1996-19 -- FOR IMMEDIATE RELEASE: August 20, 1996
Both storm areas were continuously watched by an experimental radar perched atop four seatainers (seaworthy trailers) at Front Range Airport in Watkins, the first town east of Denver on I-70. Operated throughout the summer by Boulder's National Center for Atmospheric Research (NCAR), the radar's dual-polarimetry technology proved its state-of-the-art precision on July 12 by distinguishing between the large, flat raindrops swelling Buffalo Creek and the round hailstones pounding the eastern prairie. Meanwhile, Denver's Front Range Doppler radar--part of the National Weather Service's nationwide network (WSR-88D, formerly known as NEXRAD)--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, who heads the S-Pol precipitation experiment. "Hail can fool the WSR-88D 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 a dual-polarimetric radar over the WSR-88D's 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 of S-Pol at Front Range Airport is the first in the series. Anticipating promising results from the current refinement of the now 15-year old technology, scientists at the National Severe Storms Laboratory (NSSL) are already working on how to add it to the existing WSR-88D network installed around the country in the early 1990s. Colorado State University and NSSL use dual-polarimetric radar data and are collaborating with NCAR on the S-Pol tests. Similar studies are taking place in Germany, Italy, and England.
Matt Kelsch, a meteorologist at the National Oceanic and Atmospheric Administration, parent organization of the National Weather Service, comments, "WSR-88D is set to measure maximum rainfall rates of 2.94 inches per hour in Colorado. Above that limit, hail looks like very heavy rainfall. S-Pol, however, has no set threshold."
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. 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 flood.
S-Pol is NCAR's second dual polarimetric radar. Its predecessor, CP-2, was expensive to set up because it required the construction of a concrete pad at each new site. By contrast, S-Pol 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 an hour and can be covered with a radome (protective shell) if necessary in more severe weather.
Not only is S-Pol convenient to ship and assemble, but it sports a much improved antenna, which provides more accurate measurements than CP-2 did. A new data processor using modern digital technology further supports S-Pol's state-of the-art status.
Dennis Heap, director of aviation at Front Range, donated airport grounds for the experiment. The large white 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 Authority 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.
NCAR is managed by the University Corporation for Atmospheric Research under sponsorship by the National Science Foundation.
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