WMOD00554

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

respectively. Comparison of columns 3
and 5 indicates that rainfall from
clouds of any given depth up to 10 km is
increased by silver iodide seeding as
practiced in Cloud Catcher. Expressed
as a percentage of the expected or no-
seed rainfall, the calculated effect
decreases from small to large clouds;
expressed in terms of rain volume, it
continues to increase.

It is unlikely that the linear
relationship between (RER)'~ and CDP for
silver iodide cases shown in figure 1
extends to clouds with depths as great
as 11 or 12 km. Uncertainties about
inferences based on regression lines are
greatest when one deals with the
extremes of the size range, rather than
the mid-range, and extrapolation is
considered undesirable. The situation
is made worse in this case by the fact
that the largest value of CDP among the
silver iodide cases was just 10.1 km.
In view of indications from some cloud
mOdeling studies (Orville, 1984, p.54)
that silver iodide seeding of large

st:orms can rE!duce their precipitation
efficiency, and the lack of silver
iodide cases with CDP exceeding 10 km in
the Cloud Catcher sample, WEl can not
draw any conclusions about the effects
of silver iodide seeding on such clouds.
It is for this reason that the entries
in column 5 of table 1 end at CDP equal
to 10 km.

2.3 Consideration of Dvnamic Effects
The Clou.d Catcher radar data
indicated that the additional rainfall
associated with clouds of a given depth
was not due to increases in peak
rainfall intensity, but rather to
additions in space or time, or both, of
additional precipitating areas similar
to the natural showers (Dennis et al.,
1975a). In other words, the showers
responded to seeding by becoming more
numerous, increasing in horizontal
extent, lasting longer, or some
combination of all three responses.
Such a result could be produced by
microphysical effects alone, but it is
judged more likely to represent some

Table 1. - Relative abundance of clouds, rainfall per case, and
total contribution to convective rainfall, all as functions
of cloud depth, for no-seed and silver iodide cases

(1) (2) (3) (4) (5) (6) (7) (8)
2 48.4 0.00 0 0.02 <1 1.4 70
3 22.8 0.02 <1 4.9 110 17 390
4 12.0 4.9 60 31 370 64 770
5 6.8 31 210 96 6:,0 160 1090
6 4.0 95 380 220 880 330 1320
7 2.4 210 500 420 1010 580 1390
8 1.5 410 620 700 1050 930 1400
9 0.93 690 640 1100 1020 1400 1300
10 0.59 1100 650 1600 940 940
11 0.38 1600 610 61.0 610
12 0.24 2300 550 5~ 550
100 4220 7190 9830

(1) Cloud depth (CDP) in kilometers.
(2) Relative number of cases for a = 0.35 km-', normalized to 100.
(3) Rainfall (103.m3) per no-seed case.
(4) contribution to total convective rainfall (arbitrary units) from
all clouds expected with CDP in range indicated.
(5) Rainfall (103.m3) per silver iodide case according to Eq. (2),
assuming no change in CDP due to seeding.
(6) contribution to total convective rainfall (arbitrary units) from
all clouds expected with CDP in range indicated, assuming
all clouds seeded with silver iodide and no change in CDP
due to seeding.
(7) Rainfall (103.m3) per silver iodide case according to Eq. (2),
assuming an increase in CDP of 600 m due to seeding. In this
case, the CDP shown in column 1 refers to the cloud left
unseeded. The observed clouds would be 600 m taller.
(8) Contribution to total convective rainfall (arbitrary units) from
all clouds expected with (unseeded) CDP in range indicated,
assuming all clouds seeded with silver iodide and an
increase of 600 m in CDP due to seeding for all values of
CDP up to 9 km.

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