Rain slows whipping hurricane winds

Heavy downpours put a damper on hurricanes, new research suggests.

Running simple hurricane simulations, researchers have demonstrated that descending raindrops produce significant friction as they fall along the edges of a hurricane’s eye. This friction slows the powerful winds that drive the storm, lessening the hurricane’s intensity by as much as 30 percent, the researchers report in a paper to be published in Geophysical Research Letters.

Although the finding’s ultimate benefit to hurricane forecasting remains uncertain, it demonstrates that meteorologists shouldn’t overlook rain friction, says coauthor and physicist Pinaki Chakrabortyof the Okinawa Institute of Science and Technology in Japan.

“We found that the rain drag in a hurricane is comparable to the ocean energy that fuels that hurricane,” he says. “This is not something negligible that can be thrown away.”

Meteorologists gauge a hurricane’s intensity based on the storm’s highest sustained wind speeds. In most storms, the strongest winds blow along the walls of the hurricane’s calm eye. There, inward-spiraling gusts bend into updrafts that can reach speeds of more than 300 kilometers per hour. These updrafts suck water vapor from the ocean into the atmosphere, creating the most torrential downpours anywhere in the storm.

Chakraborty and colleagues realized that no one had quantified how a hurricane’s heaviest rainfall affected its strongest winds. Using roughly 500 satellite measurements of hurricane rainfall and intensity collected from 1997 to 2011, the researchers assembled a simple mathematical simulation of how a hurricane gains and loses energy.

Falling raindrops produce enough friction to slow a hurricane’s top wind speed by 10 to 30 percent, the researchers estimate. This energy loss is on the same order of magnitude as the ocean heat that sustains and intensifies the hurricane, notes lead author Tapan Sabuwala, a physicist also at the Okinawa Institute. Current hurricane forecast simulations don’t account for the energy lost to dragging raindrops and will frequently overestimate a hurricane’s future intensity, Sabuwala says.

Forecasters had assumed that rain friction wasn’t significant enough to expend computational power on, says meteorologist Christopher Landsea of the National Oceanic and Atmospheric Administration’s National Hurricane Center in Miami. The new result, he says, is a “potentially important finding that may lead in the future toward better hurricane forecasts.”

Although worth investigating, the friction’s importance may shrink when incorporated into more complex weather simulations than those used by Sabuwala and colleagues, says meteorologist Vijay Tallapragada of NOAA’s Environmental Modeling Center in College Park, Md. “This is certainly a nice experiment,” he says. “As we make progress towards improving our understanding of hurricane intensification, the need for paying attention to the details will become important.”