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

Considerable credibility was added to the results of an earlier exploratory statistical experiment
reported by Super and Heimbach (1983) and by the physical observations reported by Super and
Heimbach (1988) for the Bridger Range, One of the high-altitude seeding generators used in the
statistical experiment was again operated and low-level aircraft observations were made in-cloud
over the target area. The basic approach was to fly normal to the wind, attempting to intercept the
plume of AgI seeding material as measured by an acoustical ice nucleus counter. Ice crystal size
spectra, habits, and IPC were monitored within and immediately crosswind of the seeded volume
The natural cloud on either side of the seeded cloud provided an almost simultaneous basis of
comparison, or control, for testing seeding effects. Dramatic increases in IPC and estimated
precipitation rate were found when the cloud contained SL W while no changes were found in the
absence of SL W. Unfortunately, resources were not sufficient for ground observations under the
seeded volume so the final step in the chain of physical events, snowfall on the surface, was not
documented. However, all available observations indicated that surface precipitation should have
been increased.

1.2.5 Cascade Project Experiments. - Among the most convincing physical experiments, and
certainly a pioneering program for development of new instruments and approaches, was the
Cascade Project work reported in the three part series by Hobbs (1975a), Hobbs and Radke (1975),
and Hobbs (1975b). Observations were made over the Cascade Mountains of Washington State
during the winters of 1969-70, 1971-72, and 1972-73. A well-instrumented aircraft released seeding
material and observed resulting changes in clouds, which were usually stratocumulus. Seeding was
done with the aid of a targeting scheme intended to affect snowfall at a small predetermined target
area. Seeded cloud volumes were tracked by aircraft, using visual observations and measurements
of ice nuclei and ice crystals. A vertical-pointing Doppler radar monitored the spectra of fallspeeds
of precipitation particles, Manned ground stations provided ice particle observations and
measurements of snowfall rate. Snow samples were analyzed for silver content and concentrations
of freezing nuclei. All together, these comprehensive physical measurements often revealed a
consistent portrayal of seeding effects reaching the intended surface target. Reference was made
to 20 case studies from 1971-72 for which the effects of seeding were clearly detected by 1 or more
airborne techniques in 80 percent of the cases, with possible verification in all other cases. For the
same cases, distinct evidence of seeding was seen on the ground on 6 occasions, suggestive evidence
on 4 occasions, and no evidence in the other 10 cases.

Further physical evidence of seeding effects was presented by Hobbs et al. (1981). Three dry ice
seeding lines were placed across the wind in a nonprecipitating altocumulus cloud deck. Aircraft
measurements clearly demonstrated an increase in larger particles in the seeded volumes compared
with nearby nonseeded cloud. This was verified by radar which showed increased reflectivities from
each seedline. The third seedline, released furthest upwind, resulted in a trace of snowfall at the
radar. No snowfall reached the radar from the earlier seedings. Each of the seed lines passed over
the radar within a few minutes of the estimated time of arrival based on the wind velocity,

1.2.6 Grand Mesa Experiments. - A comprehensive set of measurements was obtained in the
physical experiments done over the Grand Mesa of Colorado as discussed by Super and Boe (1988).
Airborne seeding with AgI was done upwind of the barrier during six experiments. The seeded
cloud volume was followed by along-the-wind passes with the cloud physics aircraft as the seeding
line passed over the mesa. An acoustical ice nucleus counter provided clear evidence of the seeded
volume on most aircraft passes. Ice particle characteristics were compared from the seeded volumes
to adjoining natural cloud. Very marked increases in IPC were observed following each seeding
event. Frequent ice particle photography at the surface revealed the effects of seeding during the

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