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.
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.
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."
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