Warming to make US conditions more ripe for tornado-making storms, study says (+video)
Global warming weakens a key ingredient for tornado-making thunderstorms, prior studies held. True, researchers now say, but the number of days in the US where the right conditions exist will increase.
(Page 2 of 2)
Using the most pessimistic scenario for future greenhouse-gas emissions the IPCC is considering for its latest global-warming assessments, the team found that the average number of days conducive to severe thunderstorms increased throughout the year in the eastern US. On a percent basis, winter saw a 50 percent increase in days with severe-thunderstorm potential, while spring sported the largest numerical increase – two additional days by the final 30 years of the 20th century.Skip to next paragraph
In Pictures Where to find greenhouse gases
Subscribe Today to the Monitor
Two days out of 365 is a small number, but depending on a storms' path, just one of those days can be disastrous, as Springfield, Mass., learned in 2011 when it was hit by one of six tornadoes that struck New England on June 1.
Indeed, depending on the season, virtually all regions in the lower 48 are projected to see some increase in days with higher storm potential. In the spring, the most pronounced increase comes to the central US, but increases also extend into the upper Midwest and Northeast as well. Summer sees the lowest increases in days with the potential for severe thunderstorms, with some regions actually projected to see decreases, depending on the model.
On high-CAPE days, shear increased at two altitudes important for the evolution of severe thunderstorms. Sufficient shear between the ground and about 20,000 feet sets a thunderstorm cell to spinning, pushing it to supercell status. Under the right conditions, supercells spawn tornadoes. One of those conditions involves vertical wind shear between ground level and about half-a-mile high in a supercell's vicinity.
This lower-level shear in the results could help researchers sort between environments conducive to severe thunderstorms and those that could add a twister to the severe storm. Even then, however, the presence of this low-altitude shear is not a perfect predictor of an impending tornado, Dr. Trapp cautions.
For Harold Brooks, a senior scientist at the National Oceanic and Atmospheric Administration's Severe Storms Laboratory in Norman, Okla., Diffenbaugh's team has taken important strides over previous efforts to estimate the effects global warming could have on severe thunderstorms in the US.
Previous work had tended to rely on one or two models using one greenhouse-gas emissions scenario, he says. And while Diffenbaugh's team uses one scenario, the team's use of output from 10 models, each producing simulations of each day's conditions at different times of day over a 100-year period "is a big deal," Dr. Brooks says. It allows researchers to explore the impact of climate change on severe thunderstorms at unprecedented levels of detail.
The results contain some oddities, he cautions. South Texas shows up as being a potential breeding ground for severe thunderstorms long after historical data show the region's spring season has ended.
Even before this latest study, previous studies also had pointed to south Texas as a hotbed of severe thunderstorm formation at times that didn't match the instrument record.
Looking closely at the details of these previous studies, south Texas's general environmental conditions were willing, but the triggers for initiation were weak, Brooks says.
"The conditions were there, but in some sense the atmosphere was wasting them," he says.