WMOD00312 ‎(2)‎

• Title: Cloud Seeding Feasibility for the Shasta-Trinity Watershed
• Date: 12/1/1993
• Weather Modification - Doc Type: Report

The numerical targeting model used in the Fixed Target Experiment was adapted for use in
ground seeding using Gaussian plume approximations. Output from this model showed the
plume position and dimensions along with predicted fallout of seeded crystals. This
information could then be compared to both aerial and ground sampling of silver. A review
of all 18 cases showed that only 4 of the 12 seeded eases had both good directional targeting
and predicted fallout of the seeding effects in the target area. The poor results were caused
mainly by warm ridgetop temperatures (>-5 OC), and winds blowing parallel to the barrier
carrying most of the seeding material to the north of the target (fig. 3.7). These results were
not surprising because previous winters had shown infrequent occurrences of temperatures
cold enough to provide rapid nucleation of the AgI, and a barrier parallel wind in the low
levels had been documented since the first year of SCPP.

The poor targeting results predicted by the model were substantiated by the silver in'snow
analyses. Of the 1681 snow samples collected and analyzed for silver, less than 10 pet had
silver above background levels (4 parts per trillion). Two locations that exceeded this average
were the northern end of the target area near Killgvale and the southern end of PG&E's
target area, where between 25 and 30 pet of the samples showed silver above background.

Aerial tracking of ice nuclei from the seeding generators showed that nuclei were moving
vertically up and in the general direction predicted by the targeting model. The results
showed the horizontal spread to be about 150 and the rate of rise about 30 em s-l. These
results are consistent with those reported by Holroyd et al. (1988) and Super and Heimbach
(1988) for AgI released from elevated locations over various mountain ranges in the
intermountain west.

3.4 General Conclusions

The extensive physical observations taken during the 10-yr SCPP program have improved
our understanding of winter mountain clouds over the Sierra Nevada. A better
understanding now exists of the temporal and spatial distribution of SLW, as well as the
quantities and temperatures of SLW potentially available for conversion to snow by seeding.
In general, SLW is highly variable both in space and time, occurs in rather low
concentrations, within a kilometer of the highest terrain, and at relatively warm supercooled
temperatures. The highest concentrations of SLW occur after frontal passage in shallow
orographic clouds with weak embedded convection. SLW exists for several hundred hours
per winter season even in years of well below normal precipitation. Radiometer data suggest
that many more hours of SLW occur than hours of precipitation. Mountaintop icing data
suggest that a portion of this SLW is above crest height because on average, only two-thirds
of the hours of precipitation have icing occurring at crest height.

If both upwind and downwind soundings are used as input, the numerical targeting model
developed on SCPP can provide useful predictions of the locations of both aerial and ground
seeding necessary to target a given location. This ability was verified by real-time application
of the model in the conduct of SCPP seeding trials.

Although direct observations of the complete link in the physical chain of effects from release
of the seeding agent to precipitation at the ground were limited, several general conclusions
can be made. Aerial seeding from one aircraft fills only a small portion of the necessary cloud
volume to sufficiently impact a target area of several hundred square kilometers. One
seedline may affect precipitation at the surface for only 10 min even after 45 min of travel

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