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Monday, September 23, 2013

Springtime severe thunderstorms could increase by as much as 40% over the eastern USA: Noah Diffenbaugh

by Doyle Rice, USA Today, September 23, 2013

Deadly and destructive thunderstorms -- and the violent tornadoes they produce -- are forecast to see a "robust" increase across parts of the U.S. in upcoming decades because of climate change, says a new study published Monday in the Proceedings of the National Academy of Sciences.
Springtime severe thunderstorms could increase by as much as 40% over the eastern U.S. by the end of the century, says lead author Noah Diffenbaugh of Stanford University.
The study is one of the first that's found such a link between climate change and severe storms. Most previous research has been inconclusive.
In recent years, severe storms and tornadoes have produced more destruction than any other weather phenomena and are typically one of the main causes of catastrophic losses in the U.S. In 2012, seven of the USA's $11 billion weather disasters were because of severe thunderstorms and tornadoes. 
Unfortunately, tornadoes and severe thunderstorms are the extreme weather phenomena that we have the least understanding of with respect to climate change, according to Weather Underground meteorologist Jeff Masters. Additionally, more than 50 years of U.S. tornado records from the Storm Prediction Center find no increase in the number of tornadoes across the country; there are also fewer strong tornadoes.
Two primary ingredients are needed to produce severe thunderstorms: The first is the fuel needed for these monster storms to develop -- what scientists call "convective available potential energy" (CAPE). CAPE is created as the air in the lower atmosphere warms: The warm air rises, carrying with it moisture to higher altitudes.
The second is vertical wind shear, a strong current of wind that helps spin up energy and moisture in the atmosphere.
Many previous studies have found that while CAPE may increase because of global warming, wind shear may decrease, so the two ingredients cancel each other out.
However, in this study, "we're seeing that global warming produces more days with high CAPE and sufficient shear to form severe thunderstorms," Diffenbaugh said. The net effect is that there will be more days overall with both high CAPE and high shear.
Diffenbaugh's study co-authors were Martin Scherer of Stanford and Robert Trapp of Purdue University. The research team used a group of complex climate computer models to look at how the atmosphere will react to global warming.
In addition to spring, the study also predicts an increase in storm days in winter and fall as well. Overall, for the entire year, the researchers forecast an increase in severe storms of 25 to 30%.
"Sadly, we have many examples of cases where a single storm has had disastrous impact," Diffenbaugh says. "So a 25 or 30 percent increase in the annual occurrence represents a substantial increase in the overall risk."
"This study is fairly robust," says Kevin Trenberth, senior scientist at the National Center for Atmospheric Research in Boulder, Colo., who was not part of the study. 
"The prospect is worrying, with the increases in severe thunderstorms and the risk of tornadoes having the potential for major impacts in the U.S."

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