WMOD00528

• Title: The Feasibility of Enhancing Streamflow in the Silver Iodide in the Sevier River Basin of Utah bt Seeding Winter Mountain Clouds
• Date: 12/1/1991
• Weather Modification - Doc Type: Report

effectiveness CUIVe for the type of AgI used, active modes of nucleation and their rates, dispersion of the
seeding agent, etc. However, stonns with higher SL W amounts tend to be wanner which compounds the
potential problem of providing insufficient ice nuclei. But these wanner, wetter stonns also tend to have
embedded convection present Impressions gained during the 1991 field season strongly suggest that
convection can transport AgI from even low-level valley generators to altitudes well above the barrier
where temperatures may be cold enough for nucleation.

Larger SL W amounts tend to occur with shallow clouds having wann tops that often are inefficient
precipitation producers due to a lack of natural ice crystals. Deep clouds with high, cold tops generally
produce high concentrations of ice crystals which can settle through the SL W condensate produced near
the mountain slopes and convert much of it to snowflakes or graupel. Huggins et al. (1989) showed
evidence for this tendency in that mesoscale bands had 2 to 4 times higher precipitation rates than adjacent
periods between bands but the latter had greater SL W amounts. This suggests considerable variability in
stonn seedability not only from stonn-to-stonn but throughout a stonn passage. Similar variability was
shown by Rauber et al. (1986) and Heggli and Rauber (1988). The bands shown by Huggins (~t al. (1989)
were only 5 to 15 kIn wide, Similar structure has been shown for winter stonns over the Cascade Range
in Washington, the Sierra Nevada of California, and the Mogollon Rim of northern Arizona..

As discussed earlier, the flux of SL W over the crest of a barrier represents the absolute maximum amount
of liquid that seeding could convert to ice crystals. Only a fraction of the ice crystals might reach the
surface as additional snowfall. Thus, seeding-induced precipitation will never exceed the SL W flux over
a barrier, but will be some fraction of that flux, The specific fraction that can be converted to
precipitation would be expected to vary with stonn conditions and seeding effectiveness and is difficult
to quantify with present knowledge.

Estimates of SL W flux were made for several Tushar data sets. Rauber and Grant (1987) estimated the
amount of SLW passing the crest for a single stonn in February 1983. This was done with rawinsonde
wind data for the elevation of the barrier crest and scanning microwave radiometer measurements collected
about 20 kIn west of the crest at a much lower elevation. It was necessary to nonnalize the radiometer
data to the zenith so the obselVations may not totally represent conditions over the crestline. However,
precipitation was very limited so most of the flux should have passed the downwind crestline as excess
SL W. The resulting estimate of total SL W flux for a 13-hour period was equivalent to approximately 13
percent of the mean annual runoff from the Beaver River which drains the west slopes of the Tushars (the
east slopes drain into the Sevier River).. This case study appeared to have substantial seeding potential.

Long (1986) reported on early 1985 data from the Tushar Mountains, also based on microwave radiometer
and rawinsonde windspeed measurements. During this season, the radiometer was at a significantly higher
elevation than during 1983, located only about 5 kIn west of the barrier crest. Similar data were presented
by Huggins (1990a) for the 1987 field season when the radiometer was located at the same site as in 1985.
Liquid water flux estimates for early 1989, given by Huggins (1990b), were again for the west slope
radiometer site used the previous two obselVational seasons.

During the above noted Tushar field programs the tenn "stonn" was used to designate a period with
significant cloud cover over the research area perceived to have some probability of producing
precipitation. As a minimum that meant a broken to overcast mid-level or lower level cloud deck.
Research periods coincided with stonn periods. It is of interest to consider a summary of SL W flux
estimates for stonn episodes during the 1985, 1987 and 1989 seasons. This provides the beginnings of
a "climatology" of possibly seed able events over the Sevier River Basin. While only three relatively short
field seasons are involved, actual SL W obselVations are much more relevant to the question of seeding

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