Central American fires may intensify U.S. tornadoes

Smoke wafting across the Gulf of Mexico from Central America can help spawn intense twisters in and around North America’s Tornado Alley, new research suggests.

Reconstructing the extreme April 27, 2011 tornado outbreak, which sired 122 twisters across the Southeastern United States, researchers found that smoke particles in the atmosphere further enhanced conditions already favorable for intense tornado formation. The smoke lowered the cloud base and increased the difference in wind velocity at different elevations —two important mechanisms in brewing severe twisters —the researchers report in a paper to be published in Geophysical Research Letters.

While more research is needed to quantify how large smoke’s effect is on tornado intensity, tracking soot in the air could one day improve tornado forecasts, says lead author Pablo Saide, an atmospheric scientist at the University of Iowa in Iowa City.

“We live in a very complex system where tiny particles such as smoke can have a measurable effect on extreme weather outbreaks,” he says. “We have to take all effects into account.”

In late April 2011, warm and cold air masses collided over North America’s Southern Plains, spawning rotating thunderstorms called supercells. When horizontally rotating winds in supercells tilt vertically, the resulting formation can transform into a tornado. On April 27 alone, 122 tornadoes touched down in the United States, including 15 with wind speeds above 267 kilometers per hour. During that day, the storms killed more than 300 people, making the outbreak the deadliest since 1974.

At the time of the outbreak, large amounts of smoke spewed by agricultural fires drifted across the Gulf of Mexico from Central America and into the United States. In the spring, Central American farmers often deliberately set fire to their fields in order to clear out old crops and fertilize the soil for planting. To see whether the smoke played a role in the record-setting U.S. tornado outbreak, Saide and colleagues assembled a computer simulation of the region. In their reconstruction, the researchers could compare atmospheric conditions with and without the smoke.

The wave of tiny smoke particles entering the United States formed the center of water droplets in clouds, the researchers found. This process increased the number of individual droplets making up each cloud, causing the clouds to precipitate less and block more sunlight from reaching the ground.

With less sunlight coming through, the ground emitted less heat. Less heat radiating from the ground increased the relative humidity, causing the bottom cloud layer to drop in elevation by roughly 100 to 200 meters, the team found. The colder ground also reduced the amount of mixing between different atmospheric layers, increasing the variance in wind speed at different elevations, known as wind shear, by about 2 meters per second. Lower cloud bases and stronger wind shear are conditions favorable for horizontally rotating winds to tip vertically and morph into a violent tornado, Saide says.

While atmospheric conditions would have triggered a severe tornado outbreak regardless of the fires, the presence of smoke promoted the development of especially intense tornadoes, Saide says. The researchers plan to investigate the effects of smoke on other tornado outbreaks to quantify how large a role the smoke plays. Including smoke particles in weather forecast simulations could improve the timeliness and accuracy of severe weather alerts, Saide says.

The April 27, 2011 outbreak wasn’t the best event to showcase the impact of smoke on tornadoes, says meteorologist Stephen Corfidi of the National Oceanic and Atmospheric Administration’s Storm Prediction Center in Norman, Okla. “On a day like this one, you could tweak and nibble at the edges of what was responsible for the event, but you’re doing window dressing basically,” he says. “Those storms were going to be potent regardless of whether there was smoke in the air.”

More studies on the impact of smoke on tornado intensity are needed before forecasters can determine whether including smoke in predictions will be worthwhile, says Paul Markowski, an atmospheric scientist at Penn State. “In a perfect world, we’d like to include everything in predictions,” he says. “But it remains to be seen whether this is a really important effect or if it takes a backseat to a number of other things.”