Weather Mod Critical Issues Report

• Project Type: General OWC
• Project Name: Weather Modification
• Title: Critical Issues in Weather Modification Research
• Date: 1/1/2004
• Water Conservation - Doc Type: Final Report

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3.2.3 Additional research accomplishments; static seeding of winter
orographic clouds

I) One of the most exciting accompl ishments in recent snowpack
augmentation research is the establishment of the direct link between the seeding activity
and the water reaching the ground in the form of snow. The mm/hr increases in
precipitation caused by silver iodide seeding have been documented several times in the
reviewed scientific literature between 1988 and 1999, The link has been established by
physical and chemical techniques. The snow precipitated at particular targeted sites is
connected directly to the seeding material and to concurrently released chemical tracers
in that snow. The big advantage of snowpack work is that the scientists are dealing with
solid-state precipitation that can be sampled during and after storm events and stored in
the frozen state until analyzed. The methodologies used to establish this direct linkage
have been described by Warburton et al. (1985, 1994, and I 995a,b) Super and Heimbach
(1992), Chai et al. (1993), Stone and Huggins (1996), Super and Holroyd (1997), and
McGurty (1999).

2) A second significant accomplishment in the snowpack augmentation
studies provides a chemical explanation for the apparent failure of some larger scale
randomized seeding experiments to achieve statistically significant increases in
precipitation. Warburton et al. (1995b) have shown that, on the average, only 20% of the
snow, which precipitated to the ground during the seeded periods of the Sierra Nevada
Truckee- Tahoe project, showed evidence of being impacted by the silver iodide seeding.
The results indicate that it would be necessary to produce very substantial changes in the
limited areas where seeding material is detected, to yield a statistically acceptable change
over the entire snowfall target area, Further studies of this type were conducted by Stone
and Warburton (1989) in other Sierra Nevada regions seeded from ground-based aerosol
generators,

3) Current physical and chemical evidence for these two significant
accomplishments comes from research projects in the northern and southern Sierra
Nevada and the Carson and Wasatch ranges of California, Nevada and Utah. Dual-
channel microwave radiometers, short wavelength radars, ice-nuclei counters, sulfur
hexafluoride gas and combinations of ice nucleating and non ice-nucleating aerosols
(silver iodide and indium sesquioxide), have enabled scientists to identify the locations
and the quantities of supercooled liquid water in winter storms and to track the seeding
aerosols from their points of release to the targeted snowfall sites, as noted above.

4) The locations within winter storm clouds where ice-phase water capture
occur have been studied by Warburton and DeFelice (1986) and by Warburton, et al.
(1993). These studies and others in the Sierra Nevada and in the Australian Alps showed
for the first time that the stable oxygen and hydrogen isotopic composition of ice-phase
precipitation are related to the microphysical processes within the clouds in which the
precipitation has formed. The work demonstrated that when orography dominated during
the post-frontal storm period, the ice-phase water substance was being captured in the
clouds between -50C and -140C with a peak around -lloC temperature. This type of

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