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.