WMOD00413

• Title: Validation of Precipitation Management by Seeding Winter Orographic Clouds in the Colorado River Basin
• Date: 9/1/1993
• Weather Modification - Doc Type: Report

The CREST will consist of physical and statistical investigations concurrently conducted at two
sites in the Basin. Recommended sites are the Grand Mesa of west-central Colorado and the
Wasatch Plateau of central Utah. Final site selection will be made only after appropriate
National Environmental Policy Act and public involvement processes are completed. These two
mountainous areas typify many mountains in the Upper Colorado Basin, and have
characteristics that make them especially suitable for experimentation. Their lack of isolated
peaks allows low-level aircraft sampling during storms, which is essential for monitoring cloud
responses to seeding. Both areas have highways and trails that provide access to ground
instrumentation sites needed for direct observations (e.g., snowfall measurements) and remote
sensing (e.g., radar monitoring). In addition, these areas have been operationally seeded in
recent winters with support from local water user groups. Recent scientific investigations have
been conducted in each area which documented the frequent existence of apparently seedable
clouds.

The CREST will be conducted in two phases with associated transferability studies. Phase I
will concentrate on collecting physical evidence to demonstrate that seeding can increase
snowfall during portions of many winter storms. Phase II will concentrate on collecting the
statistical and physical evidence necessary to quantify the seasonal snowpack increases and
resulting streamflow. Transferability studies will assist, after completion of the CREST, in
transferring the results from the two experimental areas throughout the Basin.

Phase I will consist of a series of "direct detection" experiments that use new instrumentation
to monitor key links in the chain of physical events from the release of seeding material to
precipitation reaching the ground. Each direct detection experiment will be brief (4-6 h) and
will target a small well-instrumented area. A combination of direct and remote sensing
approaches will observe the transport and dispersion of the seeding agents and resulting ice
crystals, and will follow resulting snowfall to the surlace. Supporting measurements will
document the seeding methods and atmospheric conditions for which seeding is effective and
those for which it is ineffective.

Starting in Phase I and continuing throughout the program, a sophisticated, physically based
numerical model will be used in conjunction with the observations to investigate mountain
airflow and cloud and precipitation processes. The combination of comprehensive physical
observations from the seeding experiments, and state-of-the-art numerical modeling, will
provide rapid progress in understanding seeding responses for a wide range of storm conditions.

Phase II, a statistical seeding experiment for each experimental area, will be carried out during
the final four winters. Its design will be based upon results obtained from the direct detection
seeding experiments and numerical modeling. The purpose of the statistical experiments will
be to document the magnitude of seasonal snowfall increases from seeding a large target area
A major physical monitoring effort will continue throughout Phase II to ensure that the
statistical experiments are properly designed and conducted.

Special observations and numerical modeling simulations will be accomplished for six other
mountain regions in the Basin during the CREST. These investigations will concentrate on the
availability of SLW-the "raw material" needed for seeding to be effective, and on proper
targeting of seeding material to convert excess supercooled liquid water into snowfalL The
results of this work will expedite transfer of cloud seeding technology throughout the Basin.

XLV

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