Seawinds casts a closer eye on tropical cyclones

January 14, 2002

In a new NASA-funded study, researchers have dramatically improved the warning time for tropical cyclone development in the Atlantic and Eastern Pacific hurricane basins using satellite data to access a combination of the spin of the atmosphere and wind speed data. With this new method, potential tropical cyclones can be detected more than 40 hours earlier than with traditional methods, giving more time for warnings and preparation.

Researchers from Florida State University's Center for Ocean-Atmospheric Prediction Studies, using data from the SeaWinds scatterometer on NASA's QuikSCAT satellite, have been able to detect the formation of the systems that might become tropical cyclones prior to their being classified as tropical depressions by the National Hurricane Center (NHC). This new method is based on signals from the scatterometer derived vorticity field, which highlights areas of rotating winds.

Tropical cyclones are also known as tropical depressions (winds 38 mph or less), tropical storms (winds 39 - 73 mph), or hurricanes (winds in excess of 74 mph). One of the reasons earlier detection of potential tropical cyclones is possible with this new system is that the NHC uses criteria other than vorticity, such as persistent and organized thunderstorm activity, before they classify a system as a tropical cyclone.

This system, however, uses only the wind field, in which the signal is often present prior to the materialization of the NHC criteria. Early detection of these systems will help determine those that warrant further examination by more traditional methods, and allow investigators to study the genesis of tropical cyclones by watching the full development of a storm from its very beginning.

"Earlier detection of potential tropical cyclones would give the public and maritime interests more time to prepare for a potential future threat," stated Ryan Sharp, co-author of the study and a researcher at Florida State University's Center for Ocean-Atmospheric Prediction Studies. "Advanced detection will also allow scientists more time to plan research missions into storms."

The objective technique for the detection of tropical cyclones used by the researchers was established by using data collected during the 1999 Atlantic hurricane season. This technique was then applied to the near-realtime (< 3-hour delay) data for the 2001 Atlantic and Eastern Pacific hurricane seasons in order to detect systems that had the potential to become tropical cyclones.

Of the 17 tropical cyclones that developed in the Atlantic in 2001, eight were detected an average of 43 hours before they were classified by the NHC. Some of the systems detected by this method (35 to 40 percent) never developed into tropical cyclones, but by using conventional methods of detection such as clouds in satellite pictures, researchers could eliminate most of these "false alarms" early in the study.

The results of the use of the scatterometer in the Eastern Pacific, however, are more impressive and critical. With fewer weather stations and search aircraft, the use of the scatterometer can greatly improve tropical cyclone identification and prediction. Using the technique in this study, of the 17 tropical cyclones that formed in the Eastern Pacific, 14 were identified an average of 42 hours before they were classified as tropical cyclones by the NHC. For a system developing close to land, this earlier prediction could mean the difference between life and death for those living in the region.

The SeaWinds instrument on QuikSCAT is a specialized microwave radar that measures both the speed and direction of winds near the ocean surface. Launched June 19, 1999, from California's Vandenberg Air Force Base, the spacecraft operates in a Sun-synchronous, 803-kilometer (497-mile) near-polar orbit, circling Earth every 100 minutes, taking approximately 400,000 measurements over 90 percent of Earth's surface each day.

NASA's Jet Propulsion Laboratory (JPL), a division of the California Institute of Technology, Pasadena, Calif., manages the QuikSCAT satellite for NASA's Office of Earth Science, Washington. JPL built the scatterometer instrument and provides ground science processing systems. NASA's Goddard Space Flight Center, Greenbelt, Md., managed development of the satellite, designed and built by Ball Aerospace & Technology Corp., Boulder, Colo. NASA's Earth Science Enterprise is a long-term research and technology program designed to examine Earth's land, oceans, atmosphere, ice and life as a total integrated system.
The Office of Naval Research, Secretary of the Navy Grant to Dr. James J. O'Brien at Florida State University, provided additional funding for this study.

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Editor's Note: The results of this study will be presented at 9:30 A.M. on January 14, 2002, at the 82nd Annual American Meteorological Society (AMS) meeting, Orange County Convention Center, Orlando, Fla. The presentation is entitled "Early Detection of Tropical Cyclones Using SeaWinds Derived Vorticity for the 2001 Hurricane Season."

NASA/Goddard Space Flight Center

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