WMOD00554

• Title: Seeding Summertime Convective Clouds to Increase Black Hills Rainfall
• Weather Modification - Doc Type: Report

on a test case if the updrafts lasted
long enough. There was no attempt to
overseed the clouds to produce dramatic
dynamic responses.

Eighty Cloud Catcher cases were
recorded in 1969 and 1970, with 33 of
them being no-seed cases, 18 silver
iodide cases, and 29 salt cases. The
relative lack of silver iodide cases can
be attributed in part to the lack of
sufficiently powerful blocking in the
randomization scheme. In 1970 the random
decision for the first test case of each
day was applied to subsequent cases on
that day, if any. This arrangement,
which was intended to reduce
possibilities of contamination, had the
unfortunate effect of aggravating the
imbalance among the three classes of
test cases.

2.2 Effects of Seedinq on Individual
Clouds
The radar data from Cloud Catcher
showed that cloud seeding with either
silver iodide or salt can stimulate
precipitation in cumulus congestus
clouds. "First echoes" from new
precipitation shafts in clouds seeded
with either silver iodide or powdered
salt appeared closer to cloud base than
first echoes in unseeded clouds (Dennis
and Koscielski, 1972). The average
height of first echoes above cloud base
was 3.4, 2.1, and 1.6 km for the no-
seed, silver iodide, and salt cases,
respectively. This result is evidence
that seeding speeded the formation of
precipitation in the clouds studied, as
is the fact that clouds greater than
2 km deep generally precipitated after
seeding with either silver iodide or
salt, whereas unseeded clouds did not
precipitate until they reached about
4 km depth.

Dennis et al. (1975a) presented a
statistical analysis of the Cloud
Catcher test cases of 1969 and 1970. In
particular, they studied RER as a
function of cloud depth (COP), which was
defined as the highest radar echo top
recorded in the test case minus the
height of the cloud base. They obtained
straight-line relationships by plotting
the cube root of RER against COP.
Figure 1, which is patterned after their
figure 4, has been replotted using data
from table A.2 of Dennis et al. (1974).

Dennis et al. (1975a) found the
equation of the regression line ,for the
no-seed Cloud Catcher cases to be:

2A metric ton is the mass of 1 m3 of water.

/

(RER)1/3 = -4.02 + 1.43(CDP) (1)

where RER is in kilotons2 and CDP is in
kilometers.

Dennis et al. (1975a) found an
indication that rainfall from salt cases
exceeded that from no-seed cases of the
same depth, but the statistical
significance of this result was somewhat
weak, with the p-value for the test for
differences in adjusted means being
0.06. [The test for differences in
adjusted means allowed for the fact that
the distributions of CDP varied for the
three classes of test cases. CDP
averaged 6.5 km for the no-:seed cases,
5.2 km for the silver iodide cases, and
5..8 km for the salt cases.)

RER for the silver iodide cases
exceeded tha1: from no-seed cases of the
same depth. (It should be noted that,
on figure 1, the values of COP for the
silver iodide cases are tho::;e actually
observed, ra1:her than the values that
would have been observed had the clouds
bElen left unseeded.) The difference in
adjusted means was statistically
significant, with the p-value being 0.01
under the assumption that all test cases
WE!re independent. Al though the
significance is weakened by the fact
that in 1970 all cases occurring on one
day received the same treatment, this is
a surprisingly positive result to emerge
from only two seasons of
experimentation. It is assumed, for the
purposes of the exercise presented in
this paper, that real differences exist
between the no-seed and silver iodide
cases.

Dennis et al. (1975a) derived the
following regression equation for the
Cloud Catcher silver iodide cases:

(RER)1/3 == -2.62 + 1.44(CDP) (2)

where the units are as in (1). The
difference in the intercepts of (1) and
(2) is 1.40, which is very close to the
change associated with a l-km difference
in CDP. That is, the value lof RER for a
silver iodide case approxima'tes that for
a no-seed case with CDP 1 km greater.

Table 1 has been prepared to
provide additional insight into the
implications of (1) and (2),
particularly when combined with
information to be presented l)elow on the
frE~quency distributions of shower sizes.
Application of (1) and (2) lE~ads to
columns 3 and 5 of table 1,

A kiloton is equivalent to 103 m3.

2