Midwest summer storms push smoke into stratosphere

Purdue University — 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 and very nearly as strong.

These Midwest monsoons are increasingly breaking 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. These storms allow aerosols and particles in from the lower atmosphere, new research shows.

The research was conducted in partnership with NASA using the Lockheed Martin U-2 dubbed ER-2 for Earth Resources 2, a high-altitude research aircraft taking measurements in the remote reaches of the stratosphere. Prof. Dan Cziczo of Purdue University’s College of Science led the team in conjunction with research scientist Xiaoli Shen.

“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.”

Cziczo and his team 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.

But that’s not always true. The top of the storm can become too powerful to be contained and erupt into the stratosphere in a formation called an overshooting top. It’s a fountain of cloud and 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.

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.

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, called pyrocumulus and is so strong that they catapult their own burning ash and biomass directly into the stratosphere above the fire.

To study it, NASA’s ER-2 can 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 per cent of the Earth’s atmosphere with an effective horizon of 300 miles.

The ER-2 is equipped to measure minute changes in air quality and chemistry, tracking footprints of the summer storms and fires through the stratosphere.