Large-scale tornado outbreaks increasing in frequency, study finds

The frequency of large-scale tornado outbreaks is increasing in the United States, particularly when it comes to the most extreme events, according to research recently published in Science . The study by researchers including Joel E. Cohen, a visiting scholar at the University of Chicago, finds the increase in tornado outbreaks does not appear to be the result of a warming climate as earlier models suggested. Instead, their findings tie the growth in frequency to trends in the vertical wind shear found in certain supercells'a change not so far associated with a warmer climate. 'What's pushing this rise in extreme outbreaks, during which the vast majority of tornado-related fatalities occur, is far from obvious in the present state of climate science,' said Cohen, the Abby Rockefeller Mauzé Professor at Rockefeller University and Professor of Earth and Environmental Sciences at Columbia University, who conducted the research while a visiting scholar in UChicago's Department of Statistics. Tornado outbreaks are large-scale weather events that last one to three days, featuring several thunderstorms and six or more tornadoes in close succession. In the study, published in the Dec. 16 issue of Science , the researchers used new statistical tools, including extreme value analysis'a branch of statistics dealing with deviations'to analyze observation-based meteorological estimates associated with tornado outbreaks together with National Oceanic and Atmospheric Administration datasets.