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

data set and concluded that precipitation increases of 10 pct or more could have been achieved
if seeding had been limited to only favorable cloud conditions. Also, a limited number of
physical studies during the last season of the CRBPP provided evidence that seeding potential
exists in the latter stage of winter storms over the San Juan Mountains associated with the
release of convective instability (Cooper and Marwitz, 1980).

Since the end of the CRBPP, the only major winter orographic cloud seeding project conducted
in the West was the SCPP (Sierra Cooperative Pilot Project) reviewed by Reynolds and Dennis
(1986). The main purpose of the SCPP was to determine when seeding of cold winter clouds
over the central Sierra Nevada of California produced increases, decreases, or no change in
precipitation, and to document the magnitude of the changes. In addition, extensive physical
studies were conducted of three-dimensional airflow and microphysical processes (both cloud
liquid water and ice processes) occurring in natural winter cloud systems. Examples include
Heggli et al. (1983), Heggli and Rauber (1988), Marwitz (1987a), and Marwitz (1987b). These
studies led to the development of an analytical targeting model (Rauber et al., 1988) that was
used the last 3 yr of the program (1984-1986). The latter years of the SCPP emphasized
physical observations of the effects of seeding shallow winter clouds (Deshler et al., 1990). Deep
cloud systems appeared to be naturally effective in snowfall production (Marwitz, 1987b). The
SCPP used the most extensive array of remote and in situ observations to date for monitoring
physical processes following seeding.

The SCPP attempted to document the important links in the chain of physical events from
airborne release of seeding material to snowfall on the target site. However, a number of
problems were encountered, including frequent high natural concentrations of large cloud
droplets and/or ice crystals, both associated with maritime clouds near the Pacific Ocean, which
made it difficult to discern those ice particles caused by seeding. The aircraft seeding affected
limited cloud volumes because of slow dispersion rates. Although SLW was shown to be present
for many hours in winter cloud systems, it was concentrated at low levels, near the barrier top,
where temperatures were usually too warm for AgI seeding (0 to -5 OC).

The logistics of coordinating the several observing platforms caused problems for the SCPP.
These problems included frequent airspace conflicts caused by high traffic density and, more
important, the inability to fly the observing aircraft at low levels over the rugged terrain.
Heggli et al. (1983) stated that "altitude restrictions prevented the aircraft from flying within
approximately 1 km of the Sierra terrain." As discussed later, most of the SL W over mountain
barriers exists in the lowest kilometer above the ground. The SCPP study area was not
amenable to direct detection seeding experiments. As discussed by Deshler et al. (1990), only 2
of 36 experiments were able to document the complete chain of events following seeding, ending
with snowfall at the ground. However, the research aircraft was able to document within cloud
seeding effects in 35 pct of the sampled seedlines.

2.3 Opportunity to Advance the Technology

Several factors allow significant improvements to past seeding experiments, including:

. An improved comprehension of the importance of proper targeting of seeding material and
how to achieve it.
. A better appreciation of the mountain barrier characteristics needed for practical
experimentation.

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