September 30, 2004
In the eye of the hurricane: Floats monitor conditions
Five floats loaded with instruments and deployed in the path of the eye of hurricane Frances — that’s the one after Charley and before Ivan and Jeanne — have transmitted data that may help scientists better understand ocean conditions that put a damper on tropical storms and those that pour on the gas.
UW scientists and engineers designed and built the floats, the most specialized oceanographic instruments ever deployed in the path of a hurricane, and arranged to have them dropped by aircraft into the ocean a day before the storm was to arrive. They were among numerous devices and aircraft monitoring conditions during hurricane Frances.
The floats were programmed to travel up and down in the ocean recording initial ocean conditions and then, as the hurricane passed by, measure changes in water temperature, salinity and velocity and the amounts of heat and gas given off. The floats surfaced in order to transmit data via the Iridium cell phone system after the storm passed.
While forecasting hurricane tracks has become better and better, our ability to determine the potential ferocity of such storms has not advanced nearly as far. Studying the interplay between the ocean and atmosphere in the face of hurricane-force winds is one step in improving the models used to predict the strength of tropical storms.
Scientists know, for instance, that hurricanes draw their energy from warm ocean waters, but the resulting intensification depends on such things as how thick the layer of warm water is initially, how much the growing storm winds cause the ocean surface to cool and how much cold water from below gets mixed into the warmer upper layer because of the turbulence produced by the winds.
“We need to know how much heat is transferred from the water to the air in order to know how strong a hurricane will be. Current models of this are just guesses,” says Eric D’Asaro, a principal oceanographer with the UW’s Applied Physics Laboratory.
The give and take of momentum between ocean water and the atmosphere is also important.
“Water can give energy to a hurricane if it is warm, but water can also absorb lots of energy from wind sheer, something we have yet to be able to understand,” says Tom Sanford, a principal oceanographer with the UW’s Applied Physics Laboratory.
The work is funded by the Office of Naval Research and National Science Foundation.
Devices such as temperature probes have been dropped for years in the paths of hurricanes, but most are on a one-way journey from the surface to the sea floor. Three of the floats deployed in front of Frances, on the other hand, made more than 100 trips up and down from just below the surface to depths of 650 feet and occasionally as deep as 1,625 feet.
And the floats didn’t just record temperature. For example, three were outfitted with a combination of instruments to measure temperature, salinity and velocity that is unique for autonomously profiling floats, Sanford says. The other two floats measured the flux of heat and gas from the ocean to the atmosphere, another first for these types of floats.
More about the floats:
- They were developed and built at the UW’s Applied Physics Laboratory.
- They descended to profile the ocean ahead of Frances, then ascended to near the surface to monitor heat and gas exchange as the storm passed over, and finally descended again to profile the ocean after the storm passed. The floats had to be delicately balanced to perform properly. “Each float weighs as much as a small person and we had to adjust its weight to better than the weight of a quarter,” D’Asaro says.
- They measure heat flux, important because hurricanes are fueled by warm ocean water underneath them.
- They measure gas flux because, aside from hurricanes, modelers concerned with how much carbon dioxide is absorbed by the world’s oceans would like to know how much carbon dioxide oceans take from the atmosphere and thus help control possible global warming effects.