28 June 2011—At 5:17 p.m. on 22 May, the U.S. National Weather Service forecast office in Springfield, Mo., issued a tornado warning for the nearby town of Joplin, 24 minutes before the twister hit. But this was unusually early. Tornado warnings today provide an average lead time of 13 minutes, a leap from the 5 minutes people had in the early 1980s, before the weather service employed a nationwide network of nearly 150 Doppler radars for severe weather detection.
More computing power and advanced signal-processing algorithms have maximized the warning-time gain from this radar system. Further updates are under way, but "the system is approaching its end of life," says Sebastian Torres, a research scientist at the National Severe Storms Laboratory (NSSL) in Norman, Okla. "We’re exploring the future of weather radar," says Torres.
That future is called phased-array radar, a technology that could increase warning times up to 22 minutes. Data from the new radar could eventually be used in computer models, along with factors such as temperature and humidity, in order to predict tornadoes and stretch warning times up to an hour.
Today’s radars send microwave pulses using bulky parabolic antennas. After bouncing off moving rain or hail, a pulse returns at a different amplitude and frequency; thousands of these Doppler shifts are used to measure wind velocity and direction. Forecasters examine storms, looking for adjacent air pockets moving in opposite directions—a sure sign that a tornado is brewing. Unfortunately for forecasters, they get an updated radar image only about every 5 minutes—the time it takes for the antennas to scan a fixed volume of air around them by rotating 360 degrees at multiple elevations.
"One radar image every 5 minutes is not nearly enough to accurately capture the evolution of severe storms," Torres says. "It takes time for forecasters to gain confidence in what they’re seeing and issue a warning."
A phased-array radar can capture the scene much more quickly. This type of radar system features a fixed, flat antenna composed of thousands of transceiver elements sending and receiving pulses at the same time. The relative delays between the pulses shape the direction of the beam radiating from the array. This electronically steered beam can scan the whole sky in less than a minute. Forecasters would also be able to instantly redirect it to focus on specific areas. "It gives forecasters information almost like a movie, as opposed to taking snapshots of the atmosphere," says Doug Forsyth, who leads radar research at the NSSL.
U.S. Navy ships have used phased-array radars for decades to detect missile threats. In fact, since 2003, the NSSL has been testing an antenna donated by the Navy. A group of National Weather Service forecasters are invited to participate in the trials, which include yearly mock tests on real tornado data. Last year, forecasters looking at the one-minute storm updates from the new radar were able to issue a 21-minute warning before tornado touchdown, 13 minutes before those who got the standard 5-minute updates.
But it’s still going to take another 15 to 20 years to implement the technology, Forsyth says. The biggest obstacle is cost. Until recently, each of the 4000 elements in the Navy’s phased-array antenna cost US $2000. Cheaper microwave components resulting from advances in cellphone technology have brought the price of each element down to around $100. But the price would need to fall below $50 to make the radar affordable for weather monitoring.
Cost savings could also come from combining weather radars with the Federal Aviation Administration’s 350-plus air traffic radars to make one system. Such a system would be a bureaucratic monstrosity, no doubt, but one that could save the federal government close to $5 billion, according to the NSSL.
The biggest technological hurdle, meanwhile, is making the new phased-array radar dual polarized; the National Weather Service is already retrofitting its radars with this capability. Instead of one horizontally oriented microwave pulse, the updated systems will transmit two waves: one horizontal and one vertical. The combination helps to accurately measure the size and shape of objects such as raindrops and hail, thereby improving rainfall estimates and flash-flood forecasts and helping to detect flying tornado debris. In the future, says Forsyth, this improvement could also help predict when a tornado is going to form by looking at the way different-size raindrops are moving.
But making phased-array radars dual polarized is easier said than done, given the 4000 different elements that have to work together to form the beam, Forsyth says. Researchers are now working on better antenna designs and electronic beam-forming systems to solve some of these issues.