As winds kick up, the ocean sprouts whitecaps and spews streams of spray. Past research has hinted that the ocean's aerodynamic roughness, or wind resistance, reaches a plateau in strong hurricanes. But a new study suggests otherwise: At extremely high wind speeds, blowing spray and foam create a sort of veneer that lets air glide across the waves almost without friction—an unexpected phenomenon that needs to be included in computer models of hurricanes, scientists say.
The new study includes analyses of wind data gathered from instruments that "hurricane hunter" aircraft dropped into storms between 1998 and 2005, as well as films of the ocean surface taken during low-altitude flights in hurricanes from 1966 through 1980. More than one-third of the data hasn't been published before, and much of it couldn't be collected today because current safety rules ban flights in such perilous conditions, says Leo Holthuijsen, an oceanographer at the Delft University of Technology in the Netherlands.
The aerodynamic roughness of a surface is measured using a parameter called the coefficient of drag. This is the amount of friction divided by the surface area. For wind speeds less than 35 meters per second (about 126 kilometers per hour, a Category 1 hurricane), Holthuijsen and his colleagues found that—as expected—the coefficient of drag rose as wind speed increased. But at wind speeds of about 40 meters per second (144 km/hr), the streaks of foam and spray blown from whitecap-topped waves merged to produce whiteout conditions, largely masking the ocean's surface. At the same time, the coefficient of drag began to drop.
By the time wind speeds reached 80 meters per second (288 km/hr, a Category 5-strength hurricane), the coefficient of drag had plummeted to near zero, the researchers reported last week in the Journal of Geophysical Research-Oceans. At wind speeds like those, waves in the open ocean typically measure between 20 and 30 meters high. In such conditions, Holthuijsen says, "troughs between waves are like holes in the sea, and the wind just jumps over them."
The drop in drag coefficient at high wind speeds could have profound effects. For one thing, when the surface becomes aerodynamically smooth, the wind can't transfer much momentum to the water, so waves may not grow as high as current models predict, Holthuijsen says. He and his colleagues are fine-tuning their models to see what effect super-strong winds might have on wave height and storm surge.
Notions about reduced drag in hurricane-strength winds have been wafting around for many years, says Vincent Cardone, an oceanographer and co-founder of Oceanweather Inc. in Cos Cob, Connecticut. But the new research provides more field evidence for the phenomenon, he says, especially at high wind speeds.
Holthuijsen and his team "have taken an interesting approach with a unique data set," says Klaus Hasselmann, a climate scientist at the Max Planck Institute for Meteorology in Hamburg, Germany. The suggestion that sheets of spray and foam blown from whitecaps somewhat insulate the sea from wind-induced drag "is very plausible," he says. Because extremely high wind speeds occur only rarely and fleetingly over minute patches of sea, their effects would likely be very localized. Still, he says, "if you're modeling hurricanes, those effects could be important."
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