Working amidst gale-force winds and torrential rains, a team of researchers has discovered that hurricanes whip up highly localized "rolls" of wind that bring stormy air from high in the atmosphere to the ground where it wreaks havoc. The finding, reported in today's Science, may explain why places as little as 10 meters apart vary tremendously in the damage they suffer from powerful storms.
Measurements of such tempests are made by zapping raindrops with radar to measure the speed of the winds that carry them. Usually, these measurements are made from many kilometers away, so that it is only possible to tell the average movements of large pockets of air several hundred meters across. Anything that happens at smaller scales is lost. To get a closer look, Joshua Wurman, a meteorologist at the University of Oklahoma in Norman, helped build the Doppler On Wheels--a truck equipped with radar detectors that could edge close enough to hurricanes and tornadoes to track features just 20 meters across.
When hurricane Fran hit the North Carolina coast in 1996, most people scurried out of its way. Wurman and his team, however, did just the opposite and parked themselves squarely in the middle. They measured bands of strong winds--whipping up to 216 kilometers per hour--interspersed alongside bands of slow-moving winds traveling a poky 54 kph. Such bands, say the researchers, are indicative of air whipping around in "rolls," shaped like cylinders lying on their sides. In hurricanes, the fastest moving air rides about 1000 meters above the surface. But according to Wurman, the air currents in rolls carry this fast-moving air down to Earth where it crashes into houses, trees, and cars, slows down, and then is carried aloft again by the swirling air currents.
"The fact that the rolls exist isn't new," says Robert Gall, a meteorologist at the National Center for Atmospheric Research in Boulder, Colorado, "but to see them in a hurricane environment is. We have never seen this kind of detail before." While both Wurman and Gall agree that the high-speed wind in these rolls is probably causing most localized hurricane damage, they add that researchers must brave more gale force winds before they can be sure.