Smoking may stunt human growth, but a study has found it does the opposite for tornadoes, with smoke from distant fires driving America's most destructive twister outbreak in more than eighty years.
On April 27, 2011, six American states were whipped by tornadoes, leading to 316 deaths, thousands of injuries and billions of dollars in damage. In Geophysical Research Letters, a team from the University of Iowa argue that smoke particles contributed to the severity of the event.
Professor Gregory Carmichael says that for tornadoes to form, the right underlying conditions—which have nothing to do with fires—need to exist. However, when a supercell occurs, as frequently happens in the region, an abundance of smoke particles can increase the intensity of those tornadoes that form. Most of the deaths from the more than 200 tornadoes that day came from the 19 classed as EF4 and EF5.
Credit: Martin Haas / Shutterstock.com Damage from the April 27 tornadoes was intense.
Smoke lowers the base of thunderclouds, Carmichael argues, creating a shorter distance the funnel needs to reach the ground. Moreover, it increases wind shear. Both of these increase tornado intensity.
By mapping the spread of smoke across America on that April day, and comparing this with the intensity of the tornadoes spawned, Charmichael and his co-authors show that the connection is not just theoretical. They also modelled the event, including everything known about the atmospheric conditions on that day. This was compared with another model that included everything other than the spread of smoke. The one model matches what was seen closely, while the other fails to show the observed intensity.
Credit: Brad Pierce, NOAA Satellite and Information Service Center for Satellite Applications and Research. Tornado tracks are shown as red lines, fires as yellow dots, while red also marks the thickest particulate density.
"These results are of great importance, as it is the first study to show smoke influence on tornado severity in a real case scenario. Also, severe weather prediction centers do not include atmospheric particles and their effects in their models, and we show that they should at least consider it," says Carmichael.
The reason there were so many smoke particles in the air that day is that huge areas of Mexico were on fire from land-clearing burns and the prevailing wind conditions carried the smoke across the Gulf of Mexico.
This is not an exceptional occurrence, as the paper notes: “Tornadoes in the Southeast and Central U.S. are episodically accompanied by smoke from biomass burning in Central America.” This raises the question of what can be done with the information revealed in the paper. Better forecasting may save lives, but it will do little to limit the property damage. Could the United States persuade Mexican farmers to perhaps shift the timing of their burns to reduce the downwind impact? If so, what precedent will be set for the more diffuse, but ultimately more devastating, effects of other atmospheric emissions?