Continued global warming could help create atmospheric conditions that are fertile breeding grounds for severe thunderstorms and tornadoes in the United States, a new study suggests.
The findings, detailed in this week's issue of the journal of the Proceedings of the National Academy of Sciences, are based on the most comprehensive computer modeling work done to date of the two main atmospheric ingredients that scientists think contribute to thunderstorm formation.
One of those ingredients is known as the convective available potential energy, or CAPE, that is created as air in the lower atmosphere warms. The warm air rises, whisking moisture to higher altitudes.
The second ingredient is vertical wind shear, which is essentially the change in wind speed with height.
For a severe thunderstorm to arise, CAPE must interact with strong vertical wind shear. But earlier studies concluded that while global warming will increase CAPE, it will decrease wind shear. Thus, the two ingredients were thought to cancel each other out.
But the new computer simulations by climate scientists Noah S. Diffenbaugh and Martin Scherer of Stanford University and Robert J. Trapp of Purdue University revealed a pattern that was missed in previous modeling work.
"What we've found is that the reduction in shear actually falls on days when there's low CAPE," explained Diffenbaugh.
In other words, when CAPE is high, vertical wind shear is more likely to be high as well, which means the total frequency of occurrence of severe thunderstorm environments actually increases as a result of global warming.
Diffenbaugh says his team would not have made their discovery had they not been able to access and run multiple climate models that were previously scattered across different scientific groups around the world.
"We had ten global climate data models running exactly the same experiment ... and they show high agreement on the results," Diffenbaugh said.
Why Is It Important?
The simulations all agreed that continued global warming will lead to increases in storm days over large areas of the eastern United States in the spring, winter, and autumn.
The changes mean that for springtime alone, severe thunderstorms could increase by as much as 40 percent over the eastern U.S. by the end of the century, Diffenbaugh said.
What Does This Mean?
Severe thunderstorms, which are often associated with heavy rainfall, hail, and even tornadoes, are one of the primary causes of major economic losses in the United States.
In 2012, seven of the country's $11 billion weather disasters were caused by severe thunderstorms and tornadoes.
"Sadly, we have many examples of cases where a single storm has had disastrous impact," Diffenbaugh said in a statement. "So a 25 or 30 percent increase in the annual occurrence represents a substantial increase in the overall risk."
Diffenbaugh stressed that this latest modeling work looks only at the likelihood that the atmospheric conditions that give rise to severe thunderstorms will form.
The models stop short of actually predicting whether these ripe atmospheric conditions will spawn severe thunderstorms or tornadoes. As a result, they cannot tell scientists whether future global warming–fueled storms will be more powerful or destructive than they are now.
The team hopes to create more sophisticated models that will be able to overcome this limitation. "What we need to do next is develop ways to better represent the processes that produce individual storms in the real atmosphere," Diffenbaugh said in statement.
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