rates and agents. Acknowledgments. The authors are pleased to ac- knowledge the significant contributions to this paper made by several persons. Jack McPartland of the Bu- reau of Reclamation (Bureau) was the aircraft scientist on some of the missions, while the second author flew on the others. Michael Collins of the Bureau main- tained the aircraft, and surface sensors and electronics systems. David Davalos of Aero Systems, Inc., skillfully piloted the aircraft. Marty Thorp and William Hauze of North American Weather Consultants (NA WC) operated the ground-based seeding generator and manned the Snow Lab. John Thompson, also of NA we, operated the airborne seeding generator and aircraft data acquisition system. Much of the analysis softwarle was developed by Ed Holroyd of the Bureau.