WMOD00503

• Title: Seeding of Winter Orographic Clouds: A Viable Technology for Precipitation Enhancement?
• Weather Modification - Doc Type: Report

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Integrated Liquid Water (mm)

I" Figure 1. Cumulative distribution of integrated supercooled
i liquid water as measured by a vertically pointing microwave .
i radiometer at various field site locations noted. Averaging
i interval varied from 15 min (Utah) to 1 hr (Colorado and
. California)

consistent from California to Colorado regardless of natural
ice enhancement. There is a small but sufficient and
consistent amount of residual SLW that if properly treated
could produce economically meaningful increases in
precipitation. Possibly the most significant impact of
natural ice enhancement mechanisms is to increase the
variability of cloud particle concentrations. The increased
variability masks seeding effects and extends the time
necessary for developing unequivocal physical evidence of
cloud seeding's efficacy.

,

A seeding technology is concerned with converting
SLW to meaningful amounts of additional precipitation.
Factors to be considered are the large volume of cloud over
which the SLW is distributed (several thousand cubic
kilometers) and the relatively short time available for
converting SLW to precipitation before it is evaporated in
the lee of the mountain. The most important problem
hampering the development of a seeding technology is
delivery of the seeding agent. The agent must be delivered
in sufficient quantities and over a large enough volume to
produce a significant decrease in SLW concentrations while
producing an economically meaningful increase in
precipitation. The simple seeding experiments conducted
in shallow layer clouds and supercooled fogs have led to
the idea that just a few grams of AgI or CO2 are sufficient
to seed tens of square kilometers. This idea in turn has led
to the concept that just a few ground seeding devices or a
single seeding aircraft can effectively treat an entire
watershed of several thousand square kilometers.

Limited seeding trials within shallow, widespread
orographic clouds have demonstrated that glaciogenic
seeding materials can produce dramatic increases in ice
crystal concentrations. In a few instances these in-cloud
increases have been shown to increase precipitation rates at
the ground by 0.1 to 1 mm/h. Although only a few well
documented cases exist, these observations are quite

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-- consistent with calculations of iCe particle growth for the
SLW concentrations and temperatures in which seeding was
conducted.

These studies indicate that the volume of cloud
treated by a single aircraft or by a single ground dispenser
is limited. Dispersion rates are approximately 1 m/s in the
horizontal and 0.1 mls in the vertical for a single aircraft
produced seedline. Ground releases result in plumes 15
degrees in the horizontal and 1000 m in the vertical (Super,
1990). Multiple aircraft and/or a large number of ground
seeding dispensers at relatively high elevations on the
barrier are required to fill enough cloud volume to produce
meaningful effects. This is further exemplified by
Reynolds et al. (1989) and by Super (1990) who indicate
that when snow samples are collected from a target area
treated by either aircraft or ground based AgI dispensers,
only a small percentage (10 to 20%) of the samples hav.e
silver concentrations sufficiently above background levels.
This finding not only raises the issue of volume filling but
of targeting accuracy as well.

The question of targeting has led to a great deal of
l~ffort in documenting the transport of aerosols over
(:omplex terrain through the use of tracers such as sulfur
hexaflouride (SFJ or AgI. It has also led to the
development and application of two- and three-dimensional
numerical models (Clark et al., 1992 (this conference),
Rauber et al., 1988, Orville et al., 1984 ) to simulate the
kinematic and microphysical processes in winter storms so
that seeding effects can be better predicted.

Our limited understanding of orographic clouds and
(If the results of seeding these clouds cannot refute the
published statistical results from winter seeding programs
conducted over the Sierra Nevada, the Bridger Range in
Montana, or the central Colorado Rockies. Physical
studies conducted in close proximity to these programs
provide plausibility but not proof to the reported
precipitation increases. This is why the current AMS
Policy Statement (AMS, 1985) on winter cloud seeding
supports seeding-induced seasonal precipitation increases on
the order of 10% in properly conducted programs. A
question to be asked is "Are current operational programs
being conducted properly?"

From the scientific perspective, controversy on
seeding of winter mountain clouds will continue until the
complete link in the chain of physical events from release
of the seeding agent to fallout of the augmented
precipitation is well documented and supports the pre-
experiment seeding hypothesis. These results will then
m~ to be confirmed through a randomized demonstration
program which can quantify economically useful increases
in precipitation within the intended target area.

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3. THE OPERATIONAL DECISION MAKERS
PERSPECTIVE - THE CALIFORNIA EXPERIENCE

Currently there are four documents that a water
manager could read for guidance on cloud seeding for
precipitation enhancement. The World Meteorological