| OCR Text |
Show SECTION 4. 0 RESULTS FROM FIELD MEASUREMENTS AND CASE STUDIES As indicated in Section 2. 0, numerous measurements were made in the field during the five years of project operations. Interesting and significant results have appeared from analysis of each of the several diffe rent types of observations. In this section, results and conclusions from specific measurements (ice nucleus counts, silver in precipitation, snow crystal replicas) are presented and discussed. Several case studies are then pr e sented as examples of the · way in which all types of data were utilized to de scribe and explain the results of individual seeding experiments. In Section 5. 0, data from the entire program will be considered together in an evaluation of overall results and the techniques that were employed. 4. 1 Ice Nucleus Counter Measurements Professor L. 0. Grant of Colorado State University operated a Bigg-Warne r counter at Valley View Lodge for several seasons accumulating a large body of background nuclei count data. These data show background count l e vels of 1 to 2 nuclei liter- 1 . Background nuclei count levels measured at Rabbit Ears Pass with the NCAR counter during snowfall periods were typically less than 1. 0 liter - 1 . During seeded periods with favorable wind and diffusion conditions the nuclei count level often reached:::: 100 liter- 1 at -20 °C, even during periods of moderate to heavy snowfall. This would indicate an overseeding condition near the surface, since the counter measures only unused nuclei, or nuclei that have formed ice crystals that are still small enough so as to not be excluded from the counter inlet system. However, during two dual trace r tests using the inert non-nucleating lithium stearate as a second tracer, wit½ fa vo rable wind and diffusion conditions for transport to the Rabbit Ears site, the lithium stearate tracer arrived at the site in quantity while the ice nucl e i count did not go significantly above background levels. This indicates that in these cases the nuclei may have been used in forming ice crystals too large to be ingested into the counter. The same result was noted in several other cases where diffusion and transport conditions appeared ideal, but nucl e i rel e ased at Emerald Mountain were not detected at Rabbit Ears during snowfall conditions. It is significant that the ice nuclei releases could be detected fairly predictably at a surface site some 42 kilometers away. Although ice nuclei con centrations at the surface are not as meaningful as nuclei concentrations in the cloud nucleating zone, they are the only measurements practical during moderate to heavy snowfall conditions. Counts during seeding can be corn- 121 |
