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

- I

Most experiments would be carried out in extensive, mainly stratiform, cloud systems.
Ground-based seeding experiments would use generators located well upwind of Allen Lake to
release plumes of AgI intended to merge and be transported over that target site. Repeated
crosswind passes would be made by the cloud physics aircraft through the seeded volume, and
through neighboring non seeded cloud. These observations should reveal differences in ice particle
concentrations, crystal habits and sizes, and cloud liquid water content between seeded cloud and
crosswind natural cloud. The aircraft passes should monitor the AgI plume position so that
precipitation rates in gauges likely affected by seeding could be compared with gauges measuring
only natural snowfall. The Allen Lake target would have several instrumenls to monitor snowflake
characteristics, very high resolution precipitation ra~es and silver-in-snow concentrations, Similar
observations of nonseeded snow would be made at a crosswind control site. Seeding would continue
for up to 3 h per mission after which aircraft sampling would continue as the AgI plume left the
area. This procedure would allow natural cloud to be sampled after the seeded cloud volume passes
beyond the target.

Airborne seeding experiments would be of two types. One method of seeding would have the
seeding aircraft continue to orbit over a fixed point, releasing an AgI plume intended to pass over
Allen Lake. The cloud physics aircraft and ground systems would make observations similar to
those taken during the ground-based seeding experiments. The other airborne seeding method
would release crosswind lines (seedlines) of AgI that would disperse vertically and along-the-wind
as they approached the target. Ground instruments would provide comparisons of events before,
during, and after passage of each seedline. The cloud physics aircraft would make repeated along-
the-wind passes through each seedline and natural cloud upwind and downwind of it. Several
seedlines could be released and sampled during a single experimental period, Each seedline would
be evaluated by comparison with natural conditions shortly before and after seedline passage over
the target.

Additional physical experiments are recommended for convective clouds that are sometimes present
over the Mogollon Rim during winter. These would not be seeded but observations would be made
to allow estimation of their weather modification potential. Earlier cursory examination of
convective clouds suggested they often were too short-lived or naturally efficient to be seed able, but
a more detailed study should be done.

Various investigations of a climatological na ture are recommended with some of the same observing
systems used for physical experiments, The climatological studies would aid in the design of any
future randomized seeding experiment intended to demonstrate the magnitude oflong-term snowfall
augmentation over a large area. A randomized experiment would be the next logical phase if the
physical experiments succeed in showing that snowfall can be increased over a small target with
sufficient frequency and magnitude. Climatological investigations will allow the brief physical
experiments to be considered in the context of general storm conditions over several winters.

Several analysis approaches are suggested ranging from evaluation of the individual physical seeding
experiments to general climatological investigations. The individual physical seeding experiments
are very important in testing whether key physical processes proceeded as hypothesized. When
possible, individual experiments should be pooled into similar classes to allow exploratory statistical
analysis on samples with reduced variability, using nonparametric techniques that can be applied
to small populations.

IV