Storm chasers detect tornado winds topping 300 mph in rare ‘intercept’

The tornado ripped through southern Iowa at nearly 45 mph, shredding wind turbines like string cheese.

In the town of Greenfield, it overturned cars and ripped houses from their foundations, leaving a gas of destruction visible from space. The tornado, which the National Weather Service later classified as an EF4, killed five people on May 21, making it one of the deadliest so far this year, and injured 35.

More than a dozen tornadoes touched down in the state that day. While almost everyone in the area was hiding underground, a team of nine scientists – storm chasers – sought to get as close as possible to the tornadoes.

Shortly before 3 p.m., they saw their opportunity. As a tornado began to appear on their radar screens, the group sprang into action. They rushed one of their radar trucks to a location about 10 miles west of Greenfield, a community of about 2,000 in southwest Iowa.

Another team rushed to deploy a group of scientific instruments directly in the tornado’s path.

“Debris fell on us,” said Jennifer Walton, storm chaser and team photographer.

A third truck drove through town, suddenly surrounded by trees and buildings that blocked the tornado’s radar view. They knew they were ahead of the storm; They didn’t know how much.

“This is probably the most anxious time for all of us, because we know a tornado is coming,” said Joshua Wurman, a research scientist at the University of Illinois. “We really don’t know if it’s going to happen in five minutes, three minutes or two minutes.”

It worked. The team performed an “intercept,” as they called it, collecting data on the storm with the module and two mobile radars and giving scientists a rare, detailed, close-up view of one of the world’s largest tornadoes. powerful songs ever recorded in this way.

The data they collected marks only the third time scientists have calculated wind speeds reaching more than 300 mph in a tornado. And because storm chasers took the measurements from multiple angles as the tornado struck Greenfield, the results now offer a grim view of the winds and internal dynamics of a vortex powerful enough to level homes.

The May 21 Greenfield tornado was powerful enough to overturn and crush a car. The Adair County Medical Clinic, in the background, was spared a direct hit.
Courtesy of Joshua Wurman / Flexible Radar and Mesonet Network

“Tornadoes producing this intensity and type of damage are rare in the United States. We only get a handful each year,” said Tony Lyza, a physicist at the National Severe Storms Laboratory in Norman, Oklahoma, who was not involved in the research. “This is a really important study to have a tornado of this intensity and have mobile radar observing it while the tornado is causing maximum estimated damage.”

Many fundamental elements of tornado science remain uncertain because it is very difficult to obtain high-quality data. These new findings could help resolve crucial questions about the formation and structure of tornadoes, how wind speed in the air corresponds to ground damage, and what factors can cause intensification or collapse. tornadoes.

The team of nine researchers that left for Greenfield on May 21 was led by Wurman and Karen Kosiba with the Flexible Array of Radars and Mesonets (FARM) team at the University of Illinois, funded in part by National Science Foundation. The two men are among the best collegiate storm chasers in the world.

FARM researchers started the day in McCook, Nebraska, tired after a stormy night in Colorado, where they caught only hailstones. The roving team travels with two radar trucks and several other vehicles, including a van equipped with a suite of instruments designed to survive a tornado long enough to measure temperature, pressure and other factors. They left for Greenfield, almost a six-hour drive.

Chasing a tornado is like playing a board game against nature. Researchers assess conditions, calculate probabilities and move the pieces – radar trucks, balloons and pods – so they can capture the best measurement of the storm from a relatively safe distance.

A Doppler dish on wheels sends a narrow beam of radar waves to collect data near a tornado near Duke, Oklahoma, May 23.Courtesy of Joshua Wurman / Flexible Radar and Mesonet Network

This type of work can be deadly: In 2013, three tornado chasers died while chasing a powerful tornado in El Reno, Oklahoma.

In Iowa, FARM researchers’ day started like a game of whack-a-mole. Storms were moving quickly through the area, leaving them wondering where to place their equipment and how to keep a safe distance. They found themselves zigzagging down country roads as storms appeared on radar, only to see them die out.

But as the Greenfield tornado approached the city, it unleashed 10 minutes of intense action.

“It’s not a safe sport. You have to be able to make changes on a dime,” Walton said.

Kosiba and Wurman parked their radar truck about a mile east of the center of town, about 300 yards from the edge of the tornado. They lowered the truck’s metal hydraulic legs and raised its wheels to create a stable, level platform to weather the storm.

“We don’t want to bounce around and mess up our radar,” Wurman said.

Winds blew at the truck at about 80 mph, but the researchers don’t remember much about the outward appearance: They were glued to their screens.

“I never look out the window,” Kosiba said. “I always watch the radar.”

The truck sends out a narrow beam of radar waves that strike objects flying through the air, from raindrops to two-by-fours. The device measures the energy that returns, providing data so detailed that researchers can understand the shape of falling raindrops.

Inside the truck, they saw colors flash on the screens as the tornado broke the frames of houses and tore trees.

Karen Kosiba analyzes a radar display inside a Doppler truck on wheels during the May 21 Greenfield tornado. Courtesy of Karen Kosiba/NSF BEST Project

At times, multiple vortices swirled within the tornado, which lasted about 45 minutes and traveled about 44 miles. In less than a second, researchers calculated wind speeds of more than 300 mph in part of the tornado.

“It was a violent day in Iowa,” said Tim Marshall, a veteran storm chaser based in Texas who was not involved in the search.

The weather service said the tornado was up to 1,000 feet wide at times. But when it hit Greenfield, it had shrunk. Scientists aren’t sure why this happened — it’s one of many mysteries that this kind of detailed radar data could eventually help solve.

“We’re going to think about this for years,” Wurman said.

The ingredients necessary for a tornado to form – wind shear, lift, instability and humidity – are well known and allow forecasters to set up reliable tornado watches, but beyond that, much remains mysterious .

“Once a storm forms and produces a tornado, we have very little skill in knowing whether that tornado will be large or small, long-lived or short-lived, or the exact direction that storm will take,” Wurman said .

The May 21 tornado downed wind turbines southwest of Greenfield. Tim Marshall, a veteran storm chaser, said it takes winds of 120 to 140 mph to demolish such structures.Courtesy of Joshua Wurman / Flexible Radar and Mesonet Network
“This is the type of damage that was probably rated EF4,” Joshua Wurman, a tornado researcher at the University of Illinois, said of the May 21 tornado in Greenfield. Courtesy of Joshua Wurman / Flexible Radar and Mesonet Network

Storm chasers perform their dangerous dance with the elements to advance their battlefield.

Recent measurements from the Greenfield tornado, in particular, could provide insight into how wind speeds aloft translate to damage on the ground. Kosiba plans to correlate detailed wind speed data collected by his team with ground damage surveys. She also plans to model the thermodynamics of the event, which could provide clues about the factors that could lead to more intense wind speeds.

This work could help scientists develop better tornado forecasting systems and help builders build more resilient structures.

“We don’t want to see destruction, but this is how we learn,” Marshall said. “Damage is Mother Nature’s fingerprint. This is how we assess how this happened.

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