WMOD00293

• Title: Summary of the NOAA/Utah Atmospheric Modification Program: 1990-1996
• Date: 9/1/1998
• State: UT
• Weather Modification - Doc Type: Report

7.3 Key Physical Findings

Brief answers to the above questions, based largely on the work reported herein, are now stated:

a. A considerable body of evidence from the Plateau investigations and some other work shows that
significant SLW cloud exists over western mountains in excess of that naturally converted to ~
snowfall. This "excess" SL W flux represents a large fraction of seasonal snowfall amounts.
While the existence of "excess" SL W water cloud has been assumed for decades, aqd is necessary
for operational seeding to have any potential, adequate documentation has been provided only
during the past several years. Field deployment of microwave radiometers has been especially
important in this documentation. '

b. Orographic cloud SL W varies rapidly in time and space. Some of the greatest SL W amounts have
been found during storms with strong synoptic support which are naturally very efficient snow
producers during some phases but inefficient during other phases. Conversely, weaker localized
storms typically produce lesser SL W amounts but these persist over many hours per winter. Both
storm types are important in total seasonal SL W flux production.

"'.

c. Orographic cloud SL W is usually found over the windward slopes and crests and rapidly
diminishes further downwind, even as cloudy air moves across the relatively flat Plateau top,
about 10 kIn wide. The SL W is depleted by a combination of snowfall production and subsidence
of the airflow.

d. The SL W cloud is confined to a shallow layer above the terrain. Most SL W condensate exists in
the lower 500 m above the terrain and SL W amounts are usually' negligible at an altitude of
1,000 m above the terrain. Forced orographic uplift, weak embedded convection, and gravity
waves all combine to produce the liquid condensate.

e. The SL W cloud found near the mountainous terrain is typically mildly supercooled over Utah's
mountains. Frequently, the SL W cloud is too warm for significant ice nucleation with AgI, except
perhaps in its upper portions. Often natural ice nucleation processes become efficient as cloud
temperatures become cold enough for effective AgI nucleation. Consequently, the "window of
opportunity" for effective AgI seeding is limited to a fraction of the periods with "excess" SL W.
To restate this important point, most SL W periods cannot be effectively seeded with the present
type of operationally applied AgI, especially when it is released from the ground with resulting
limited vertical dispersion.

f. The frequency of successful transport of AgI plumes over the Plateau is directly related to
generator elevation relative to the mountain barrier. Plumes released from high altitude sites
within 300 to 500 m of the Plateau top were routinely transported over the barrier when winds had
a cross-barrier component, necessary for significant SL W production. Similar results have been
demonstrated at several other mountainous locations, including Montana, Colorado, and Arizona.
High altitude release s.ites on the Plateau were usually just below or just ~bove cloud base.

. .

g. While experimental cases are limited, adefinite impression developed over the course of the
experiments that canyon mouth releases have a significantly greater probability of over-Plateau
tr:ansport than valley releases.

h. Plumes released from the valley floor are less likely to be transported over mountain barriers than
plumes released from higher elevation sites. . A number of experimental periods showed that AgI

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