NASA/Goddard Space Flight Center

Public release date: 30-Aug-2007

Contact: Lynn Chandler

lynn.chandler-1@nasa.gov

301-286-2806

NASA study predicts more severe storms with global warming

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.

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The Goddard Institute for Space Studies is a leading center in the

study of Earth's past, present and future climates.

http://www.nasa.gov/centers/goddard/news/topstory/2007/moist_convection.html

Written By: Leslie McCarthy

posted to ClimateConcern