Climate study projects more severe thunderstorms

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Lightning flashes over Peak 1 in Frisco, Colorado, during a late-summer thunderstorm. bberwyn photo.

Eastern U.S. may see more winter and autumn storms

By Summit Voice

FRISCO — A warmer and wetter atmosphere is likely to drive up the number of severe thunderstorms in coming decades, potentially resulting in more economic losses associated with extreme weather.

The new study led by Stanford University scientists shows  that global warming is likely to cause a robust increase in the conditions that produce these types of storms across much of the country over the next century, including more severe weather during the spring, fall and winter.

To date, efforts to project thunderstorm activity under various global warming scenarios has been hampered by sparse historical data. But the Stanford-led team, headed by professor Noah Diffenbaugh, was able to use a complex ensemble of physics-based climate models to produce the most comprehensive projections of severe storm conditions for the next century.

The formation of severe thunderstorms is based on two main ingredients: Convective available potential energy, created as the air in the low atmosphere warms. The warm air rises, carrying with it moisture to higher altitudes, and strong vertical wind shear — essentially a moving wind current that organizes the atmospheric energy and moisture such that it can sustain a storm.

Climate researchers have previously hypothesized that global warming will increase CAPE and cause an overall decrease in wind shear, which created uncertainty about the net effect, but the new models developed by Diffenbaugh and colleagues suggests that those changes will play out in unexpected ways.

Global warming is expected to decrease wind shear on days that aren’t as prone to thunderstorm formation to begin with. The net effect is that the increases in CAPE on other days drive increases in the occurrence of severe thunderstorm environments.

“We’re seeing that global warming produces more days with high CAPE and sufficient shear to form severe thunderstorms,” said Diffenbaugh, who is also a senior fellow at the Stanford Woods Institute for the Environment.

The study was scaled to look at boxes about 60 miles per side, assessing the climate conditions that could emerge over the next century. The analysis showed the biggest changes occurring in the spring season, with each box in the central United States experiencing about two-and-a-half additional storm days per spring by the late 21st century.

The researchers also reported that sustained global warming is likely to cause robust increases in storm days over large areas of the eastern United States not only in spring but also in winter and autumn. While the summer season also showed increases over the region as a whole, those increases were the least robust within the region and across the different climate models.

An additional few days of severe storm conditions might not seem like a large change, but Diffenbaugh emphasized that the projected increases are in fact substantial compared to the frequency of occurrence in the current climate.

“We are looking at the conditions that produce severe events, which are relatively rare at present,” Diffenbaugh said. “For example, the changes during spring represent an increase of about 40 percent over the eastern U.S. by the late 21st century.”

Diffenbaugh also emphasized even a single severe storm can cause very high levels of damage.

“The severe thunderstorms we experience now can result in very high economic losses,” Diffenbaugh said. “Sadly, we have many examples of cases where a single storm has had disastrous impact. So a 25 or 30 percent increase in the annual occurrence represents a substantial increase in the overall risk.”

More tornadoes?

Such storms also create conditions that can lead to tornado formation, although the researchers stress caution in drawing conclusions specifically about the effect of global warming on tornadoes.

“We have tried to analyze the atmospheric conditions that are associated with tornadoes,” Diffenbaugh said. “Although we do see that those conditions increase in occurrence in response to global warming, it is important to bear in mind that we are not resolving tornadoes in these experiments.”

Diffenbaugh hopes to build on this research to improve the understanding of the atmospheric dynamics that lead to the development of severe thunderstorms, and to better incorporate those processes into climate models.

“These are rare but significant events,” Diffenbaugh said. “This new set of global climate model experiments has provided some important new insights. What we need to do next is develop ways to better represent the processes that produce individual storms in the real atmosphere.”

The study is published in the current issue of Proceedings of the National Academy of Sciences.

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