Weather Radar Technology Beyond NEXRAD - the NAS report
Radar Research and Development - National Severe Storm Lab
National Weather Service Weather Safety pages
The Weather Room - for elementary school students
They provide the nation with life-saving storm warnings, but they are no longer state-of-the-art.
As this ScienCentral News video reports, the National Weather Service wants what the military has.
The nationwide network of 160 Doppler radar sites is the pride of the National Weather Service. They have also become "the main bottleneck" to improving warning times for severe storms, says Doug Forsyth, Chief of Radar Research & Development at the National Severe Storms Laboratory (NSSL) in Norman, Oklahoma.
The radars are called WSR 88-Ds (for Weather Surveillance Radars, 1988, and D for Doppler). They currently provide a precious eleven-minute warning to get out of a tornados path. If that amount of warning time doesnt sound like much, Forsyth says the radars are credited with saving countless lives. But upgrading them could double that lead time.
Forsyth is leading several projects studying how upgrading the radar network could achieve that goal. The National Academy of Sciences (NAS) Board on Atmospheric Sciences and Climate published a report this summer encouraging the studies, which so far include research partnerships with the Weather Service, Lockheed Martin Corporation, the U.S. Navy, University of Oklahomas Department of Meteorology and College of Engineering, and the Federal Aviation Administration.
Lockheed Martin built the 88-Ds and says the biggest advance is the antennas. New military radars, like the ground-based air surveillance radars employed by the US Air Force, use electronic antennas called phased arrays, instead of mechanical dishes.
"The dish thats currently in use uses a single beam, the transmitter produces a radio-frequency signal that is then... directed out into the volume scan area," says Kevin Hines, program manager for NEXRAD weather systems at Lockheed. "Each time it performs that scan it then has to move mechanically up—has to be tilted up until the volume scan is reached—and then it goes back down and starts that over again." That process takes six to seven minutes.
Phased arrays use multiple beams so the antennas never need to tilt. "Generally well think about it in terms of there being six beams, and you quickly divide the radar up into thirds," Hines explains. "As you rotate, those beams are constantly moving from the bottom third to the middle third to the top third as it goes around the scan, so it takes... essentially 30 seconds, as opposed to six or seven minutes."
Phased array, says Forsyth, "will also give us the capability to be able to do what we call adaptive scanning by being able to go back quickly and look at a phenomenon that we see with the radar. This gives us a greater chance of improving the lead times across the nation for severe weather warnings."
Rain, Snow, Sleet, Hail?
Forsyth says another needed improvement would clear up meteorologists views of precipitation. Military radars have (and the 88-Ds lack) a capability called dual polarization. The NAS report instructed the Weather Service to investigate adding dual polarization to the existing antennas, as well as including it in future antennas.
When an 88-D receives a returning RF signal, it can only receive waves that are polarized in one direction, either vertically or horizontally. "It cant do them simultaneously," Hines says. "So if you have the radar set for vertical reception mode, and then the precipitation is oblong such that its more horizontal... it would in fact essentially fool the meteorologist or distort his ability to determine how much precipitation is there."
Dual polarization will allow meteorologists to determine "exactly how big that precipitation is, what shape it is, and how much is coming," Hines says.
National Weather Radar Testbed
Construction is now underway at the NSSL for a new National Weather Radar Testbed that will be built around a phased array antenna called a SPY-1 to be contributed by the US Navy.
"I liken it to when we first got the radars that led to the Doppler radars," Forsyth says. "It was military technology that we converted to run as a research tool to determine this Doppler radar capability. And now were going to get military technology again from the Navy to actually test out this phased array technology, so were very excited here and its a new frontier for us."
Theyll need to sustain that excitement for quite some time. Forsyth says at current funding levels, upgrading the antennas could take 15 to 20 years to get underway. The precursors to the 88-Ds were the WSR-57s—a 31-year technology gap—and the last 88-D wasnt installed until 1996.
While the NSSL anticipates several participating agencies might share in an upgraded network that could track weather, air targets, and commercial air traffic at the same time, Forsyth also seems resigned to the past pace, in spite of the bounding pace of technology today. A recent presentation he authored on phased array was titled, "Will Phased Array Radar become the WSR-22P?"