America’s network of high-tech Doppler radars, which were first deployed in the early 1990s to detect severe weather, has just become much more sophisticated. According to the National Weather Service (NWS), the agency has completed upgrades to its 122 Doppler radars, turning them from conventional Doppler radars to dual-polarization, or dual-pol, radars that provide more information to forecasters to improve weather warnings.
What the 2011 Phoenix dust storm looked like from the local NWS forecast office.
By sending out both horizontal and vertical pulses of energy, which are then reflected back to the radar by precipitation, the dual-pol radars are able for the first time to give forecasters a more informative 2-dimensional picture of what is going on in the atmosphere. The radars can differentiate between precipitation types, spot airborne debris from large tornadoes, and make far more precise estimates of the amount of precipitation that has fallen in a particular area compared to conventional Doppler radars. The dual-pol technology, which consists of new software and a hardware attachment to the radar dish, can clearly distinguish between hail and rain, for example, or between rain and snow.
The radars have already proved their value to NWS forecasters. In February, when an EF-4 tornado struck Hattiesburg, Miss., dual-pol radar products allowed local NWS forecasters to spot the tornado on their radar screens and determine that it was likely a large and powerful twister. More specifically, the dual-pol radar detected the airborne debris generated by the tornado, and the debris signature prompted forecasters to include specific and strong language in the warnings they issued to the community.
Despite the extensive damage it caused, no lives were lost.
“It is amazing what we can see with dual-pol technology,” said NWS director Louis Uccellini in a press release. “This game-changing technology has already helped forecasters issue more accurate and timely warnings to the public and has saved lives.”
Weather forecasters have also taken advantage of dual-pol technology for forecasting winter weather events. During the major blizzard that struck New England in early February, paralyzing parts of Southern New England with up to 40 inches of snow, weather forecasters on Long Island were able to determine the locations where rain, snow, and mixed precipitation were falling, enabling them to detect a rapid transition to heavy snow across New York City and Long Island. More than 2 feet of snow fell on Long Island, and a foot of snow accumulated in New York.
Previously, the Doppler radars that the NWS installed in the early 1990s only sent out horizontal pulses of energy, which allowed them to see where precipitation was falling, and to detect rotation in the atmosphere, which is key for spotting developing tornadoes. Those capabilities were considered to be revolutionary advancements at the time, and they allowed forecasters to make tornado warnings with far more lead time.
What the Phoenix dust storm looked like on dual-pol radar.
Click image to enlarge. Credit: NWS.
However, the older-generation Doppler radars did not give forecasters much insight into the precipitation type, nor did they allow them to detect airborne tornado debris, or make reliable estimates of how much rain had fallen in a particular area.
Dual-pol technology, for example, enabled forecasters in Phoenix to detect the massive dust storm that struck the city on July 5, 2011. The radar data showed that the dust reached a height of 6,000 feet, with a width of nearly 100 miles. It traveled for more than 150 miles, reducing visibility to near zero, and bringing wind gusts of greater than 50 mph.
The NWS is working with the FAA and the military to upgrade 37 airport and military Doppler radars, a process that should be finished this summer, the NWS said. The dual-pol radars are still limited, though, in that they take a few minutes to complete each scan of the sky, during which time a tornado could form, touch down, and lift back up. While upgrades have helped improve the update time of each radar scan, the NWS is researching Phased Array Radar technology, which can scan the skies much faster than the current generation of Doppler radars. However, that won't be deployed anytime soon, especially given recent budget cuts.
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