WMOD00285 ‎(2)‎

• Title: Feasibility Study of Wintertime Cloud Seeding to Augment Arizona Water Supplies
• Date: 1/1/1987
• State: AZ
• Weather Modification - Doc Type: Report

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Executive Summary

The most important factor determining cloud seeding potential centers on
the presence of SLW (supercooled liquid water) in sufficient amounts and
durations and on enough occasions to attain cost effective operations.
While SLW is the most essential ingredient for precipitation augmentation
by cloud seeding, singularly it is not sufficient, according to current
theory.

Clouds that are inefficient at producing precipitation occur in most winter
storms. In these clouds excess SLW generated naturally on the upwind side
of mountain barriers is swept over the crest and evaporates in the
descending air over the lee slope. Cloud treatment conceptual models
exploit this natural phenomenon based on the premise that timely delivery
of a seeding agent, such as silver iodide, to the proper cloud volume will
create additional ice crystals which then grow by consumption of the SLW,
1 eadi ng to additi onal prec i pi tati on.

Determination of cloud seeding potential is a complex task and requires
local measurement of a number of parameters. However, in the absence of
these data, a preliminary assessment is possible with the use of climato-
logical observations including precipitation and upper air measurements of
the wi nd, temperature and moisture to infer treatment potential. These
data should be available in time and space resolutions adequate to describe
conditions important to cloud development. The data base developed for
this study included historical daily upper air and precipitation data.

A limited study was performed of winter storm types leading to mountain
precipitation. Results of weather map investigations revealed four promi-
nent types of winter storm systems that led frequently to precipitation
over the Mogollon Rim and, at times, over the southern mountains.

The first storm type, producing moderate to heavy amounts of mountain pre-
cipitation in Arizona, is characterized by strong southward advection of
cold air over the western U.S. coastline that evolves into a closed cir-
culation over California and moves slowly eastward. A second storm type,
which yields moderate precipitation, is typified by moderate cold air

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