WMOD00116

• Sponsor Name: MWDSC
• Project Name: Weather Modification White Paper
• Title: Weather Modification for Precipitation Augmentation and Its Potential Usefulness to the Colorado River Basin States
• Prepared For: Colorado River 7 Basin States
• Prepared By: Tom Ryan - Metro Water District of Southern California
• Date: 10/1/2005
• Weather Modification - Doc Type: Report

Appendix A

Melt and runoff from snowpack contributes most of the reservoir storage in large portions of
the Western U.S., so snowpack augmentation is an attractive target in the region. Accordingly,
operational cloud seeding to augment snowpack in the mountains has a long history in several
western states. Prior studies suggest that seeding of supercooled orographic clouds has worked,
yielding seasonal precipitation increases on the order of 10% (AMS, 1998). More research and
documentation is needed to buttress these findings, however.
Winter orographic seeding has been dominated by the use of silver iodide (AgI), delivered
from ground-based generators or aircraft The last federally-assisted cloud seeding project was a
winter experiment in Utah during the 1990s (Super 1999). There are a number of other seeding
technologies that have been put forward but not tested extensively. New chemical compositions,
such as silver chloride-iodide complexes, may act more efficiently to produce ice particles at
temperatures warmer than _50C, where AgI is ineffective. Similar warm temperature results may
be achieved by cost-effective liquid propane generators (Medina 2000). The Utah WDMP
experiment operated such generators.
The use of tracer chemicals with the seeding materials affords a method to verify the
targeting of those materials. Such chemicals were employed in the Nevada WDMP experiment,
which even attempted to distinguish ground-based and aircraft seeding signatures. This
experiment also included a Lagrangian particle dispersion model and windflow from the MM5
cloud model to further characterize seeding plume trajectories (Koracln et al. 1998). The
Colorado experiment also used a Lagrangian transport model (Uliasz 1994) coupled with the
RAMS cloud model (Cotton et al. 1994) to evaluate targeting of seeding material.

2.2 Convective Seeding

Research into warm-season seeding of convective clouds in the Western U.S. was
conducted mostly in the late 1960s through early 1980s, notably through projects such as the
National Hail Research Experiment (NHRE) and the High Plains Experiment (HIPLEX; Cooper
and Lawson 1984). These experiments had mixed results for hail suppression and rainfall
augmentation, respectively. Rosenfeld and Woodley (1989; 1993) reported increases in radar-
estimated rainfall with Agl seeding in West Texas during the 1980s. Tracer studies were
conducted in North Dakota in 1989 (Boe et al. 1992).
More recently, positive findings regarding hygroscopic seeding have been reported in
several experiments, e.g., in South Africa (Mather et al. 1997), Thailand (Silverman and
Sukarnjanaset 2000), Mexico (Bruintjes et al. 2001; Fowler et al. 2001), and India (Murty et al.
2000). Vin et al. (2000; 2001) have conducted modeling studies of hygroscopic seeding that offer
plausible arguments for why such seeding may lead to rain increases.

2.3 Individual Project Goals

The states conducting WDMP research are shown in Fig. 1. Three projects, in Nevada,
Utah and Colorado, are involved in cool-season, orographic seeding. The other two projects, one
in North Dakota and the other in Oklahoma, Texas and New Mexico, seed convective clouds
during the warm season. Specifically, individual project principal goals are as follows:
. Nevada - (1) Remotely sense supercooled cloud water to quantify cloud seeding
potential in a selected watershed; (2) Apply mesoscale atmospheric and dispersion
modeling to evaluate seeding effectiveness under a variety of storm conditions; (3)
Evaluate seeding effectiveness through physical and chemical analyses of snow
packs; (4) Use hydrologic modeling to estimate impacts of seeding-induced increases
in snow packs on streamflow; (5) Characterize natural and seeded cloud regions with
in situ aircraft microphysical measurements.
. Utah - (1) Conduct randomized cloud seeding on the Wasatch Plateau using propane
dispensers to develop embryonic ice particles; (2) Explore impacts on precipitation by
the cloud seeding.
. Colorado - (1) Configure the Colorado State University cloud model (RAMS) and conduct
modeling over operational cloud seeding areas; (2) Implement algorithms simulating
cloud seeding generators as sources of ice nuclei, (3) Simulate Lagrangian transport

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