WMOD00554

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

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CLOUD DEPTH
scatter diagram comparing cube root of radar-
estimated rainfall (RER) to cloud depth (COP)
for no-seed and silver iodide cases of
. project Cloud Catcher in 1969-70.
dynamic effects as well, perhaps an be observed rather than the no-seed
intensification of secondary cloud values as shown. Comparison of columns
towers around the major precipitation 5 and 7 shows that the addit.ional impact
cells. of a 600-m increase in cloud height upon
rainfall would be comparable: to that
revealed by the regression analysis.

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- Ag I TREND

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Figure 1. -

The analysis to this point has
taken no account of possible increases
in COP, the depth of the tallest
precipitation echo in each test case.
An increase in COP due to seeding would
shift a point on figure 1 corresponding
to a seeded cloud some distance to the
right. Analysis of the Cloud Catcher
radar and aircraft data with the aid of
cloud models showed no increases in COP
associated with salt seeding, but
suggested increases in COP due to silver
iodide seeding, with maximum cloud top
in a test case rising, on average, by
perhaps 600 m (Dennis et al., 1975a).
The model analysis showed no skill in
assigning increases in COP to particular
days; however, the increases must vary
from day to day, or even from cloud to
cloud, depending on the ambient
conditions. Any changes in rainfall
associated with changes in COP would be
in addition to those indicated in column
5 of table 1. Column 7 of table 1
provides an estimate of the total
rainfall increases produced by the
combination of increases in COP (a
possible dynamic effect) and changes in
the relationship between RER and COP
(probably a combination of dynamic and
microphysical effects). The numbers in
column 7 were obtained by adding 600 m
to COP and applying (2); it would be the
enhanced cloud tops that would actually

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We repeat, for emphasis, that the
increases in COP would not be uniform.
Cloud model runs using North Dakota
pilot Project data (Dennis et al.,
1975b) suggest that the increases in
cloud height are greatest for clouds
with (unseeded) top temperatures in the
range from -10 to -30'C, that is, to
clouds with COP from 2 to 6 or 7 km, and
nonexistent for most cases with COP
greater than 10 km (Smith et al., 1986).
Because of the many uncertainties, there
are no entries in column 7 for clouds
whose depth is 10 km or greater.

2.4 Potential Effects of Seedinq
Assemblaqes of Convective Clouds
In order to determine the potential
impact upon Black Hills summer rainfall
of seeding all available convective
clouds, it is necessary to develop some
statistics regarding the size
distributions ,of natural clouds and the
contributions of the different size
ca'tegories to the natural rainfall. For
this purpose, it is not sufficient to
look at the size distribution of the
Cl,:)ud Catcher cases, because many
factors influenced their selection,
including location with respect to the

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