By Steve Koppes
Photo courtesy of Anthony Boccaccio/National Geographic Stock
“ What’s unusual about this year is the intensity and the number of tornadoes that we observed.”
Professor in geophysical sciences
Nature’s might created a “perfect storm” for Noboru Nakamura’s 2011 winter quarter Natural Hazards course.
“We had this remarkable snowstorm in Chicago and our own midterm had to be postponed,” says Nakamura, professor in geophysical sciences, referring to the University’s closure following the Feb. 2 blizzard. “And then just before the final exam, the earthquake and tsunami in Japan happened.”
Disasters may seem like sporadic and unusual events, “but they’re frequent and all over the place,” Nakamura says. Earlier this year, major earthquakes also struck New Zealand, a Category 5 hurricane hit Australia, and in the United States, drought, flooding, and record tornadoes have become commonplace.
UChicago scholars have made significant strides in the study of natural disasters, thanks in part to the historic contributions of the late Tetsuya “Ted” Fujita, who in 1971 developed the widely used Fujita Scale (F Scale), which rates tornado strength. A refined version of Fujita’s work, the Enhanced Fujita Scale, remains the standard used by the National Oceanic and Atmospheric Administration. Fujita also discovered the dangers that microbursts—damaging columns of sinking air in thunderstorms—posed to aircraft.
Fujita’s legacy still influences UChicago researchers such as Nakamura, as well as Fujita’s former students who have risen to prominence in the field of geosciences. One of Fujita’s students, Roger Wakimoto, PhD’81, has been tracking the effects of this year’s violent tornado season as director of the National Center for Atmospheric Research in Boulder, Colo. He says this is one of the most destructive seasons he has experienced during his meteorological career. “The death total has been staggering,” says Wakimoto.
Super outbreak of tornadoes
Among the meteorologists and engineers who formulated the EF Scale was another Fujita student, the Weather Channel’s severe weather expert, Greg Forbes, SM’73, PhD’78, profiled by UChicago Magazine in 2008. Forbes devoted his doctoral dissertation at UChicago to the super outbreak of 148 tornadoes that tore through 13 states on April 3-4, 1974, which resulted in 315 fatalities.
Although eclipsed in multiple categories by the tornado outbreak of April 26-28, 2011, “the 1974 Superoutbreak still holds several records, including the numbers of tornadoes rated F2 and higher, F4 and higher, and killer tornadoes,” Forbes wrote in a June 13, 2011 blog posting.
Fujita died in 1998, but if alive today he would undoubtedly be taking a keen interest in studying the aftermath of this year’s powerful tornadoes. “I can picture him performing aerial and ground damage surveys and probably documenting features that others missed,” Wakimoto says.
April to June is the season for tornadoes in the United States, with the most powerful ones typically occurring in the “Tornado Alley” region of the Great Plains. “What’s unusual about this year is the intensity and the number of tornadoes that we observed,” Nakamura says.
The jet stream, a high-altitude “river” of air moving eastward at an average speed of approximately 80 miles an hour, has packed considerable winds over Tornado Alley this year. “The consensus among meteorologists is that this has something to do with the La Niña condition,” Nakamura says. The opposite of El Niño, La Niña is a multi-year temperature pattern that brings unusually cold sea surface temperatures to the eastern tropical Pacific Ocean.
“Even though La Niña is a tropical phenomenon, it has a ripple effect over the global circulation pattern, and in particular, over the United States,” Nakamura says. At the start of spring, cold Arctic air begins to break up and migrate toward lower latitudes, where warmer temperatures predominate near the ground.
Jet stream energy transfer
This combination creates an unstable, actively convecting atmosphere over Tornado Alley. As the atmosphere overturns, it transfers the extra energy of the jet stream to lower altitudes, where it can feed extra power to developing tornadoes.
“When you compare the La Niña years with the El Niño years, you do see significant statistical differences,” Nakamura says. “The La Niña years tend to have stronger tornadoes and more frequent and more violent tornado outbreaks.”
Although the most severe part of the tornado season is winding down, NCAR’s Wakimoto notes, “tornadoes can occur any time of the year.” This especially applies to the peak hurricane season in the Atlantic, which runs from July through November, when landfalls often become associated with twisters, he says.
Nakamura has developed a keener interest in severe weather since he began teaching his Natural Hazards course three years ago, but he specializes in theoretical and computational research on Earth’s large-scale fluid dynamics. Fujita took a more hands-on, storm-chasing approach to the study of localized severe-weather phenomena.
“He was really a field meteorologist, and I learned a lot about atmospheric science from a very different perspective because his approach was so different from mine,” Nakamura says. “I really looked up to him as the type of atmospheric scientist that I can aspire to be. He’s gone but his legacy continues, not just here but elsewhere.”
Wakimoto also counts himself among Fujita’s numerous admirers. “Ted has always been my guiding light, and for others too,” he says. “The fact that he was referred to as ‘Mr. Tornado’ says it all.”
By Steve Koppes