WMOD00265

• Project Name: Evaluation of Operational Seeding in Western Kansas
• Title: Evaluation of Perational Cloud Seeding in Western Kansas
• Prepared For: State of Kansas
• Prepared By: USBR
• Date: 11/1/1987
• State: KS
• Weather Modification - Doc Type: Report

ever in the target area, counting as seed years only those years when that particular cell was being
seeded. The results were not any different from those presented in figure 4.3 and so they are not
shown here.

4.4 Examination of Radar Records

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Interpretation of patterns like those of figure 4.3 requires some understanding of how
thunderstorms form and move across western Kansas. MicrofIlmed radar records for the seed
years (1975 to 1985 inclusive) were obtained from the National Weather Service to determine the
prevailing storm motions. To make the task manageable, three days were selected at random from
each month from May through August of each year and the microfIlm records for those days were
viewed to determine the storm motions. In cases where thunderstorm cells were grouped into
convective complexes or squall lines, the motions of the complexes or lines were recorded rather
than those of the individual cells.

In most cases it was possible to assign a mean motion that applied to all storms occurring between
noon and about 2200 local time, the period which accounts for most crop-hail damage. In a few
cases, such as when an important front passed during the afternoon, storm motions changed
radically. Usually the change corresponded to a veering of the steering winds, for example motion
from southwest to northeast early in the day and motion from north to south later on. No attempt
was made to divide or weight such days; we simply recorded two mean motions for each such day,

In agreement with earlier researchers, we found that most storms affecting western Kansas moved
out of the southwest or out of the northwest. A special analysis was then done on stonns that
reached to more than 45,000 ft above sea level, as previous studies have shown that most hail in
the Great Plains is associated with tall thunderstorms. The results are shown in figure 4.4. The
predominance of storms moving from the southwest or from the northwest is apparent. In general,
their motions can be related to upper-air troughs of low pressure over the region; storms occurring
to the east of (ahead of) a trough move from southwest to northeast and those occurring behind a
trough move from northwest to southeast. '

The speeds of the tall thunderstorms observed in the radar data sample varied widely, from 3 to 20
mis, but averaged about 11 mls (22 knots). The average speed appeared to decline from about 12
mls in May and June to just WIder 10 mls in August.

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4.5 Attempts to Use Control Areas

Control areas in the evaluation of hail suppression operations are of limited value. Hailfalls tend to
be spotty and therefore target-control correlation coefficients tend to be low. Nevertheless, Hsu
and Chen [1] reported that the use of control areas sharpened the significance of their evaluation of
hail suppression effects in the Western Kansas Weather Modification Project.

In the present study, loss-cost ratios for grid cells throughout the study area were correlated with
the average loss-cost ratio in the target area. The highest correlations were found for loss-cost
ratios recorded in the regions northwest and southwest and northeast of the target area. These
results generally agree with the expectations based on the radar data, but there was no region of
high correlations southeast of the target area The correlations are not high in any case, with the
largest ones calculated being in the range of 0.4 to 0.6. The correlation coefficients calculated are
comparable to those observed in other parts of the Great Plains.

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