Aug 30, 6:58 PM EDT
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Study Predicts More frequent and  Severe U.S. Storms*

By SETH BORENSTEIN
AP Science Writer

WASHINGTON (AP) -- As the world warms, the United States will face more
severe thunderstorms with deadly lightning, damaging hail and the
potential for tornadoes, a trailblazing study by NASA scientists suggests.

While other research has warned of broad weather changes on a large
scale, like more extreme hurricanes and droughts, the new study predicts
even smaller events like thunderstorms will be more dangerous because of
global warming.

The basic ingredients for whopper U.S. inland storms are likely to be
more plentiful in a warmer, moister world, said lead author Tony Del
Genio, a NASA research scientist.

And when that happens, watch out.

"The strongest thunderstorms, the strongest severe storms and tornadoes
are likely to happen more often and be stronger," Del Genio said in an
interview Thursday from his office at the Goddard Institute for Space
Studies in New York. The paper he co-authored was published online this
month in the journal Geophysical Research Letters.

Other scientists caution that this area of climate research is too
difficult and new for this study to be definitive. But some upcoming
studies also point in the same direction.

With a computer model, Del Genio explores an area that most climate
scientists have avoided. Simple thunderstorms are too small for their
massive models of the world's climate. So Del Genio looked at the forces
that combine to make thunderstorms.

A unique combination of geography and weather patterns already makes the
United States the world's hottest spot for tornadoes and severe storms
in spring and summer. The large land mass that warms on hot days, the
contours of the atmosphere's jet stream, the wind coming off the Rocky
Mountains and warm moist air coming up from the Gulf of Mexico all combine.

Del Genio's computer model shows global warming will mean more strong
updrafts, when the wind moves up and down instead of sideways.

"The consequences of stronger updrafts are more lightning and bigger
hail," he said.

On a normal sunny day, updrafts are less than 1 mile per hour. In a big
rainstorm that is not severe, it's about 2 mph. In a severe storm they
could be 20 to 30 mph. The faster that updraft, the worse the storms.

The Southeast and Midwest lie in the path of most of the most dangerous
of these storms.

However, the new study also forecasts danger for the Western United
States. It predicts lightning will increase about 6 percent as the
amount of carbon dioxide - the chief global warming gas - doubles.

Previous studies have shown that the West will get drier, making it a
tinderbox for more wildfires. This study shows that there will be more
matches in the form of lightning strikes to start those fires, Del Genio
said.

One general benefit of global warming is decreased wind shear, which is
the speed of side-to-side wind as the altitude rises, Del Genio said.
That would moderate the effects of updrafts.

However, during certain times of the year and under the right conditions
in the Midwest and Southeast, wind shear will increase. Combine wind
shear and updrafts, and damaging winds result, the scientist said.

Other pending and recent research, especially from the National Oceanic
Atmospheric Administration, point in the same general direction, said
several scientists who weren't involved in Del Genio's study. But they
said research in this area is so new that the NASA study is not the
final word.

"It's certainly a plausible result," said Leo Donner, a climate modeling
scientist at NOAA's Geophysical Fluid Dynamics Lab in Princeton, N.J.
Donner earlier this year came out with a study predicting more heavy
rain as temperatures rise.

Harold Brooks, a top scientist at NOAA's severe storms laboratory in
Norman, Okla., has soon-to-be-published studies finding results similar
to the new NASA study, especially when it comes to hail. Some of the
severe hail that should be increasing could be baseball-sized and come
down at 100 mph, "falling like a major league fastball," he said.

He said it's not possible to predict more tornadoes will result from
climate change, however.

Jerry Mahlman, who used to be NOAA's top climate model expert, said that
a decade ago then-Vice President Al Gore asked if global warming could
cause more tornadoes. Then as now, Mahlman said that's something that's
just too detailed to derive from complex climate models.

Mahlman, a scientist who has long warned about the dire consequences of
global warming, cautions against going overboard on climate change
links: "I'm beginning to suspect that global warming is dynamically much
less sexy than people want it to be."

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On the Net:

NASA's Goddard Institute of Space Studies:

http://www.nasa.gov/centers/goddard/news/topstory/2007/moist-convecti...