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

EXECUTIVE SUMMARY

This report documents planning for comprehensive physical cloud seeding experiments to be
conducted on the Mogollon Rim of Arizona. The goal of these experiments is to validate
hypothesized physical processes following seeding, The planning included the development of
(1) experimental designs for both ground-based and airborne seeding of winter clouds, (2) analysis
approaches to be applied to the resulting measurements, and (3) cost estimates for the overall
program. The entire physical experiment program, from equipment procurement and contracting
through field observations, final analysis and reporting, would require 5 years. This assumes a
separate analysis group will work simultaneously with the field group and that measurement
programs will be carried out during years 2 through 4" The total cost of the 5-year program is
estimated to be $9.5 million.

Past attempts at physical seeding experiments with winter clouds were reviewed and briefly
described. Some common factors affecting either success or failure were identified. For example,
airborne identification and tracking of the seeded volume were very important in the successful
experiments, especially in the complex airflow near mountains. These factors require that mountain
experimental areas be carefully chosen to permit low level aircraft sampling during storms. Changes
in ice particle concentration were the most detectable characteristic of seeding at ground level.
Frequent monitoring of the silver content of the snowfall helped validate targeting in silver iodide
(AgI) seeding experiments. Negative factors included attempts to "piggy-back" physical experiments
on statistical designs and inadequate analysis during and between observational programs so that
needed design improvements were recognized too late.

The entire Mogollon Rim, from near Flagstaff to the White Mountains on the New Mexico border,
was considered in choosing an experimental area for physical seeding experiments. By a process
of elimination the "best" target site was determined to be Allen Lake, 60 km south-southeast of
Flagstaff. It is near the rim's crest, has few peaks to interfere with low level flight, has good road
access and electrical power, and is in a large enough clearing for operation of key instrumentation
systems. A Doppler weather radar could be operated north of Allen Lake in a position that would
permit scanning from cloud tops to near the ground. The project operations center would be
colocated with the radar while project aircraft would be based at Scottsdale.

The various scientific uncertainties concerning Arizona winter clouds are addressed. These include
limited knowledge of the spatial and temporal distributions of supercooled liquid water over the rim,
and of ice crystal formation, growth and fallout trajectories, especially when many crystals aggregate
into large snowflakes. Whether conditions exist such that seeding might sometimes reduce snowfall
is an open question. These and other important issues will be investigated during the course of the
experimental program.

Several sophisticated observing systems are recommended for use in the physical experiments.
These include a microwave radiometer for detection of water vapor and liquid water; a Doppler
acoustic sounder to monitor winds in the lowest kilometer; a scanning Doppler radar for storm
structure, cloud top, and wind observations; two rawinsonde systems to obtain vertical profiles of
wind, temperature, and moisture; and various surface sensors including high resolution precipitation
gauges and an ice particle imaging probe. An instrumented cloud physics aircraft would monitor
the seeding agent and a variety of cloud characteristics. A seeding aircraft would dispense AgI ice
nuclei and also make some cloud measurements.

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