WINTER CLOUD SEEDING WINDOWS AND
POTENTIAL INFLUENCES OF TARGETED
MOUNTAIN BARRIERS
(Note: Paper with this title will be published in the WMA’s
Journal of Weather Modification, 2013 Edition)
DON A. GRIFFITH
NORTH AMERICAN WEATHER CONSULTANTS
NAWC
SANDY, UTAH
WMA ANNUAL CONFERENCE
APRIL 9-12, 2013
SAN ANTONIO, TEXAS
PRESENCE OF SUPERCOOLED LIQUID WATER AND/OR POSITIVE
CLOUD SEEDING EFFECTS IN WINTER OROGRAPHIC PROGRAMS
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Mooney and Lunn, 1968: Lake Almanor, CA., seeding increases in
“cold westerly” conditions, post-frontal with lower cloud tops.
Grant and Elliott, 1974: Climax I and II, CO., positive seeding effects
with inferred cloud top temperatures from -10 to -24 C.
Hobbs, 1975: Cascade Mountains, WA., higher supercooled liquid
water contents post-frontal with lowering cloud tops.
Vardiman and Moore, 1978: Generalized positive seeding effects with
cloud top temperatures from -10 to -30 C.
Cooper and Marwitz, 1980: San Juan Mountains, CO., supercooled
liquid water in the latter part of the storm sequence (typically
associated with declining cloud tops)
Hill, 1980: Utah, highest seeding potential when cloud top
temperatures are > -22 C.
Shaffer, 1983: Colorado River Basin Pilot Project, CO., most
significant seeding effects when cloud top temperatures were > - 20 C.
Rauber and Grant, 1986: Colorado Orographic Seeding Experiment,
CO., most supercooled liquid water present with cloud top
temperatures > - 22 C.
Reynolds, 1988: Sierra Cooperative Pilot Project, CA., most
supercooled liquid water present in split front conditions (a region
with lowering cloud tops).
Manton, 2011: Snowy Mountains research program, Australia,
indications of increased effects of seeding when cloud top
temperatures were from -7 to -20 C.
Average Distribution of Liquid Water (g/m3) in Clouds over
the Cascade Mountains as a Function of Altitude
Conceptual Model of a Split Front and Indicated
Regions of Seeding Opportunity
Conceptual Depiction of a Preferred Supercooled Liquid
Accumulation Water Zone Over a North-South Oriented
Orographic Barrier in Wintertime
LOCATIONS OF GROUND BASED ICING RATE METERS IN UTAH
(2-3 winters of data collection through 2011-2012)
BRIAN HEAD: ICING, WIND, TEMPERATURE AND
PRECIPITATION INSTALLATION
ICING RATE METER
ICING EVENT AT BRIAN HEAD SKI AREA
ICING EVENT AT BRIAN HEAD SKI AREA
RAINFALL RATE, TEMPERATURE ICING STACKED PLOT
SKYLINE SITE, FEB. 4-5, 2011
Synoptic Pattern Classification for Three Winter
Seasons during Icing Periods at Brian Head Ski Area,
Utah (elevation 10,900 feet MSL)
WIND ROSE FOR ICING PERIODS AT BRIAN HEAD,
THREE SEASONS
Brian Head site wind distribution for the 54 heaviest icing periods
in the data set. Radial scale is wind speed (meters/second), with
lighter winds near the center of the plot and stronger winds near
the outer edge
Synoptic Pattern Classification during Icing Periods
for Three Winter Seasons at Skyline, Utah
(elevation 9,330 feet MSL)
Temperature Distribution during Icing Periods at Brian
Head, Three Winter Seasons
Temperature Distribution during Icing Periods at
Skyline, Two Winter Seasons
Brian Head Icing Distribution with Respect to Precipitation
Periods of 0.01”/hr (0.1”/hr snowfall) or Greater during Storm
Events, Three Winter Seasons
Estimated Results in Winter Orographic Cloud Seeding Programs
Estimated Results in Winter Orographic Cloud Seeding
Programs (Cont.)
INFLUENCES OF THE ORIENTATION OF MOUNTAIN
BARRIERS UNDER POST-FRONTAL WIND FLOW
WESTERN UNITED STATES TOPOGRPHY
WINTER OROGRAPHIC CLOUD SEEDING
CONCEPTUAL MODEL
Research programs, physical observations, analyses of operational
cloud seeding programs and recent feasibility studies have indicated
some of the earlier thinking regarding winter cloud seeding potential
has considerable merit. The concept that deep systems with cold
cloud tops often appear to be naturally efficient with little or no SLW
content is especially important. The activity of naturally occurring ice
nuclei is known to increase as ambient temperatures decrease.
Consequently, cold cloud top events will naturally nucleate large
numbers of supercooled water droplets near their tops. As these
nucleated ice crystals grow into snowflakes that then descend
through the clouds, they will often “sweep out” the lower-level
supercooled cloud droplets through the riming process. It appears
from a variety of earlier sources of information and more recent
observations that post-frontal, relatively shallow, orographically
induced clouds often contain SLW and therefore offer good cloud
seeding potential. Some post-frontal orographic clouds may develop
without any or with only scattered clouds upwind of the mountain
barriers. These post-frontal orographic clouds are often relatively
young and therefore contain little natural ice.
SUMMARY
• NUMEROUS PREVIOUS STUDIES HAVE INDICATED EITHER
THE PRESENCE OF SUPERCOOLED LIQUID WATER AND/OR
POSITIVE SEEDING EFFECTS WITH LOWER CLOUD TOPS
AND WARMER TEMPERATURES.
• THE ABOVE CONDITIONS FREQUENTLY OCCUR UNDER
POST-FRONTAL OR EVEN POST UPPER-TROUGH
SITUATIONS
• THESE CONDITIONS ARE FAVORABLE FOR THE RELEASE OF
SILVER IODIDE FROM GROUND BASED GENERATORS DUE
TO LOWERING TEMPERATURES AND FEWER ATMOSPHERIC
STABILITY ISSUES.
• PUBLISHED RESULTS FROM WINTER OROGRAPHIC
PROGRAMS AND FEASIBILITY STUDIES SUGGEST THAT
NORTH-SOUTH ORIENTED BARRIERS OFFER THE HIGHEST
SEEDING POTENTIAL.
• THESE RESULTS MAY BE IMPORTANT IN THE DESIGN AND
CONDUCT OF FUTURE WINTER OROGRAPHIC CLOUD
SEEDING PROGRAMS
ACKNOWLEDGEMENTS
The funding for the operation and evaluation of the groundbased icing meter data in Utah was provided by major water
interests in the three lower Colorado River Basin States
(Arizona, California and Nevada). This funding was
administered through agreements between the Utah Division of
Water Resources, North American Weather Consultants and the
Lower Basin States. Special acknowledgment goes to Mr. Todd
Adams and Mr. David Cole of the Utah Division of Water
Resources and Mr. Tom Ryan of the Metropolitan Water District
of Southern California for their support in these data collection
and evaluation efforts.
PUBLICATION AND NAWC WEB SITE INFORMATION
• PAPER ENTITLED “WINTER CLOUD SEEDING WINDOWS AND
POTENTIAL INFLUENCES OF TARGETED MOUNTAIN
BARRIERS” RECENTLY ACCEPTED FOR PUBLICATION IN THE
WEATHER MODIFICATION’S JOURNAL OF WEATHER
MODIFICATION, 2013.
• NORTH AMERICAN WEATHER CONSULTANT’S WEB SITE:
www.nawcinc.com. Several relevant publications listed under
publications tab.