Although Doppler radar has been transformative, it is not perfect. It leaves meteorologists like Forsyth blind to the shape of a given particle, which can distinguish, say, a rainstorm from a dust storm. Ironically, the trajectory of his career path changed when a failed eye exam led him from U.S. Air Force pilot ambitions to a career in meteorology. Since then, Forsyth has focused on radar upgrades that give forecasters a better view of the atmosphere.
One critical upgrade is called dual polarization. This technology allows forecasters to differentiate more confidently between types of precipitation and amount. Although raindrops and hailstones may sometimes have the same horizontal width—and therefore appear the same in Doppler radar images—raindrops are flatter. Knowing the difference in particle shape reduces the guesswork required by a forecaster to identify features in the radar scans. That understanding helps to produce more accurate forecasts, so residents know they should prepare for hail and not rain, for example.
Information about particle size and shape also helps to distinguish airborne bits of debris lofted by tornadoes and severe thunderstorms, so meteorologists can identify an ongoing damaging storm. Particle data are especially important when trackers are dealing with a tornado that is invisible to the human eye. If a tornado is cloaked in heavy rainfall or is occurring at night, dual polarization can still detect the airborne debris.
The National Weather Service is integrating dual-polarization technology—which is also helpful for monitoring precipitation in hurricanes and blizzards—into all 160 Doppler radars across the nation, expecting to finish by mid-2013. At the same time, NOAA personnel are training forecasters to interpret the new images. The Weather Forecast Office in Newport/Morehead City, N.C., was the first to scan a tropical cyclone using such radar when Hurricane Irene made landfall in North Carolina in 2011. During that storm, dual-polarization radars proved more accurate in detecting precipitation rates, and therefore predicting flooding, than conventional Doppler radars farther north. The improved capabilities surely saved lives in the Carolinas; farther up the coast, without this technology, Hurricane Irene was deadlier despite early warnings, claiming nearly 30 lives.
NOAA research meteorologist Pam Heinselman believes another advanced radar technology used by the U.S. Navy to detect and track enemy ships and missiles has great potential to improve weather forecasting as well. Heinselman leads a team of electrical engineers, forecasters and social scientists at the National Weather Radar Testbed in Norman, Okla., focused on a technology called phased-array radar.
Current Doppler radars scan at one elevation angle at a time, with a parabolic dish that is mechanically turned. Once the dish completes a full 360-degree slice, it tilts up to sample another small sector of the atmosphere. After sampling from lowest to highest elevation, which during severe weather equates to 14 individual slices, the radar returns to the lowest angle and begins the process all over again. Scanning the entire atmosphere during severe weather takes Doppler radar four to six minutes.
In contrast, phased-array radar sends out multiple beams simultaneously, eliminating the need to tilt the antennas, decreasing the time between scans of storms to less than a minute. The improvement will allow meteorologists to “see” rapidly evolving changes in thunderstorm circulations and, ultimately, to more quickly detect the changes that cause tornadoes. Heinselman and her team have demonstrated that phased-array radar can also gather storm information not currently available, such as fast changes in wind fields, which can precede rapid changes in storm intensity.
Heinselman and others believe phased-array technology alone could extend tornado warnings to more than 18 minutes, but much more research and development needs to be done. Ideally, the phased-array system would have four panels that emitted and received radio waves, to provide a 360-degree view of the atmosphere—one each for the north, south, east and west. Researchers in Norman have made only one-panel systems operable for weather surveillance, and it is likely to be at least a decade before phased arrays become the norm across the country.