WMOD00512

• Title: Special Report to the Commissioner - Precipitation Management for the Colorado River Basin
• Date: 1/1/1980
• State: CO
• Country: United States
• Weather Modification - Doc Type: Report

Mountain range east of che Animas River. The projecc design
consisted of a 24-hour experimental unic selecced on the basis of
a forecast that met specific criteria. Seeding was conducted with
ground-based silver-iodide/acetone generators located various
distances upwind of che target area.

Evaluation was conducted by Aerometric Research, Inc. 14/ and
employed the Mann-Whitney rank sum test to assess che significance
of tbe ratio of seeded to nonseeded precipitation. The
pre-specified analysis for tbe 71 seeded and 76 nonseeded days
showed no significant difference attributable to seeding. This
evaluation also revealed two important operational problems.
First, the design required that seedahle conditions be forecast
up to 24 hours in advance. It was found, however, that accurate
forecasts could not be made over this timeframe. As a result,
unseedable clouds were seeded on some occasions, and seedable
clouds uere not seeded on some occasions. The second problem
arose because,under stable air mass conditions, ground-released
seeding material was trapped near the surface. Therefore. the
seeding material did not enter the clouds during the seeded
period. but was released later to contaminate a nonseeded period.
The result of these two operational problems, i. e., inefficient
opportunity recognition and uncontrolled targeting of ground
seeding material, was that positive seeding effects were balanced
by negative seeding effects -- yielding no significant overall
effect according to the! priori design.

An ! posteriori analysis of 6-hour cime intervals that met the
seed ability criteria indicated seeding had increased precipitation
under certain conditions and decreased it under others. It
reaffirmed the role of cloud-top temperature in determining
seeding potential and the importance of the wind speed acroas the
barrier. This analysis showed that, if seeding vere restricted to
thick clouds with warm tops and light vinds. seasonal increases of
10 to 12 percent could be obtained.

Sierra Cooperative Pilot Project

The Sierra Cooperative Pilot Project is an ongoin6 Skywater program
vhich began field studies in the winter of 1976-77. It is
located in the American River and Tahoe-Truckee River Basins of
the Sierra Nevada. The program objective is to develop a better
understanding of physical processes of precipitation formation
and to identify those conditions which provide the best potential
for winter precipitation enhancement. The primary emphasis has
been to obtain physical measurements which lead, through analysis,
to an increased understanding of the precipitation process and

IV-3