As if tornadoes weren’t fearsome enough, new research has suggested that they may be stronger near the ground than previously thought. The actual speeds of destructive winds are therefore probably being missed by weather forecasts meant to help people brace for impending storms.
“One surprising thing about tornadoes is, for such a well-known phenomenon, the fundamental details are poorly known,” said Joshua Wurman, a meteorologist at the University of Illinois at Urbana-Champaign.
Until recently, most of what had been known about these low-altitude wind speeds was inferred from the destruction wrought when they blustered through a town. “The information that we can infer from that damage is crude and has known biases,” Wurman said, due to the relative rareness of tornadoes crossing through populated areas and the fact that they damage weak structures.
In the United States, tornadoes are typically observed by the National Weather Service’s network of stationary radars. But the height at which the radar beam measures increases with distance from the twister, said Jana Houser, a meteorologist at the Ohio State University who wasn’t part of the new work. Unless the tornado is very close, the beam doesn’t collect data near the surface.
When radar observations are available, they’re typically from far above the ground—higher than 100 meters (330 feet). They don’t reveal the nature of winds near the surface, where damage happens, said Karen Kosiba, a meteorologist also at the University of Illinois at Urbana-Champaign. Early mathematical and computer models had suggested that the strongest blasts occurred above the heights of buildings, she said.
To get the lowdown on wind speeds near where people live, Kosiba and Wurman have observed hundreds of tornadoes using weather radars on trucks—systems they call Doppler on Wheels. When a supercell thunderstorm spins up a tornado, the pair and their colleagues race to get their instruments in front of it to collect radar data from about 2 to 3 kilometers (1 to 2 miles) away.
“Basically, we do what storm chasers do, but with a very different goal,” Wurman said.
Wurman and Kosiba manage the Flexible Array of Radars and Mesonets facility, which maintains, among other equipment, several Doppler on Wheels vehicles available for the research community. (The researchers are continuing their work after a 2021 settlement with the federal government over alleged misspending of grants under a prior project.)
In the latest study, Kosiba and Wurman dug through radar data from 73 tornadoes that occurred between 1999 and 2009 in the Great Plains of the United States. For each storm, they constructed a vertical profile of wind speeds using measurements at heights of between 15 and 100 meters (50–330 feet) aboveground. This revealed that the tornadoes spun 30% faster at 15 meters (50 feet) than radars had indicated higher up.
The finding suggested that “the true average intensity of tornadoes may be even stronger than we thought,” Wurman said.
The Business End of the Tornado
The swirling dust a tornado kicks up makes quite an impression, “but the business end of the tornado really is very close to the ground,” said Richard Rotunno, an atmospheric scientist at the National Center for Atmospheric Research in Boulder, Colo., who was not involved with the research. That the strongest gusts often occur close to the ground isn’t surprising, Rotunno said, as this is consistent with current theory and modeling of tornadoes.
Friction is important near the ground, Houser said. Though friction does slow winds to a degree, it also causes the air swirling on the outside of the twister to turn toward the upward moving column in the center. This makes the tornado’s radius smaller and causes it to rotate faster.
“It’s like the ice skater effect,” Houser said, referring to how twirling ice skaters speed up when they draw their arms or legs nearer their body.
There are still many questions around how tornadoes form, Houser said, and real-world observations like those in this study help to untangle the physics of what’s happening inside storms. Kosiba and Wurman are continuing to collect data on tornadoes to investigate the factors—such as the roughness of a landscape—that might affect a tornado’s longevity or wind speeds.
Knowing how wind speeds vary with height is crucial for designing buildings and structures to withstand storms. Risk from tornadoes is increasing, said Wurman, especially as suburbs sprawl in tornado-prone areas. “Understanding the true intensity of tornadoes is the only way to really quantify that risk in a meaningful way,” he said.
—Carolyn Wilke (@CarolynMWilke), Science Writer