NASA Study Predicts More Severe Storms With Global Warming*

The central and eastern areas of the United States are especially prone
to severe storms and thunderstorms that arise when strong updrafts
combine with horizontal winds that become stronger at higher altitudes.

by Staff Writers
Greenbelt MD (SPX) Sep 04, 2007

NASA scientists have developed a new climate model that indicates that
the most violent severe storms and tornadoes may become more common as
Earth's climate warms.

Previous climate model studies have shown that heavy rainstorms will be
more common in a warmer climate, but few global models have attempted to
simulate the strength of updrafts in these storms. The model developed
at NASA's Goddard Institute for Space Studies by researchers Tony Del
Genio, Mao-Sung Yao, and Jeff Jonas is the first to successfully
simulate the observed difference in strength between land and ocean
storms and is the first to estimate how the strength will change in a
warming climate, including "severe thunderstorms" that also occur with
significant wind shear and produce damaging winds at the ground.

This information can be derived from the temperatures and humidities
predicted by a climate computer model, according to the new study
published on August 17 in the American Geophysical Union's Geophysical
Research Letters. It predicts that in a warmer climate, stronger and
more severe storms can be expected, but with fewer storms overall.

Global computer models represent weather and climate over regions
several hundred miles wide. The models do not directly simulate
thunderstorms and lightning. Instead, they evaluate when conditions are
conducive to the outbreak of storms of varying strengths. This model
first was tested against current climate conditions. It was found to
represent major known global storm features including the prevalence of
lightning over tropical continents such as Africa and, to a lesser
extent, the Amazon Basin, and the near absence of lightning in oceanic
storms.

The model then was applied to a hypothetical future climate with double
the current carbon dioxide level and a surface that is an average of 5
degrees Fahrenheit warmer than the current climate. The study found that
continents warm more than oceans and that the altitude at which
lightning forms rises to a level where the storms are usually more vigorous.

These effects combine to cause more of the continental storms that form
in the warmer climate to resemble the strongest storms we currently
experience.

Lightning produced by strong storms often ignites wildfires in dry
areas. Researchers have predicted that some regions would have less
humid air in a warmer climate and be more prone to wildfires as a
result. However, drier conditions produce fewer storms. "These findings
may seem to imply that fewer storms in the future will be good news for
disastrous western U.S. wildfires," said Tony Del Genio, lead author of
the study and a scientist at NASA's Goddard Institute for Space Studies,
New York. "But drier conditions near the ground combined with higher
lightning flash rates per storm may end up intensifying wildfire damage
instead."

The central and eastern areas of the United States are especially prone
to severe storms and thunderstorms that arise when strong updrafts
combine with horizontal winds that become stronger at higher altitudes.
This combination produces damaging horizontal and vertical winds and is
a major source of weather-related casualties. In the warmer climate
simulation there is a small class of the most extreme storms with both
strong updrafts and strong horizontal winds at higher levels that occur
more often, and thus the model suggests that the most violent severe
storms and tornadoes may become more common with warming.

The prediction of stronger continental storms and more lightning in a
warmer climate is a natural consequence of the tendency of land surfaces
to warm more than oceans and for the freezing level to rise with warming
to an altitude where lightning-producing updrafts are stronger. These
features of global warming are common to all models, but this is the
first climate model to explore the ramifications of the warming for
thunderstorms.