WMOD00473

• Title: Evaluation of Bridger Range Winter Cloud Seeding Experiment Using Control Gages
• Date: 12/12/1983
• Weather Modification - Doc Type: Report

,

DECEMBER 1983

ARLIN B. SUPER AND JAMES A. HEIMBACH, JR.

1999

TABLE 2. Ridge temperature partitions: fractions of all days. Wilcoxon (W) and MRPP one-tailed probabilities are for 3 zones of Fig. 1.
Experimental days Zone I Zone 2 Zone 3
Range
(DC) All sl/ns2 W MRPP W MRPP W MRPP
-26.0/+5.0 185 90/95 0.17 0.22 0.35 0.36 0.12 0.12
-9.4/+5.0 93 48/45 0.31 0.36 0.22 0.30 0.50 0.50
-26.0/-9.5 92 45/50 om 0.01 0.09 0.11 0.07 0.05
-7.3/+5.0 62 29/33 0.33 0.34 0.42 0.50 0.45 0.50
-11.5/-7.4 61 32/29 0.30 0.41 0.10 0.11 0.34 0.21
-26.0/-11.6 62 29/33 0.02 O.oz 0.02 0.02 0.03 0.02
~6.0/+5.0 47 22/25 0.33 0.36 0.33 0.30 0.26 0.24
-9.4/-6.1 46 26/20 0.35 0.25 0.08 0.12 0.22 0.24
-13.3/-9.5 47 24/23 0.11 0.15 0.24 0.31 0.11 .0.14
-26.0/-13.4 45 18/27 0.01 0.Ql. 0.01 0.02 0.01 om
I seeded.
2 nonseeded.

amount in Zone 3 was only 0.15 mm as compared
with 0.40 mm for the 22 seeded days. Obviously, pre-
cipitation amounts were often limited in this zone for
both seeded and nonseeded days. Interestingly, only
two seeded and four nonseeded days had zero precip-
itation at all gages for the partition. Thus, snowfall
usually occurred on these experimental days, but it
was very light at these gages, which were generally
located in a dry valley to the lee of the Bridger Range
(see Fig. 1).
A number of partitions were examined with the
highest temperature set .at -9oe but with the lowest
temperature limit varied. These showed no improve-
ment over inclusion of all the cases colder than -90C.
This suggests that the apparent seeding effect was
maintained for the coldest portion of the available
cases; however, the range in question is not large. Of
the 100 days with mean Ridge temperature ~ -9oe,
82 were in the range of -16 to -9oe. It is noteworthy
that this contains the dendritic ice crystal range around
-15oe where growth rates are maximum. It should

also be noted that few cases with temperatures colder
than -.20oe existed so these data are not appropriate
for testing the cold side of Grant and Elliott's (1974)
conceptual cloud seeding temperature window, which
should be applied to AgI plume top rather than cloud
top in the particular situation of the Bridger Range.
The total population of 185 days in Table 2 was
further divided by consideration of data from each of
the two winters separately. It was reasoned that in-
creased credibility could be given to the results if they
were similar for each winter. This is a demanding test,
since the population size is approximately halved.
Table 4, Part A for the 1970-71 winter indicates
probabilities of 0.06 or less in all 3 zones by both
statistical tests for temperatures ~ -11 oC. Part B of
Table 4 shows the results for the winter of 1971-72.
Probabilities of 0.05 or less were achieved by both tests
in Zone 1 for the 24 days with temperatures colder
than or equal to. -13 oC. Probability values for Zone
2 are 0.05 and 0.08 for the Wilcoxon and MRPP,
respectively with higher values in Zone 3. While these

TABLE 3. Ridge temperature partitions: colder days by 2DC increments. Probabilities and double ratios are for 3 zones of Fig. 1.

Experimental
days Zone I Zone 2 Zone 3
Probability Double ratio Probability Double ratio Probability Double ratio
Range
(DC) All sins W* MRPP Mean/Median W MRPP Mean/Median W MRPP Mean/median
~ -7.0 131 65/66 0.04 0.04 1.35 1.37 0.33 0.25 1.11 1.08 0.20 0.13 1.17 2.15
~ -9.0 100 44/56 0.01 0.01 1.56 1.29 0.13 0.14 1.38 1.46 0.09 0.08 1.43 3.35
~ -11.0 68 31/37 0.01 0.02 1.69 2.02 0.06 0.08 1.78 2.01 0.02 0.02 1.90 5.45
~-13.0 49 22/27 0.01 0.01 1.90 2.35 0.01 0.01 2.93 3.40 0.005 0.004 3.24 5.96
~ -15.0 26 10/16 0.30 0.15 1.06 1.18 0.05 0.09 2.02 2.81 0.02 0.02 2.08 7.86
* Wilcoxon.