WMOD00427

• Title: Design of Physical Cloud Seeding Experiment for the Arizona Atmospheric Modification Research Program - Final Report
• Date: 2/1/1991
• Weather Modification - Doc Type: Report

1. PHYSICAL DESIGN AND FIIELD OPERATIONS

1.1 Introduction

1.1.1 Background. - The ADWR (Arizona Department of Water Resources) and Reclamation
(the Bureau of Reclamation) have been cooperating since 1985 on an assessment of snowpack
augmentation potential known as the Arizona Program. The purpose of the program is to develop
a cloud seeding technology capable of enhancing the winter precipitation on the higher portions of
Arizona's Mogollon Rim, which in turn can be expected to increase water supply in the local areas
for direct use and ground-water recharge and to increase streamflow. The technology must be
scientifically sound and socially acceptable, the latter implying that no significant deleterious
environmental consequences result. Both ADWR and Reclamation want to proceed in a methodical
fashion, documenting the extent to which cloud seeding can enhance Arizona's limited water
supplies, and the social and environmental consequences of the seeding, prior to any consideration
of operational seeding.

The first effort under the Arizona Program was a feasibility study (Reclamation, 1987), which
investigated Arizona winter cloud seeding potential and ramifications using existing data sources.
It was recognized at the onset that such a study would not yield conclusive answers, but the
investigations did find reasons for optimism concerning cloud seeding potential, and laid the
framework for further onsite studies. A number of approaches were suggested for further work.

Two-month field efforts were conducted at two different sites on the Mogollon Rim during early
1987 and early 1988, respectively. These measurement programs were intended to document the
characteristics of winter clouds and precipitation with emphasis on the availability of SL W
(supercooled liquid water), which is required for seeding to have any practical potential. The
feasibility of seeding with ground-based AgI generators was also addressed using a tracer gas. No
cloud seeding was conducted during these field programs.

A final report by Super et al. (1989) discussed the findings of the two field programs. Briefly, SL W
was observed during portions of most storm episodes. The winter total flux of SL W was estimated
at 30 to 100 percent of the mean annual streamflow from high elevation watersheds, suggesting that
this necessary (but not sufficient) ingredient for cloud seeding to succeed is available in relative
abundance. The SL W was concentrated in a few large storms that were naturally efficient in
snowfall production during portions of their passage, but inefficient during other storm stages. The
seeding simulation studies revealed that ground-released! seeding material would usually reach the
lower cloud regions where much of the SL W is found, but temperatures would frequently be too
warm for formation of many ice crystals using conventional types of AgI and generators. It was
recommended that experiments be designed to document results of particular seeding treatments,
including field testing of recently developed types of AgI which produce significant ice crystal
concentrations in cloud chambers warmer than -10 oc.

1.1.2 Future Investigations. - Analyses of the 1987 and 1988 field observations have indicated
winter cloud seeding potential likely exists over the Mogollon Rim. The next step for the Arizona
Program is to demonstrate, with convincing physical evidence, that particular seeding approaches
applied to specific cloud types can augment precipitation on the surface at a preselected target site.
The goal of this report is to provide practical experimental designs and subsequent analysis
approaches that should provide a credible demonstration of seeding effectiveness. If the proposed
physical experiments succeed, a logical followup phase would be to design and conduct a