WMOD00567

• Title: A Report on Winter Snowpack-Augmentation
• Date: 11/11/1988
• Weather Modification - Doc Type: Report

1298

The utility of two types of numerical models will be reviewed
here.
The first to be discussed is the diagnostic models utilized
for determining appropriate sel~ding locations. Hobbs (1975a)
described such a model used in the Cascades and Rauber et al.
(1988) describe a similar two-dimensional model utilized in
SCPP. The SCPP model utilizes upper-air soundings obtained
near the foothills and at the crest of the Sierra Nevada to provide
the input necessary to calculate the u, u, and w, wind profile
over the barrier. This model-produced wind field has been
shown to represent the wind flow as verified by in situ aircraft
survey flights. Parameterized microphysics, using the results
of Ryan et al. (1976) (now one could use the more explicit
results shown by Prasad et al. 1989, pers. comm.) and observed
fall velocities (e.g. Locatelli and Hobbs 1974), allow calcu-
lation of particle trajectories. The combination of these two
processes allows determination of the seeding-line location
needed to correctly target a sp1ecific area on the ground. Sim-
ilar types of calculations, using appropriate dispersion estimates,
are being tested for ground rekases.
More sophisticated models such as Clark and Fadey's (1984)
three-dimensional time-dependent, primitive-equation, nested-
grid model offer the promise of modeling the more complex
thermodynamic and microphysical processes taking place over
the mountain. These models are beginning to show promise in
reproducing the observed horizontal and vertical windfields for
storm situations over mountains, and further showing that liq-
uid-water regions should appear low over the mountain as ob-
served. Unfortunately, we are a long way from understanding
the complex microphysical processes occurring in even simple
orographic clouds. Additional laboratory studies combined with
field observations and seeding experiments will be needed be-
fore these models can be sufficiently improved to address such
issues as "extra-area". effects associated with seeding.
Having details of the three-dimensional windfield may allow
selection of appropriate ground generator sites to improve seed-
ing material delivery. This might help address the feasibility
of attempting a seeding program in a given location. As com-
'puters increase in speed and capacity and our knowledge in-
creases, more sophisticated models will be developed. Once
these models can be appropriately verified, as is being at-
tempted by the WMO (1986) for currently available models,
they could be applied to new geographic locations in an attempt
to understand the complexities of flow fields and precipitation
mechanisms in these areas. Unfortunately these models are
probably many years away.

6. Summary and future direction

[ .

~~

In this review, we have simplified and generalized the very
complex processes taking place in winter orographic clouds.
The variability of SLW and cloud ice (including primary and
secondary nucleation processes [see Hobbs and Rangno 1985])
are significant. However, these are problems that tend to mask

6 "Extra-area" refers to seeding effects occurring outside the in-
tended target area. Those effects might be related to both increases or
decreases in precipitation as a result of the seeding.

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Vol. 69, No. 11, November 1988

or compete with the seeding effects. We have tried to describe
what has been observed and to show that a physically consistent
picture is beginning to emerge. This picture may be less op-
timistic than one would like, due mainly to low and highly
variable liquid-water contents observed and the temperature
regime of SLW. However, those programs defined in the in-
troduction that have statistically inferred precipitation increases
from seeding are consistent with the physical observations taken
in close proximity, and are further substantiated by theoretical
particle growth models. With a better understanding of nu-
cleation mechanisms and transport and dispersion processes the
stage is set for developing new strategies for seeding-agent
delivery .
There is a continuing need for research efforts to document
the physical-links in the chain of effects leading to winter snow-
pack augmentation. Also, studies such as those conducted in
the Bridger Range are very important. These types of studies
are providing the critical scientific plausibility to statistically
inferred precipitation increases. A great deal of progress has
been made to date and results presented here have begun to
provide limits to anticipated effects from seeding, at least for
the types of clouds and liquid-water contents described. Larger
sample sizes are needed to confirm the reproducibility of these
results. With the current level of sophistication in measuring
equipment, it is possible to document the effects taking place
in individual aerially released seedlines. Therefore, one could
randomize on a per seedline basis, thus greatly expanding the
sample size necessary to reach statistical significance without
a IO-year randomized program, which is not possible in today's
budgetary environment. Tracer studies incorporating both AgI
and SF. are providing information useful in determining the
potential effectiveness of ground-based seeding. The combi-
nation of these tracer studies with diagnostic target schemes
(section 5) may allow reliability in targeting specific areas on
the ground. Adaptation of PRO FILER (Hogg et al. 1983) tech-
nology to winter cloud seeding is also needed. A network of
PRO FILERs will allow excellent monitoring of changing wind,
temperature, and liquid-water fields that can be incorporated
into these simple models for updating operational decisions.
Another major step forward has been the application of an
icing-rate meter to ground operations for monitoring icing near
the barrier. By providing a real-time readout of icing plus
temperature and wind information, better implementation of
ground seeding can be performed.7
Seeding delivery remains a critical problem. This along with
the lack of a time-dependent nucleant at higher temperatures
(> - 5aC) still limits the utility of ground and aerial seeding
in the warmer and more moist storms. There is obviously much
work Ita be done in this important area.

Acknowledgments. This manuscript was prepared as an invited paper
to the IVGG XIX Conference on Weather Modification. Funding for
this work was provided by the Bureau of Reclamation.

7 The State of California is now designing a winter cloud-seeding
program utilizing liquid propane as a seeding agent (Vardiman et aL
1971) released along the crest of the Sierra Nevada. Icing-rate data,
along with temperature, wind speed, and wind direction telemetered
to the project headquarters will be utilized to make real-time seeding
decisions.