Tornadoes swept across southern Iowa at nearly 45 mph, shredding windmills like string cheese.
In the town of Greenfield, it overturned cars and ripped houses from their foundations, leaving a trail of destruction visible from space. The twister, later rated an EF4 by the National Weather Service, killed five people on May 21, one of the deadliest so far this year, and injured 35 others.
More than ten cyclones hit the state that day. While everyone in the area huddled in the basement, a team of nine scientists – the storm chasers – tried to get as close to the twisters as possible.
Before 3pm, they saw their chance. When a tornado started brewing on their radar screens, the team sprung into action. They rushed one of their radar trucks to a location about 10 miles west of Greenfield, a town of about 2,000 in southwestern Iowa.
Another team attempted to position scientific instruments directly in the path of the twister.
“Debris fell on us,” said Jennifer Walton, a storm chaser and photographer on the team.
A third truck drove through town and was suddenly surrounded by trees and buildings that blocked the Twister’s radar view. They knew they were ahead of the storm; They don’t know how much.
“It’s probably the most anxious time for all of us because a tornado is coming,” said University of Illinois research scientist Joshua Wurman. “We don’t know if it’s going to come in five minutes or three minutes or two minutes.”
And it worked. The team, as they call it, collected data on the storm and two mobile radar devices, giving scientists a rare, detailed and up-close view of one of the most powerful twisters ever recorded.
This is the third time scientists have calculated wind speeds reaching 300 mph within a hurricane, the only data they’ve collected indicates. As storm chasers took measurements from multiple angles as the twister hit Greenfield, the findings now provide a stark view of the internal dynamics of a vortex powerful enough to level winds and houses.
Courtesy Joshua Wurman / A flexible array of radars and mesonets
“Tornadoes that develop this intensity and do this type of damage are rare in the United States. Tony Lysa, a physicist at the National Severe Storms Laboratory in Norman, Oklahoma, who was not involved in the research. “This is a very important study to have a tornado of this intensity and monitor with mobile radar when the tornado is causing peak damage. .”
Because high-quality data are so difficult to obtain, many fundamental elements of tornado science remain unresolved. These new findings will help unravel important questions about hurricane formation and structure, how wind speeds in the air correspond to damage on the ground, and the factors that cause hurricanes to intensify or break up.
The team of nine researchers that left for Greenfield on May 21 was led by Wurman and Karen Kosiba with the University of Illinois Flexible Radars and Mesonets (FARM) group, which is funded in part by the National Science Foundation. Both are among the world’s best academic storm chasers.
Farm researchers began the day in McCook, Nebraska, exhausted after a night of storm chasing in Colorado, where all they caught were hailstones. The roving team travels with two radar trucks and several other vehicles, including a pickup truck with instruments designed to stay in the hurricane long enough to measure temperature, pressure and other factors. They set off for Greenfield – a nearly six-hour drive.
Chasing a hurricane is like a board game against nature. Researchers assess conditions, calculate odds and move the pieces — radar trucks, balloons and pods — so they can capture the best measure of the storm from a relatively safe distance.
This kind of work can be dangerous — in 2013, three tornado chasers died while chasing a powerful tornado in El Reno, Oklahoma.
In Iowa, Farm Researchers’ Day began as a game of whack-a-mole. Storms moved quickly through the area, which left them guessing about where to position their equipment and how to keep a safe distance. As storms appeared on radar, they found themselves zigzagging down country roads only to watch them pass away.
But as Cyclone Greenfield neared the city, it hit 10 minutes of intense action.
“It’s not a safe game. You can make change 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 twister. They lowered the truck’s metal hydraulic legs and raised its wheels to create a stable, flat surface to ride out the storm.
“We don’t want to jump around and mess up our radar,” Wurman said.
Winds blew through the truck at about 80 mph, but the researchers couldn’t remember what it was like outside—they were glued to their screens.
“I don’t look out the window,” Kosiba said. “I’m always looking at the radar.”
The truck sends out a narrow beam of radar waves that hit airborne objects, ranging from two to four raindrops. The device measures the rebound energy, providing detailed data so researchers can understand the pattern of raindrops falling.
Inside the truck, colors flash across the scenes as the tornado shatters house frames and rips up trees.
At times, multiple eddies spun into the tornado, which lasted about 45 minutes and traveled about 44 miles. In less than a second, researchers measured wind speeds of 300 mph in one part of the hurricane.
“It was a violent day in Iowa,” said Tim Marshall, a veteran storm chaser based in Texas who was not involved in the research.
The weather service said the twister was sometimes up to 1,000 feet wide. But when it hit Greenfield, it shrank. Scientists aren’t exactly sure why that happened—one of the many mysteries this kind of detailed radar data will eventually help solve.
“We’ll be chewing on this for years,” Wurman said.
The ingredients necessary to form a tornado—wind shear, lift, instability, and moisture—are well known and allow forecasters to issue reliable tornado watches, but, beyond that, remain a mystery.
“Once a storm forms and develops into a hurricane, we have very little ability to know whether that hurricane will be large or small, long-lived or short-lived, or the exact path that the storm will take,” Wurman said.
Storm chasers perform their dangerous dance with the elements to push their field forward.
Recent measurements of Greenfield hurricanes, in particular, can provide insight into how wind speed translates to damage on the ground. Kosiba plans to correlate the detailed wind speed data his team has collected with ground damage studies. He also plans to model the thermodynamics of the event, which could provide clues as to what factors lead to more extreme wind speeds.
This work could help scientists develop better hurricane forecasting systems and help builders build more resilient structures.
“We don’t want to see destruction, but this is what we’re learning,” Marshall said. “Damage is the fingerprint of Mother Nature. Here’s how we rate it.
