by The tornado raced across southern Iowa at nearly 45 mph, shredding wind turbines like string cheese.
In the town of Gleanntach, he overturned cars and tore houses off their foundations, leaving a trail of destruction that can be seen from space. The twister, later rated an EF4 by the National Weather Service, 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 most everyone in the area was walking in basements, a team of nine scientists – storm chasers – tried to be as close as possible to the twisters.
Just before 3 pm, they saw their chance. As a tornado began to brew 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 tackled deploying a science instrument pod directly in the path of the twister.
“Debris was falling on us,” said Jennifer Walton, a storm forecaster and photographer on the team.
A third truck drove through town, suddenly surrounded by trees and buildings that blocked its radar view of the twister. They knew they were ahead of the storm; they didn’t know how much.
“That’s probably the most anxious time for all of us, because we know a tornado is coming,” said Joshua Wurman, a University of Illinois research scientist. “We don’t really know if it’s coming in five minutes or three minutes or two minutes.”
It worked. The team achieved an “intercept”, as they call it, gathering data about the storm with the pod and two mobile radar devices and gave scientists a rare, detailed and close-up view of one of the most powerful loops ever recorded on this way.
The data they collected represents only the third time scientists have calculated wind speeds that reached more than 300 mph inside a tornado. And because storm chasers took the readings from multiple angles as the twister pumped Greenfield, the results provide a grim picture of the winds and internal dynamics of a vortex powerful enough to level houses.
“Tornadoes that produce this intensity and this type of damage are rare in the United States. We only get a handful of these every 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 to have mobile radar watching when the tornado is doing estimated peak damage.”
Many fundamental elements of tornado science are still in disarray because it is so difficult to obtain high-quality data. These new findings could help resolve critical questions about tornado formation and structure, how wind speeds in the air respond to damage on the ground and the factors that can intensify or break down tornadoes.
The team of nine researchers that left for Greenfield on May 21 was led by Wurman and Karen Kosiba and the University of Illinois Flexible Array Radar and Mesonets (FARM) team, which is funded in part by the National Science Foundation. Both are among the best academic storm candidates in the world.
FARM researchers started the day in McCook, Nebraska, tired after a night of storm chasing in Colorado, where all they encountered was hail. The roving team travels with two radar trucks and several other vehicles, including a pickup truck with an instrument pod designed to survive a tornado long enough to measure temperature, pressure and other factors. They went to Greenfield – almost a six hour drive.
Tornado popularity is like playing a board game against nature. Researchers assess the conditions, calculate odds and move pieces – radar trucks, balloons and pods – so they can capture the best measure of the storm from a relatively safe distance.
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 began like a game of whack-a-mole. Storms were moving through the area quickly, leaving them wondering where to place their equipment and how to maintain a safe distance. They found themselves zigzagging down country roads as storms popped up on the radar, only to watch them drift away.
But as the Greenfield tornado approached town, it experienced 10 minutes of intense activity.
“This is not a safe sport. You have to be able to change on a dime,” Walton said.
Kosiba and Wurman positioned their radar truck about a mile east of downtown, which was 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, level platform to ride out the storm.
“We don’t want to be bouncing around and messing up our radar,” Wurman said.
The winds reached the truck at about 80 mph, but the researchers do not remember much how it looked outside – they were glued to their screens.
“I never 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 objects flying through the air, everything from raindrops to two-by-fours. The device measures the energy that comes back, providing such detailed data that researchers can understand the shape of falling rain.
Inside the truck, they watched the colors flash across displays as the tornado split house frames and split trees.
Sometimes, multiple vortices swirled within the tornado, which lasted for about 45 minutes and traveled about 44 miles. For less than a second, the researchers calculated wind speeds of more than 300 mph in part of the tornado.
“This was a violent day in Iowa,” said Tim Marshall, a Texas-based storm watcher who was not involved in the research.
The weather service said the twister was up to 1,000 feet wide at times. But when Greenfield hit him, he was encroached upon. Scientists aren’t quite sure why that happened – it’s one of many mysteries that this kind of detailed radar data could finally 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, much remains a mystery.
“When 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 what the exact track of that storm will be,” Wurman. said.
Storm seekers perform their dangerous dance with the elements to push their pitch forward.
The recent measurements of the Greenfield tornado, in particular, could provide insight into how high wind speeds translate into damage on the ground. Kosiba plans to correlate the detailed wind speed data collected by her team with land damage surveys. She also plans to model the thermodynamics of the event, which could provide clues about the factors that could lead to stronger wind speeds.
This work could help scientists develop better tornado prediction systems and help builders build more resilient structures.
“We don’t want to see the destruction, but this is how we learn,” Marshall said. “Damage is Mother Nature’s fingerprint. That’s how we assess how it happened.”
This article was originally published on NBCNews.com
