WMOD00293

• Title: Summary of the NOAA/Utah Atmospheric Modification Program: 1990-1996
• Date: 9/1/1998
• State: UT
• Weather Modification - Doc Type: Report

. .

systems to augment snowfall in mountainous regions, and ultimately to increase runoff in the spring and
summer months.

'-

Prior to 1990 the project focused on the evolution of supercoole~ liquid water (SL W) in winter storms
over the Tushar Mountains. Long et al. (1990) and Sassen et al. (1990) documented the importance of
synoptic, mesoscale and local topographic forcing in SLW development. Super and Huggins (1993)
summarized SL W flux estimates from four seasons of microwave radiometer measurement in the Tushar
Mountains and the Wasatch Plateau, finding that nearly all winter storms contained supercooled liquid
water that was not being converted to precipitation augmentation potential of winter storms by comparing
SL Wand precipitation fluxes. All these studies found that significant snowfall augmentation potential
existed, provided an appropriate cloud seedingteqhnique could be applied at the proper time and location
to convert excess SL W to ice crystals, and ultimately to snowfall.

Beginning with a field program in 1991 on the Wasatch Plateau, the Utah AMP shifted its emphasis to
studies of cloud seeding aerosol dispersion from silver iodide ground-based generators in valleys upwind
of the plateau, and from generators positioned partway up the windward slopes of the plateau (referred to
later as high altitude generators), and to studies designed to detect the effects of seeding. Supercooled
liquid water studies focused on the spatial distribution of SL W across the plateau using a new mobile
radiometer technique, Huggins (1995) summarized the first mobile radiometer experiments from 1991,
finding an expected maximum in SL W depth over the windward slopes, with decreasing depths across the
top of the plateau due mostly to removal by precipitation. Griffith et al. (1992), Heimbach and Hall
(1994), Holroyd et al. (1995), and Super (1995a) report the results of plume dispersion studies, including
the modeling of plumes, and the detection of microphysical seeding effects during seeding experiments in
1991 and 1994. Aerosol plumes from high altitude and valley-based generators have been detected
frequently (somewhat less frequently for valley releases) by ground-based platforms on the plateau, and
by aircraft flying above the plateau.

...

There remains a need for ,further evaluation of seeding plumes, particularly to determine the frequency
with which they reach appropriate altitudes (temperatures) and liquid water conditions for ice crystal
nucleation by the silver iodide compound currently used in Utah's operational project. Although some
indications of microphysical effects from ground seeding have now been documented, it remains to more
fully quantify the impact of seeding on precipitation under various winter storm conditions.

This paper focuses on the use ofthe mobile radiometer, in conjunction with another ground-based mobile
instrument platform, to document cloud conditions within seeding plumes, and to show further evidence
of the natural depletion of SL W across the Wasatch Plateau. Results are presented for one case study
where the impacts of high altitude seeding were apparently detectable on radar.

SUMMARY OF RESULTS

A multiple radiometer technique was used to demonstrate upwind/downwind differences in SLW depth,
which had previously been indicated by mobile radiometer passes across the same mountain parrier. An
example from 7 February indicated downwind depths averaged about 0.5 mm less'than upwind depths
during a two hour period of comparison over a 10 km distance. Converting this depth difference to a flux
. difference, and then converting the flux to a precipitation rate over the 10 km distance, indicates a
1.62 mm h-I precipitation rate would have been required to account for the difference in SL W across the
plateau top. .

.
. ,

. .

74