Taming the Unpredictable Tempest

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Robert Irion
1999-10-13 19:00
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Few winds are more fickle than those of a hurricane. The strength of a cyclone can grow unpredictably, with dire consequences for anything in its way. Now, a new model of hurricane dynamics suggests that a storm's intensity hinges on a few simple variables, most importantly the temperature of the ocean in the storm's path. According to a report in tomorrow's Nature, the model may help forecasters predict a hurricane's power as accurately as they can now predict where it will strike land.

Satellite images, more realistic models, and faster computers have spurred steady improvements in forecasting the tracks of hurricanes. But predictions of sustained wind speeds have not followed suit because of what meteorologists saw as an impossibly complex interplay among the atmosphere, the ocean, and the storm's internal motions. As a result, they didn't foresee sudden jumps in intensity that have turned some average hurricanes into killers--or equally sudden weakenings that have rendered storms impotent before they hit shore.

The secret lies in probing the ocean's upper layer, says atmospheric scientist Kerry Emanuel of the Massachusetts Institute of Technology in Cambridge. Hurricanes draw strength from the temperature difference between warm tropical waters and the cool atmosphere at the cloud tops. But when a storm churns over the ocean, it stirs up cold water from deeper layers, which saps a storm's engine more quickly than any other factor, Emanuel found. If the ocean's warm layer extends to great depths--as it does along the Gulf Stream off the southeastern United States--no cool water rises, and the storm can grow mightier.

Emanuel plugged data about the temperature and the depth of the top layer into his model, as well as about the storm's initial strength, its speed, and the rate of cooling with altitude in the atmosphere. The model successfully reproduced most of the intensity variations seen in eight major hurricanes since 1969. "It's so simple that I'm surprised it worked," Emanuel says. "Intensity prediction may no longer be a hopeless case."

Emanuel says that much work remains to make the model useful to forecasters, such as accounting for wind shear that can weaken some storms. Those improvements are feasible, says meteorologist Hugh Willoughby of the National Oceanic and Atmospheric Administration's Hurricane Research Division in Miami, Florida. "It's exciting that this model works on real cases of past hurricanes," Willoughby says. "[Emanuel] may be onto something."

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