Gazing Inside The Supercell Thunderstorms That Spawn Tornadoes
A tornado spawned by a supercell thunderstorm cut a swath through north-central Wisconsin near Chetek on May 16, 2017, killing one person and injuring 25 others. This tornado stayed on the ground for more nearly 90 minutes leaving the longest recorded damage path in Wisconsin, according to the National Weather Service's Milwaukee/Sullivan office.
These kinds of thunderstorms are by far the most deadly, but they aren't well understood despite advances in meteorology. Why do some supercells spin up tornadoes such as the one in Chetek, while most others don’t?
To answer that question, University of Wisconsin-Madison atmospheric scientist Leigh Orf uses sophisticated computer modeling to analyze supercell storms in his lab. Orf and his team of researchers at the Cooperative Institute for Meteorological Satellite Studies draw on data from real-world weather events — air sampling from near a storm that includes wind speed, air pressure, temperature and moisture — and apply the laws of physics to replicate the conditions and watch how the simulated storms behave. Orf has based much of his work on a 2011 tornado outbreak near El Reno, Oklahoma.
Orf described the process of creating such high-resolution visualizations at a Wednesday Nite @ the Lab lecture on June 28, 2017, recorded for Wisconsin Public Television's University Place. The end goal of his work, he said, is saving lives through knowing better when a tornado is going to form.
"Wouldn't it be nice if, when you heard a tornado warning there was an actual tornado, and you should really take shelter? That's where we want to go," Orf said.
Orf's stated research goals are to better understand how tornadoes form in supercell thunderstorms, how tornadoes are maintained by processes occurring inside the thunderstorm, and why some supercells produce devastating, long-lasting tornadoes while others do not.
To create his supercell storm models, Orf used a program developed by a scientist at the National Center for Atmospheric Research. He also wrote software that creates highly detailed data visualizations and animations, then ran the simulations on the Blue Waters supercomputer, housed at the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign.
The process allows Orf to get inside the storms and see the individual parts and how they work in ways that observational science cannot, he explained.
Storm chasers — the intrepid souls who drive into the heart of dangerous storms with their field instruments — provide essential information to researchers like Orf, including compelling videos that resemble his simulations. Those visuals tell Orf that he may be on the right track.
"We're just starting to get to the quantitative analysis part," he said. "This means when you get into the real numbers, not just movies. And then, of course, we also always want to compare our results to observed storms because the real atmosphere is really guiding us on this."
One thing that surprised him, Orf said, was the strength of the simulated storms’ updraft near the ground. He was also surprised to see the role that cold air plays in the formation of a tornado. In one of the computer-generated visualizations he showed to the audience, the storm’s updraft sucked up the heavier cold, rotating air and tilted it into the vertical updraft. That, he theorized, helps to drop the air pressure and make the updraft stronger, which contributes to the formation of a tornado.
Orf's computer visualizations also revealed the presence of multiple vortices, mini-tornadoes that he likened to the circling blades of a rotary electric shaver. As a funnel cloud develops, he said, the mini-tornadoes merge with it to intensify the power of the storm. These structures are impossible to see during an actual tornado, he said.
"We think that most strong tornadoes, especially, have multiple vortices in them," he said.
Orf's research also identified another feature of these superstorms: what he calls the "streamwise vorticity current," or SVC. He describes it as a horizontal stream of air at the leading edge of the storm that that then tilts up and rotates around the updraft. He thinks the SVC provides an energy source for tornadoes.
"Sometimes you'll observe something and then you have to model it. In this case, we modeled something and now we want to go observe it. So there's this nice give-and-take in our field where you have the theoretical, the modeling and the observational, and they all help to move the field forward."
- Supercells are the least common type of thunderstorm, but they are the most devastating. More than 90 percent of all fatalities occur during EF-4 or EF-5 tornadoes, the strongest on the Enhanced Fujita scale.
- About one in four to one in five supercells produce a tornado, with most tornadoes short-lived and not very damaging.
- All thunderstorms contain updrafts, air that rises like smoke billowing from a campfire, because it's lighter than the air surrounding it. In a supercell storm, the updraft interacts with the atmosphere around it that contains a lot of wind shear and begins to rotate. This rotating updraft is called a mesocyclone.
- Computer modeling showed an underlying structure in supercells Orf calls the "streamwise vorticity current," or SVC. It's a horizontal column of cool air that forms at the leading edge of the storm and then rises around the mesocyclone, rotating around it. To observe the SVC in the modeling, he releases simulated air parcels in a certain region of a storm to see where they go.
- Wisconsin Public Radio's Central Time interviewed Orf on March 30, 2017, when he discussed how this thunderstorm computer modeling is adding to understanding about tornadoes. A major part of his work involved learning to how to manage massive amounts of data effectively.
- On the purpose of supercell thunderstorm modeling research: "Despite advances in observational and computational meteorology, such as what I do, we still don't really know why some storms produce tornadoes and others don't. So one of the ways to approach this is just to simulate a whole bunch of storms and look at what happens in the ones that do produce these long-lived tornadoes compared to the ones that don't."
- On the difficulty of modeling supercell thunderstorms on computers: "We've got a lot to do, but the fact that we actually did this is a big deal because it's just really hard to wrestle a supercomputer into submission and get it to do this. It's really hard. I can’t tell you how many times I failed at this before I got success."
- On the advancing complexity of meteorological computer modeling: "I am by far not the first person to try to do this sort of thing. We've had people doing cloud modeling like this since the late 1970s. The pioneers of cloud modeling, some of them were here at the University of Wisconsin, actually. Robert Schlesinger in the department of meteorology was one of the first meteorologists to simulate a supercell in three dimensions. Robert Wilhelmson at the University of Illinois, who's also one of my collaborators, he also made a model that was three dimensional, and we learned so much about supercells. So I'm doing the same basic thing those guys started a long time ago, only to the tenth power. We're using a more refined model that has better physics, better numerics. We're exploiting new computers."
- On the existence of the streamwise vorticity current that has been modeled: "This SVC thing and some of the other things we've identified, do they exist in nature? Probably they do. We haven't looked for them because we didn't know what to look for."
- On the seemingly arbitrary nature of tornado damage: "When you hear about tornadoes hitting your region and one house gets obliterated and the house across the street doesn't, it’s because you've got these tiny little vortices, and it's almost random. It's chaotic. So that's the reason why, I think, sometimes you see almost random damage when it comes to populated areas … We're pretty sure that this is not something that is just happening in models. This kind of activity where you have all these multiple vortices, spinning cyclonically."
- On the relationship between computer modeling and actual weather: "We always have to look to Mother Nature when we do this theoretical work. We have to validate our data."