WEST LAFAYETTE, Ind. — Gully warsher. Duck drownder. Toad strangler. Cob floater. Sod soaker. Whatever their names, summer in the Midwest isn’t summer without strong, sudden storms with towering clouds. While the Indian subcontinent is famous for its monsoon season, what many people don’t know is that the Midwestern United States has its own monsoon season, nearly as powerful.
These Midwest monsoons, increasingly, are breaking through the ceiling of the sky and into the stratosphere, a typically undisturbed layer of the atmosphere. This phenomenon introduces burning biomass and aerosols from western wildfires, potentially threatening the ozone layer and climate stability. New research shows that these storms allow aerosols and particles from the lower atmosphere to penetrate the stratosphere.
Scientific Breakthroughs and Concerns
The research, conducted in partnership with NASA, utilized a high-altitude research aircraft to take measurements in the remote reaches of the stratosphere. Dan Cziczo, a professor in the Department of Earth, Atmospheric, and Planetary Sciences at Purdue University, led the team alongside research scientist Xiaoli Shen. Their findings were published in Nature Geoscience.
“In the summer, here in the Midwest particularly, we get all these air quality warnings from wildfires because the climate is getting warmer and the land is getting drier,” Cziczo explained. “That’s becoming more common, but that’s all close to the planet’s surface, where we thought it was staying. We flew this research aircraft up into the stratosphere, the next layer up of the atmosphere, which should be separate. Stratosphere means stratified; it should be separate. But what we found is that during these big wildfire seasons, the lower part of the stratosphere is just littered with these biomass particles.”
The Mechanics of Atmospheric Disruption
Cziczo and his team focus on the mechanics of the atmosphere, particularly how, why, when, and where clouds and storms form. They are especially interested in the way warm, wet air moves up from the Gulf of Mexico, crashes against the Rocky Mountains, and forms severe summer storms and rain, similar to the summer monsoon in India when warm, wet winds collide with the Himalayas.
Typically, clouds cannot expand beyond the pressure and wind that mark the transition between the troposphere and the stratosphere. However, powerful storms can erupt into the stratosphere in a formation called an overshooting top, bringing with them aerosols, pollutants, and burning biomass.
The stratosphere, home to the ozone layer, acts as a buffer absorbing much of the sun’s radiation. Typically, only rare and dramatic events like violent volcanoes and massive meteors introduce particles into this layer. The incursions identified in this study, while not yet catastrophic, could signify microfractures in the planet’s atmospheric armor.
“This could be a really big deal for a number of reasons,” Cziczo noted. “For one thing, for so long, we’ve assumed the stratosphere is a pristine area. But what this shows is that human impacts through a changing climate can affect the chemistry and the radiative ability of the stratosphere.”
Wildfires and Their Atmospheric Impact
It’s not just summer storms contributing to this phenomenon. Wildfires themselves can become so large that they create their own weather, generating storm clouds called pyrocumulus. These clouds can propel burning ash and biomass directly into the stratosphere. Cziczo observed this during the 2019 Australian bushfires, and as storm seasons grow warmer and drier, this effect is becoming more frequent.
“There are actually two ways for this stratosphere puncture to happen,” Cziczo said. “It can be the one severe fire, but it can also be a bunch of little fires that are just constantly perturbing the stratosphere in a way that we didn’t recognize before.”
Exploring the Stratosphere
The stratosphere is typically the domain of military aircraft, weather and research balloons, and spacecraft. To study it, NASA built a variant of the Lockheed Martin U-2 aircraft, dubbed ER-2 for Earth Resources 2. This aircraft can reach altitudes of 70,000 feet, equipped to detect aerosols, particles, and shifts in pressure, temperature, humidity, and wind.
Based in California at the NASA Armstrong Flight Research Center, the ER-2 temporarily transferred to Kansas to study the Midwest storms. “What’s kind of interesting about this, and this is one of these things that I’m not sure that everybody knows about, is that North America has a monsoon,” Cziczo said. “Warm, wet air from the Gulf of Mexico comes up and gets hung up on the Rockies. That’s what creates a lot of those powerful thunderstorms over the Midwest and through the Great Plains area.”
The ER-2, active since the 1980s, allows researchers to track the footprints of summer storms and fires through the stratosphere. “Using these very sophisticated tools, we were able to tell that it’s not that we’re just throwing a bunch of tropospheric air and putting it in the stratosphere,” Cziczo explained. “Putting this particulate matter in the stratosphere changes the dynamics; it changes the chemistry, and it changes the way that part of the atmosphere works.”
This research was funded by NASA’s Earth Science Project Office.
Looking Forward
The implications of these findings are significant. Understanding how particulate matter affects the stratosphere’s chemistry and stability is crucial. As Cziczo emphasized, “We went to all this trouble to save the ozone layer.” The research team’s ongoing efforts aim to further investigate these atmospheric changes and their potential impact on global climate patterns.
Purdue University, a leading public research institution, continues to support groundbreaking studies like this one, contributing to a deeper understanding of our planet’s complex systems.
