OCR Text |
Show area included Rabbit Ears, and (3) se eding had c e ased. There is goo::l evidence that 1 cph seeding was reflect e d in a positive 1 cph seedin g effect on this day's operation. 4. 5. 2. 5 Observed Crystal Types Replicas taken at Rabbit Ears Pass during this operation i ndicat e d a predon _,_inance ( > 50 % of total crystal count) of hexago nal plates with many of the small plates exhibiting inner stellar structure (a phenomenon found to be associated with seeding - - see Section 4. 2 ). The minimum temperature to which Agl was indicated to reach in time to produce fallout of seeded pr e cipitation at Rabbit Ears Pass was - 10°C. Thus, AgI-seeded crystals should have been predominantly columns, n eedles or hexagonal plates, with a minimum of large dendrites, as observed. 4. 5. 2. 6 Summary and Conclusions From the data studied, it is concluded that : (1) winds were sufficiently steady and large-scale precipitation-producing mechanisms were sufficiently weak to allow isolation of a positive 1 cph seeding effect in this case, (2) the observed predominant crystal type at Rabbit Ears Pass agreed well with predicted seeded crystal type, and (3) sounding structure changes were reflected in ice nucleus count changes and precipitation rate variability. 4. 5. 3 Mt. Harris Ground Releases 4. 5. 3. 1 Case Study of Operations on 7 February 1969 The seeding events on 7 February 1969 were selected for detailed analysi --· since the collected precipitation rate data and supporting physical measurements are of good quality and th e prevailing synoptic weather pattern was unusually steady. 4. 5. 3. 1. 1 Synoptic Pattern, Moisture and Stability The synoptic pattern featured a developing le e- side surface low in southeastern Wyoming with a closed circulation extending to 700 mb (Figure 104). Winds were quite light furth e r aloft (Figures 105 and 106) with most of the transbarrier flow occurring below 14, 000 ft. MSL. This simplified the area precipitation pattern with most of th e snow being orographically produced from a thin layer. The depth of the orographic cloud laye r gradually incr eased during the day (Figures 107, 108, 109, and 110) with cloud top t e mp eratures co rresponding ly de creasing from -l4°C at 0503 MST to -22 °C at 1153 IV[ST and 1603 MST. Th e 173 |