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, very nearly as strong.
And those Midwest monsoons, increasingly, are breaking through the ceiling of the sky and into the stratosphere, a typically undisturbed layer of the atmosphere, introducing burning biomass and aerosols from western wildfires with potentially concerning consequences for the ozone layer and the climate.
Like a hole in the hull of a boat leaking in dirty seawater, these storms allow aerosols and particles in from the lower atmosphere, new research shows.
The research was conducted in partnership with NASA using a high-altitude research aircraft taking measurements in the remote reaches of the stratosphere. Dan Cziczo, a professor in the Department of Earth, Atmospheric, and Planetary Sciences in Purdue's College of Science , led the team in conjunction with research scientist Xiaoli Shen. The paper 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 said. "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."
A rent in the vault of heaven
Cziczo and his team study the mechanics of the atmosphere, especially how, why, when and where clouds and storms form. They are especially interested in the way that warm, wet air moves up from the Gulf of Mexico, crashes against the Rocky Mountains and forms severe summer storms and rain, much like summer monsoon in India forms when warm, wet winds collide with the Himalayas.
Big storms and clouds typically can't expand beyond the layer of pressure and wind that marks the change between the troposphere, the layer of the atmosphere closest to the ground, and the stratosphere — it's why so many clouds look like buttes or mesas with flat tops.
But that's not always true. Like a titan punching up through the cloud layer, the top of the storm can become too powerful to be contained and erupt into the stratosphere itself in a formation called an overshooting top. It is a fountain of cloud, a geyser of storm that erupts into the peaceful protective layer of the stratosphere. As it gushes up, it brings with it a burst of air, along with currents of aerosol, and anything in the air below it — including pollutants, aerosols and burning biomass.
Earth's atmosphere is the sheer bubble that protects of our planet like a snow globe. The stratosphere is the realm of the ozone layer, the buffer that absorbs so much of the sun's radiation and helps keep Earth from turning into a Venusian hothouse.
Typically, the only particles that make it up into the stratosphere come from rare, globally notable and dramatic events — violent volcanoes and massive meteors. The incursions scientists found in this study aren't necessarily chinks in the planet's armor — yet. But they might be microfractures. And scientists aren't sure yet what kind of effects these alterations might have.
"This could be a really big deal for a number of reasons," Cziczo said. "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.
These particles can interact with sunlight and heat up, warm the stratosphere. It could affect its stability — which is vital to the planet."
It's not just the summer storms, either. Sometimes the wildfires themselves get so large that they create their own weather — directly generating their own storm clouds, called pyrocumulus , so strong that they catapult their own burning ash and biomass directly into the stratosphere above the fire. Cziczo notes that they observed this in the fires over Australia in the 2019 bush fire season, but that, as storm season warms, dries and increases in severity, 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."
Up and away into the wild blue yonder
The stratosphere is a high and lonely place — usually the domain only of military aircraft, weather and research balloons, the grounded Concorde, and spacecraft passing through on their way up or down, as well as a few remarkable weather phenomena including red sprites and blue jet lightning.
To study it, NASA built a variant of the Lockheed Martin U-2 aircraft — dubbed ER-2 for Earth Resources 2 . Equipped to sniff out aerosols, particles, and shifts in pressure, temperature, humidity and wind rather than adversarial forces and resources, the plane can reach altitudes of 70,000 feet — higher than 95% of the Earth's atmosphere with an effective horizon of 300 miles. (In comparison, the reduced gravity aircraft — also called "vomit comets" — which frequently help train astronauts and conduct low-gravity science experiments, only reach altitudes of about 35,000 feet.)
Those two planes are based in California at the NASA Armstrong Flight Research Center, and the storms were happening in the Midwest, prompting one to temporarily transfer to Kansas.
"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. "Most of us have heard about the Asian monsoon over the Indian subcontinent; these powerful storms that crash up against the Himalayas and drop all this rain. The Midwest has something analogous to that, and it is called the North American monsoon. 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. That's why we wanted to be in Kansas during the summertime; you can reach all these different systems from there. We flew up into Wisconsin, Illinois, Indiana, Upper Midwest, Great Plains and all over. I think we even got as far as Texas."
The ER-2, which has been active since the 1980s, is equipped to measure minute changes in air quality and chemistry, allowing Cziczo and his team to track the footprints of the 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 said. "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. It changes the way it handles heat — it heats it up faster. And that's what we're worried about. That's what we really need to investigate, to understand. We went to all this trouble to save the ozone layer."
This research was funded by NASA's Earth Science Project Office.
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