WMOD00270

• Title: The Southern Plains Experiment in Cloud Seeding of Thunderstorms for Rainfall Augmentation Phase II ‎(SPECTRA)‎
• Prepared For: The Texas Department of Licensing and Regulation
• Prepared By: Woodley Weather Consultants
• Date: 12/28/2005
• State: TX
• Weather Modification - Doc Type: Report

Figure 11. Plots of the flights tracks of the seeder in yellow (Seed 3) and cloud physics in green (Seed 1) aircraft
for the single seeded case obtained on May 25, 2005. The short segment of the green flight track that appears in
red is the region where the SF gas was detected.
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Documentation of the pass during which the SFgas was detected is provided in Figure 12. In
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constructing this graph the SF gas plot was adjusted to take into account the 7-sec lag in the response of the
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instrument. Upon accounting for this lag note that the large peak in the gas plot coincides nicely with the plot of
the largest measured droplet size, indicating that this is a real peak in the SFgas. During the latter portion of
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the overall plot, however, there is an indication that the SFgas was measured outside the confines of the cloud.
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This is not real. It is due to the confounding effects of updrafts on the SFdetector, since updrafts have been
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shown to produce increases in the measured voltage by the detector, producing false indications of the presence
of the SF gas.
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The 1-sec plots of the droplet distributions shown in the right panel for the segment of the overall plot
highlighted in yellow in the left panel of Figure 12 show a tight clustering of the plots except for the right tail of
the plots where larger drops were measured. It should be noted that this figure includes the measurements of
both the CDP and CIP instruments such that the entire droplet distribution from the tiniest cloud drops to
raindrops has been measured. The measurements of the CIP instrument were not available on May 14, 2005.
Although the droplet measurements in the portion of the flight were the SFgas was detected probably
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represents an effect of hygroscopic seeding. It is virtually impossible to prove that this is the case here, because
similar droplet measurements were made deeper into the cloud where the SF gas was not present. The cloud
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was much taller during the latter half of the cloud pass and thislikely explains the presence of the larger drops.
This is illustrated in Figure 13 for the highlighted portion of the pass where the larger drops were measured
upon entry into the downshear rain shaft. Similar figures are available for the entire pass, but there was no point
in reproducing them here. The single illustration for the downshear rain shaft in Figure 13 should suffice.
Figure 12. Documentation of the droplet sizes (on the right) in the seeded cloud on May 25, 2005 in the portion
of the overall cloud pass that is highlighted in yellow (on the left). The CDP and CIP instruments provided the
measurements in this upshear portion of the cloud pass about 5,000 ft above cloud base. The plot for the SFgas
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has been adjusted to account for the 7-sec temporal lag in the response of the detector.
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