NSF PR 96-41 - August 22, 1996
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High-Tech Radar Sees Through Colorado 'Weather Tricks'
If you want to know the difference between hail and
rain, says Jewel Prendeville, program director in
the National Science Foundation (NSF)'s division of
atmospheric sciences, ask scientists at the NSF-funded
National Center for Atmospheric Research (NCAR) in
Boulder, Colorado.
Early in the evening of July 12, 1996, an estimated
3.5 inches of rain fell in Colorado, creating a 12-foot
wall of water along Buffalo Creek southwest of Denver.
The creek flooded its banks, washed away a bridge
and road, and left two people dead. At the same time,
a second storm pelted the Colorado plains with hail.
Both storms were continuously watched by four NCAR
experimental radar instruments, called S-Pols, perched
atop seatainers (seaworthy trailers) at Front Range
Airport in Watkins, the first town east of Denver
on I-70.
Operated throughout the summer by NCAR, S-Pol proved
its state-of-the-art precision on July 12th. It distinguished
between the large, flat raindrops swelling Buffalo
Creek and the round hailstones pounding the eastern
prairie. Meanwhile, Denver's Front Range Doppler radar
showed both areas as having similarly heavy rain and/or
hail, without distinguishing between the two.
"Measuring heavy rains accurately is important for
anticipating flash floods," explains NCAR scientist
Jim Wilson. "Hail can fool Doppler radar into 'thinking'
it's raining harder than it actually is, thereby introducing
uncertainty into the issuance of flash flood warnings.
S-Pol can more accurately measure the size and shape
of raindrops. This helps us spot areas of heavy rainfall
and predict the resulting runoff."
To quantify the advantages of the experimental dual-polarimetric
radar over more conventional technology (single polarization),
NCAR is planning a series of experiments over the
next few years in various seasons and locations around
the United States. This summer's deployment at Front
Range Airport is the first in the series.
"The dual polarimetric radar measures the average
height and width of raindrops in a measured volume
of air about the size of a city block," explains Prendeville.
"Its precision reveals the raindrop's shape, a big
clue as to how much water is actually falling." Small
drops tend to be round, while larger drops flatten
into hamburger shapes as they fall. The larger the
drops, the heavier the rainfall and the greater the
risk of flooding.
The experimental radar can be placed on a base of
four seatainers-the same ones it's shipped in-for
assembly at any stable, accessible site in the world.
The 28-foot aluminum dish is sturdy in winds up to
50 miles per hour and can be covered with a radome
(protective shell) if necessary in more severe weather.
Dennis Heap, director of aviation at Front Range Airport,
donated airport grounds for the experiment. The large
white radar dish has been scanning the horizon from
its seatainer base on the former north-south runway
at the airport's west side. Says Heap, "The Front
Range Airport provided the land at no cost to NCAR
because it saw the long-range benefits to the aviation
community, which heavily depends on accurate weather
information." About 125 airplanes from the Denver
metro area are based at Front Range, and many more
from around the country stop there for fuel.
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