WMOD00554

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

where the predicted increases come from;
this can be seen in figure 2, which
shows the rainfall contributions as a
function of CDP for both no-seed and
silver iodide cases, as well as in
table 1. Small clouds are numerous and
respond well to seeding (percentage-
wise), but they produce very little rain
whether seeded or not, so the impact of
seeding them is not great. On the other
hand, large clouds are scarce and the
effect of seeding (percentage-wise)
drops off as one moves to large clouds.
The contribution to the additional
rainfall peaks for CDP between 6 and
7 km. Clouds in this size range are
significantly smaller than the ones that
produce the bulk of the natural
rainfall.

[ . SEED 2

--------~--~- ---- -

Wi SEED 1 mill NO SEED i

_____~ .~__~~_.____J

1500 I ;' .- .....
1250 ,"
R ..-
A 1000 .' . ,
I ,.
N 750 ."
F ..
~.
A ... ~
. /
L 500
L . .
... .. /
250 .
~Il ,
"

o
2 3 4 5 6 7 8 9 10 11 12
CLOUD DEPTH (km)
Figure 2. - contributions to total
shower rainfall from clouds
of different depths under
assumptions of no seeding,
silver iodide seeding
without changes in cloud
depth (Seed 1), and silver
iodide seeding with 600-m
increases in cloud depth for
clouds with depth less than
10 kIn (Seed 2).

The entries in column 8 of table 1
for all values of CDP up to 9 km are the
products of the corresponding numbers in
columns 2 and 7. They account for the
possibility of increases in CDP of 600 m
as a dynamic response to silver iodide
seeding. On the assumption that there
would be no increases in maximum height
for clouds with CDP of 10 km or greater
(Smith et al., 1986), the entries of
column 6 are repeated in column 8 for
these large clouds. The differences
between columns 4 and 8 of table 1 (see
also fig. 2) suggest a 2.3-fold increase
in total shower rainfall through a
combination of microphysical effects,
with the maximum contribution coming

frClm clouds or cloud clusters with
values of CDP around 6 kIn.

In summary, the Cloud Cat,cher
results, combined with some modeling
studies and data on size distributions
of shower echcles, indicate that silver
iodide seeding could increasE~ total
rainfall from isolated convective clouds
or cloud clusters near the Black Hills
by a factor somewhere between 1.5 and
2.5" with much of the increase coming
from moderate shower clouds or small
thunderstorms with tops from 7 to 10 km
above sea level.

In order tCl draw conclusions about
the impact of seeding isolated
convective clouds upon total summer
rainfall near the Black Hills, one
requires information on the contribution
that isolated convective systems make to
that rainfall. It is also necessary to
consider the possibility that seeding
one convective cloud might suppress
rainfall from other clouds in the
vicinity through dynamic effects (e.g.,
Simpson and Dennis, 1974). Some
information about both of these points
can be drawn from the results of the
Rapid Project although, as we shall see,
some questions remained unans'wered
following its completion.

3. THE RAPID PROJECT: AN EXPERIMENT ON
AREA RAINFALL

3.1~xperimental Desiqn
The Rapid Project of 1966-68 has
been described by Dennis and Koscielski
(1969). It used a randomized crossover
design, with two pairs of target areas
laid out east of the Black Hills, one
pair for days with southwest flow and
one pair for days with northwest flow.
This arrangement minimized cr()ss-target
cont:amination. Each target area
encompassed about 2500 km2.

Days were classified in advance as
southwest-flow or northwest-flow days.
They were also classified according to
the synoptic situation into four types,
of which the most important were shower
days and storm days. Shower days and
storm days had the same requirements
regarding precipitable water and upper
winds as determined from the Rapid City
rawinsonde. storm days were
characterized by forcing by large-scale
weather systems, as evidenced by
positive vorticity advection at the
500-mb level; they normally produced
substantial rainfall, often from squall
lines. Shower days were characterized
by absence of positive vorticity
advection at the 500-mb level; they
normally produced only isolated showers,

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