Distant pollution may intensify U.S. twisters

Sooty smoke drifting across the Gulf of Mexico may be able to boost the strength of twisters forming in and around North America’s “Tornado Alley.” That’s the finding of a new study.

It comes from analyzing a particularly severe April 27, 2011, tornado outbreak. On that one day, 122 twisters plowed across the southeastern United States. Fifteen especially violent ones roared through with wind speeds reaching 270 kilometers per hour (167 miles per hour) or greater. Eight of the day’s twisters took paths along the ground that were each more than 80.5 kilometers (50 miles) long. When the debris settled, 316 people lay dead.

Researchers soon began looking for what might have prompted this outbreak, one of the deadliest in U.S. history. And they became curious about smoke that had been in the air. It had enhanced conditions that already favored the development of intense tornadoes, they now claim.

The smoke had arrived from fires in Central America. Its sooty particles lowered the cloud base in the southeastern states. It also increased the difference in wind velocity — something known as wind shear — at different elevations. Both of those factors, cloud base and wind shear, play a big role in brewing severe twisters, the researchers note.

While more research is needed to quantify how large an effect this smoke had on tornado strength, tracking soot in the air could one day improve tornado forecasts, says lead author Pablo Saide. He’s 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.”

His team’s new findings appear in a paper to be published soon in Geophysical Research Letters.

The smoke’s role

In late April 2011, masses of warm and cold air collided over North America’s Southern Plains. (This region runs from eastern New Mexico, Oklahoma and parts of Texas into Louisiana and Arkansas.) The mixing air masses spawned rotating thunderstorms called supercells. Their winds rotate horizontally, like a rolling pin. But some conditions can tilt upright those rotating supercells. And this is the first step in tornado formation.

As storm supercells were forming in late April, large amounts of sooty smoke were spewing into them from fires in Central America. Farmers there often deliberately set their fields ablaze in the spring. This can clear out old crops and fertilize the soil for planting. But the fires’ hot smoke also can climb high into the sky and travel long distances — certainly across the Gulf of Mexico.

Saide and his colleagues knew that. So they assembled a computer program to simulate those Central American fires and how their smoke might travel. Such programs are referred to as computer models. To evaluate the role that smoke played, the researchers ran those computer models again and again. Each time they varied the conditions somewhat. In this way, the scientists could compare what might happen with and without the smoke.

Tiny smoke particles entering the United States formed the centers of water droplets in clouds, the researchers found. This increased the number of individual droplets making up each cloud. More smoke also decreased the chance that the clouds’ water would rain out. And bulkier clouds would cut how much sunlight could get through to heat the ground.

If the ground was cooler, the model predicted, the relative humidity of the air would have increased. These conditions would have dropped the elevation of the bottom cloud layer by roughly 100 to 200 meters (330 to 660 feet).

Cooler ground temperatures also would have reduced the amount of air mixing, creating strong wind shears. Lower cloud bases and stronger wind shear are conditions favorable for tipping horizontally rotating winds into an upright, rotating column. And from there, this storm system can morph into a violent tornado, Saide says.

Smoke helped, but wasn’t necessary

Conditions in late April 2011 would have triggered a severe tornado outbreak regardless of the fires, Saide says. The smoke just boosted their intensity. His team now plans to look at smoke’s role in other tornado outbreaks. If a link proves strong, he says, adding smoke particles into the computer models used to make weather forecasts might improve the accuracy of severe weather forecasts.

But some scientists think it’s too early to link smoke to severe twisters. The April 27, 2011 outbreak wasn’t the best event to look for such a link, says Stephen Corfidi. He’s a meteorologist at the National Oceanic and Atmospheric Administration’s Storm Prediction Center in Norman, Okla. “Those storms,” he argues, “were going to be potent regardless of whether there was smoke in the air.”

Paul Markowski agrees. More studies are needed before forecasters will know if adding smoke into tornado predictions is useful, he says. Markowski is an atmospheric scientist at Pennsylvania State University in State College. “In a perfect world, we’d like to include everything in predictions,” he says. But it’s still not clear if smoke “is a really important effect.”

Power Words

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cloud     Airborne water droplets and ice crystals that travel as a plume, usually high in Earth’s atmosphere. Their movement is driven by winds.

computer  An electronic device that processes information based on rules stored in the device.

computer model  A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.

computer program  A set of instructions that a computer uses to perform some analysis or computation. The writing of these instructions is known as computer programming.

humidity  A measure of the amount of water vapor in the atmosphere.

meteorologist  Someone who studies weather and climate events.

outbreak   The sudden emergence of disease in a population of people or animals. The term may also be applied to the sudden emergence of devastating natural phenomena, such as earthquakes or tornadoes.

plume   (in environmental sciences) The movement of some gas or liquid, under the direction of gravity, winds or currents. It may be in air, soil or water. It gets its name from the fact that it tends to be long and relatively thin, shaped like a large feather.

smoke    Plumes of microscopic particles that float in the air. They can be comprised of anything very small. But the best known types are pollutants created by the incomplete burning of oil, wood and other carbon-based materials.

soot      Also known as black carbon particles,these are the residues of incompletely burned materials, from plastics, leaves and wood to coal, oil and other fossil fuels. The particles can be quite small — nanometers in diameter. If inhaled, they can end up deep within the lung.

supercell A rotating thunderstorm that can produce a violent tornado.

tornado A violently rotating column of air extending from the ground to a thunderstorm above.

wind shear The effect of winds at different levels above the ground blowing in different directions or at different speeds.