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SEPTEMBER 1996 REYNOLDS 1443 4 T ~~ E o Q. == W I- -4 -8 i? 0.4 E S 0(,), 0.2 ~ 0.0 2700 :[ ~ 2400 c( 1752 1754 1:756 /~ 1758 1800 i=' Q. !!::. co lL. III 2100 80 18.05 40 o 17.85 17.90 17.95 Time (UTe) FIG. 11. Variables observed or calculated from aircraft observa- tions made during plume tracing experiments during VFR conditions on 9 March along the valley track. From top to bottom: temperature and dewpoint, cloud liquid water, aircraft altitude (time in hours and minutes given with altitude), and SF6 concentration. Distinct SF6 plumes are seen from site 7 and then site 9. Pattern 2 is shown in Fig. 8. These days show the lack of the strong downdraft just after the balloon launch. It appears that the crest of the lee wave has moved toward the mountain with good updrafts where downdrafts were located in pattern 1. Pattern 2 is typ- ical of the lighter wind cases, which normally occur postfrontally or near upper-level closed lows. In these situations, the atmosphere is less stable, which damp- ens the amplitude of the lee wave. Note that lighter winds are indicated by the fact that the balloon has translated only 6 or 7 km downwind by the time it reaches 5000 m, in contrast to the 15 km for pattern 1. The 5 February 1993 case shown, representative of this pattern, was a case when the aircraft observed the high- est vertical transport of SF6 of the nine releases made in 1993. Tracer studies and balloon vertical velocities indicate that the higher wind and more stable situations that normally occur prior to frontal passage are less con- ducive to ground-based seeding. The lighter wind and less stable environment, which most often occur post- frontally, are more conducive to seeding (targeting). These results complement the results summarized by Reynolds (1988), which noted that postfrontal cloud conditions over the Sierra Nevada have the highest liq- uid water content and the coldest supercooled cloud temperatures, which would also be more conducive to glaciogenic seeding. We will examine the 1993 tracer studies in more detail to see if these results can be con- firmed. 3. Aircraft tracer studies a. Experimental procedures The procedure followed was to remotely release the SF6 at dispenser site 7 and/or 9. Release rates varied from 20 to 25 kg h -) . The transport and horizontal dis- persion of the gas was monitored by aircraft and ground samplers. The ground-based sampling procedures were similar to those used during the previous two winters except that the ground-based continuous analyzer was operated at a fixed location (ICC) instead of a mobile van dlriven within the valley. The NOAA aircraft flight pattern used during the 1993 research missions is shown in Fig. 9. The flight leg flown between PI and P2 was the valley track flown at a nominal flight level of 2500 m. The flight leg be- tween P3 and P4 was the ridge track flown at 2680 m. If SF6 was observed at these nominal altitudes, the air- craft would fly the next set of flight legs 200 m higher. The flight altitudes would be incremented at 200-m in- tervals until no tracer above the background was ob- served. During VFR (visual flight rules) flights (4 days), these same tracks were flown but at flight levels down to 2100 m. At times the aircraft would fly short legs farther downwind between P3 and P6, and from P6 to N. During IFR (instrument flight rules) condi- tions, these legs were flown at 2600 m. At an airspeed of 80 m s -), it would take about 15 min to complete 90 80 70 " f60 e:. so ~40 " 0 ~30 20 10 0 2100 2200 " " " " " " ". " 2300 2400 2500 ALTIIUDE (M) 2600 2700 FIG. 12. Average SF6 concentrations plotted with respect to altitude for all SF6 plume penetrations from site 9 along the valley track.