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Hurricanes Start With Tiny Droplets, Study Hints

Brian Handwerk
for National Geographic News
July 27, 2005
 
Tropical storm season brings annual angst for coastal dwellers, but it
also poses new challenges for scientists pondering how hurricanes and
other storms can build such staggering wind speeds.

New research suggests that a prominent force behind devastating storms is tiny drops of water: the ocean spray from storm-surge waves.

In intense storms, wind-driven waves create a cloud of water droplets suspended above the ocean surface.

This spray acts to dissipate turbulence in the air, the research suggests, allowing airflow to increase rapidly to the extreme wind speeds seen in tropical storms and hurricanes.

Alexandre Chorin, a mathematician at the University of California at Berkeley, describes this model in the current issue of the Proceedings of the National Academy of Sciences. He collaborated with Berkeley mathematician Grigory I. Barenblatt and V. M. Prostokishin of the Shirshov Institute of Oceanology in Moscow.

"This in an intriguing thought," Chorin said. But he cautioned that "deducing that this actually happens in real hurricanes is a long way [off]. Yet, it is possible."

The concept could lend modern scientific support to the ancient sailor's practice of spreading oil on stormy waters to calm them, which might have reduced spray.

The theory could also boost efforts to curb growing storms by dropping environmentally benign liquids or foams into ocean waters. Such products have been tested but have yet to meet with any widespread success.

Storm-Building "Sandwich"

Chorin's research builds on the "sandwich model" of tropical storms developed by the late mathematician Sir James Lighthill.

Lighthill's work in the 1960s and '70s suggested that ocean spray was an important layer between air and sea, a cloud of droplets that constituted a "third fluid."

Ever since Lighthill developed this concept, scientists have struggled to determine just what role an ocean spray layer might play in storm formation.

"There's a pretty strong feeling that spray is a key to understanding this problem [of wind intensity], but just how it works is still an open question," said Kerry A. Emanuel, atmospheric scientist at the Massachusetts Institute of Technology in Cambridge.

Chorin's model shows that spray may lower drag, or friction, in the air, allowing storm winds to ramp up.

Emanuel, meanwhile, conducted research independent of Chorin that suggests spray may play an important role in transferring heat from the sea to the air. This transfer helps power tropical storms.

"A lot of spray could enhance the transfer of heat from the ocean to the atmosphere," Emanuel said.

Chorin's study adds an intriguing piece to the puzzle, but the mathematician is quick to emphasize what the research hasn't done: definitively answer the question of how such storms gain intensity.

"We've done something very specific," he said. "We've suggested that drops in spray can inhibit turbulence in the layer between the ocean and the air and make it possible for extremely high winds to form. We didn't prove that hurricanes are caused by these drops, but it's not that far-fetched."

MIT's Emanuel added, "Getting a handle on this problem is absolutely essential for, among other things, forecasting the intensity of hurricanes."

"Forecasts of [storm] tracks have improved quite dramatically over the past 30 years," he said. "But during the same period intensity predictions have hardly improved at all."

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