Better storm, flood warnings eyed

Technique being tested uses radar to map water vapor

Jim Erickson, Rocky Mountain News

Published August 2, 2006 at midnight

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BOULDER - A new technique that uses radar beams to track wet air could lead to more accurate and timely warnings of severe thunderstorms and flash floods, scientists at the National Center for Atmospheric Research said Tuesday.

For the first time, radar is being used to map the near-ground water vapor that helps fuel thunderstorms, said NCAR meteorologist Rita Roberts.

The innovative technique is being tested at four radar locations in northeastern Colorado, including NCAR's Marshall Field research site, just south of Boulder.

"We can really map the moisture moving around in fine detail," Roberts said while staring at the brightly colored moisture map displayed on a computer screen. Outside, the white, 28-foot-diameter dish of NCAR's S-Pol Doppler radar scanned the horizon.

"Nobody's done this before," she said.

National Weather Service radars can detect precipitation and winds, but not water vapor. Air-moisture data are collected at weather stations and with weather balloons, but those sites can be separated by 100 miles or more.

As a result, there is no fine-scale monitoring of low-level air moisture in the United States.

The new radar project, known as REFRACTT, could change that.

The REFRACTT project started June 5 and ends Aug. 11. It is funded with $500,000 from the National Science Foundation.

It uses four northeastern Colorado radar dishes that scan the horizon, sending out bursts of energy at the speed of light.

When the radar beams strike a stationary object - a house, a silo, power lines, a mountain - some of the energy bounces back to the radar dish.

Researchers look for minute changes in the speed of the radar beam - wet air slows the beam slightly because it's denser.

The four radars collect moisture data from the layer of air below about 1,700 feet.

It is used to create detailed moisture maps of northeastern Colorado, information that can be fed into computerized forecasting models.

Forecasters at the Boulder office of the National Weather Service have tested the REFRACTT data this summer, but it is not used routinely in forecasts.

"Low-level moisture is the key to our weather here, especially during the summertime," said Larry Mooney, head meteorologist at the weather service's Boulder office.

"We're really excited about the -REFRACTT data," he said. "I think it's a great example of how you can move technology into the operational realm pretty quickly if you're committed to it."

The idea behind REFRACTT was developed by Frederic Fabry of McGill University while he was a visiting researcher at NCAR in the late 1990s.

Since then, he has collaborated with NCAR researchers to refine the technique.

REFRACTT relies on a computer program that can be used with the nationwide network of National Weather Service Doppler radars. Roberts, the project's lead scientist, said she hopes that happens soon.

"This is a demonstration to show that it works, and we really have demonstrated that this summer," she said.

KMGH-TV meteorologist Richard Ortner, who visited the NCAR radar site Tuesday morning for a news briefing, said the new moisture data "would be a huge asset for short-term forecasting."

"The ability to see moisture pooling up against the Front Range, in real time, is almost like having X-ray vision," he said.

REFRACTT stands for Refractivity Experiment For H2O Research and Collaborative operational Technology Transfer.

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