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Friday, September 07, 2007

New Faraway Sensors Warn of Emerging Hurricane's Strength

A new study supported by NASA and the U.S. Office of Naval Research takes forecasters one step further to improving their ability to predict just how powerful an oncoming storm may become by using highly-sensitive sensors located thousands of miles from the storm to detect lightning outbreaks within a hurricane’s most dangerous area.

Researchers can now investigate with greater accuracy how the rate of lightning strikes produced within a hurricane's eyewall is tied to the changing strength of that hurricane. A hurricane’s eyewall is the inner heat-driven region of the storm that surrounds the “eye” where the most intense rainfall and most powerful winds occur. By monitoring the intensity of lightning near a hurricane’s eye, scientists will be able to improve their forecasts of when a storm will unleash its harshest conditions.

During the study, researchers used data from a growing network of new, long-range, ground-based lightning sensors, a NASA satellite and aircraft-based sensors. They explored the relationship between eyewall lightning outbreaks and the intensity of two of the most severe Atlantic storms on record before they made U.S. landfall: category five hurricanes Katrina and Rita. An article on this research, also supported by the U.S. Office of Naval Research, will be published in the American Meteorological Society's Monthly Weather Review later this year.

"There are very few observing systems that offer a broad view of a storm over the open ocean where hurricanes tend to build or lose strength," says lead author Kirt Squires, a recent graduate of the meteorology program at the University of Hawaii in Honolulu. "What’s really compelling about the new sensors is their increased sensitivity to pick up lightning's electromagnetic signal over water from such a long distance. As a result, we can see thunderstorm activity over the ocean from thousands of miles away for the first time. This development is essential to improving the way meteorologists can look at a growing storm to judge just how harsh it will be."

When water condenses from vapor into a cloud droplet, latent or hidden heat is released, which in turn builds updrafts – air moving upwards in a cloud. Latent heat provides the energy that fuels hurricanes. If the ensuing updrafts are strong enough, they can cause the separation of charge that produces lightning. The tight correlation between the rate of lightning strikes, the amount of rainfall and the heat released in the eyewall of a storm allows the lightning rate data to be useful in computer models that forecast hurricane track and intensity.

"Hurricane forecasters and researchers are very interested in developing methods that allow a continuous examination of the structural growth of the eyewall within hurricanes," says co-author Steven Businger, a senior professor of meteorology at the University of Hawaii. "The fact that lightning is directly linked to the heat energy released in the eyewall makes it a priority for us to examine the evolution of lightning within a storm."

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