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New Radar Technique Locates Storm-Fueling Moisture

August 1, 2006

BOULDER—People planning ball games, picnics, and other outdoor events may soon have more precise short-term forecasts of rainfall, thanks to an observing strategy now being tested by the National Center for Atmospheric Research (NCAR). An NCAR field project this summer is, for the first time, using multiple Doppler weather radars to track water vapor in the lower atmosphere. Measuring the low-level moisture is expected to help forecasters pin down the locations and timing of storms that might rage a few minutes to a few hours later.

NCAR's S-Pol radar gathers refractivity measurements as part of the REFRACTT project. Click here or on photo to enlarge. (Photo by Carlye Calvin, ©UCAR.)

The project is named REFRACTT (Refractivity Experiment For H2O Research And Collaborative operational Technology Transfer). Researchers are measuring changes in the speed of radar signals caused by refraction, which in turn reveal the presence or absence of atmospheric moisture. If the project proves successful, this refractivity technique could be added in the next few years to the national network of Doppler radars operated by NOAA's National Weather Service (NWS).

"Nobody's ever seen such high-resolution data on moisture before. We believe this could greatly help forecasters predict where heavy rains might develop," says NCAR scientist Rita Roberts, the lead principal investigator for REFRACTT.

REFRACTT runs from June 5 to August 11 and is being funded by the National Science Foundation, NCAR's primary sponsor. Along with four radars, scientists are using computer models, satellites, NCAR radionsondes (weather balloons), and ground-based sensors that intercept Global Positioning System signals and infer atmospheric moisture.

Jim Wilson and Rita Roberts
Jim Wilson (left) and Rita Roberts discuss refractivity data. Click here or on photo to enlarge. (Photo by Carlye Calvin, ©UCAR.)

Strong contrasts in moisture can help to spawn intense storms, but the exact location of these contrasts is often hard to identify before storms develop. Currently, NWS radars detect rainfall and winds but not water vapor. Moreover, weather stations and weather-balloon launches that do measure water vapor are often separated by 50-100 miles or more. As a result, there is no regular monitoring of low-level moisture in between surface stations.

When meteorologists use Doppler radar to track storms, they normally monitor signals that strike raindrops, hailstones, or snowflakes and bounce back toward the radar. The strength of the returning signals indicates the intensity of rain, hail, or snow, while the change in signal frequency holds information on wind speed. During REFRACTT, scientists are adding a third variable: the speed of the radar signals. They are using fixed targets such as power lines and silos to see how much the radar signal is sped up or slowed down by variations in water vapor. The resulting data on refractivity is plotted on a map that shows scientists where the moisture is located.

The idea behind REFRACTT was developed by Frederic Fabry of McGill University while he was a visiting researcher at NCAR in the late 1990s. He has since collaborated with NCAR to refine the technique.

Forecasters at the Denver NWS office have been using REFRACTT data this summer to monitor the weather across northeast Colorado, including the risk of weak tornadoes that often spin up east of the Front Range. After the field project ends, the NWS will consider including refractivity as part of a larger upgrade to the national radar network.

"Low-level moisture is the key to our weather here, especially during the summertime," says Larry Mooney, meteorologist in charge at the Denver NWS office. "We're really excited about the REFRACTT data. 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."

Above:  These two images show refractivity data for northeast Colorado as compiled for REFRACTT by one or more of the radars involved. Cooler colors indicate greater amounts of water vapor in the lower atmosphere (roughly the lowest 3000 feet).  The image on left is from July 18, 2006, at 1:54 p.m. CDT, when moisture had pooled against the Front Range (green band).  At right is the image from June 14, 2006, at 2:37 p.m. CDT, when hot, dry air was moving east from the foothills across the plains. Major roads and highways are visible as faint lines. Click on images to enlarge. (Images, ©UCAR.)

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