WMOD00413

• Title: Validation of Precipitation Management by Seeding Winter Orographic Clouds in the Colorado River Basin
• Date: 9/1/1993
• Weather Modification - Doc Type: Report

experimentation can provide a powerful approach to furthering knowledge of cloud responses to
seeding as demonstrated in summer convective clouds during the HIPLEX-1 (High Plains
Cooperative Program) Experiment (Cooper and Lawson, 1984; Mielke et al., 1984).

Direct detection experiments, which monitor the important links in the chain of events
following seeding, have been made increasingly practical by recently developed
instrumentation. Reclamation scientists have participated in such experiments starting with
the HIPLEX summer program in the late 1970s and, more recently, in winter projects in
California, Colorado, Montana, and Utah. Although some direct detection experiments strongly
indicated increased precipitation, others failed to do so, at least at the intended point target. In
the latter cases, identification of plausible reasons for the apparent failure often was possible
because of the comprehensive nature of the observations. These probable explanations led to
improved understanding and better experimental design. The sophistication of these
experiments has increased dramatically in recent years.

In addition to the direct detection experiments, a series of experiments with more limited
objectives will investigate specific processes. One example is investigation of the T&D of
seeding agents which can be accomplished with a specially instrumented aircraft and limited
supporting observations. These T&D experiments can be conducted even when full storm
conditions do not exist, with cloud base above the terrain permitting very low level sampling.
Another example ofa limited experiment is study of the concentration of ice particles created by
release of a seeding agent under particular temperature, moisture, and turbulence regimes.
Again, such observations can be made primarily by an instrumented aircraft with no attempt to
follow the seeding signature to the surface.

Both the direct detection and more limited physical experiments will markedly increase
understanding of the physics involved in winter orographic storms. This knowledge is required
before a statistical experiment can be properly designed.

The direct detection experimental approach is essential to understanding and verifying the
chain of events from release of seeding material to resulting snowfall on a small, well-
instrumented target. However, this approach does not permit accurate assessment of the
precipitation changes that would result from routine seeding over the long-term (several
winters) and over a significantly larger target area. Accurate assessment is impractical even if
results are available from several comprehensive (but short duration) direct detection
experiments conducted over a wide range of storm conditions. As stated by Hobbs (1975b), ''The
(direct detection) method should not be considered as an alternative to statistical evaluation
but rather as a logical precursor which permits determination of whether a particular seeding
technique can modify certain types of clouds and precipitation. Armed with such knowledge,
the procedures and statistical design and evaluation of further cloud seeding experiments can
be appreciably sharpened."

The results of the direct detection experiments will be used to design and carry out statistical
experiments to estimate average seasonal snowfall changes over two relatively large target
areas. The statistical experiments will be designed for confirmatory analysis, but exploratory
analysis will also be employed. Using the definitions of Gabriel (1981), "A confirmatory analysis
presupposes a rigidly carried out sequence of experimentation-measurement-analysis. For valid
confirmation or repudiation of the original hypothesis there has to be faithful adherence to the
original design." He later notes that "A confirmatory analysis may, however, also

14

I
I
I
I
I
I
'I
I
l
I
I
I
I
I
I
I
1
I
I