Satellite Data Puts New Spin on Forecasting Tropical Cyclones

Florida State University 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. Forecasters will be using SeaWinds data during the 2002 hurricane season, which officially begins on June 1.

With this new objective method, potential tropical cyclones can be detected more than 40 hours before they are traditionally classified as tropical cyclones, giving more time for investigation, warnings and preparation. Professor James O'Brien, research associate Mark Bourassa and graduate student Ryan Sharp, all of FSU's Center for Ocean-Atmospheric Prediction Studies, outlined their findings in the June issue of the Bulletin of the American Meteorological Society.

"Earlier detection of potential tropical cyclones would give the public and maritime interests more time to prepare for a potential future threat," Sharp said. "Advanced detection will also allow scientists more time to plan research missions into storms."

Using data from the SeaWinds scatterometer on NASA's QuikSCAT satellite, FSU scientists 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). Typically the NHC monitors these systems and looks for persistent and organized thunderstorm activity as well as the winds near a storm system before classifying it as a tropical cyclone. However, using only characteristics of the surface wind field as criteria, the researchers were able to identify potential tropical cyclones before they developed.

This early detection will help determine those storms 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.

The researchers established the objective technique for the detection of tropical cyclones by using data collected during the 1999 Atlantic hurricane season. They applied the technique to near-realtime data for the 2001 Atlantic and Eastern Pacific hurricane seasons 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. About 35 to 40 percent of the systems detected by this method 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 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. 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 in June 1999 from California's Vandenberg Air Force Base, the spacecraft operates in a Sun-synchronous, 497-mile, near-polar orbit, circling Earth every 100 minutes, taking approximately 400,000 measurements over 90 percent of Earth's surface each day.

O'Brien and Bourassa are members of the NASA Science Team for SeaWinds, which funded the research. The Office of Naval Research, Secretary of the Navy Grant to O'Brien provided additional funding for the study.

NASA's Jet Propulsion Laboratory (JPL), a division of the California Institute of Technology in Pasadena 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 American Meteorological Society is the nation's leading professional society for scientists in the atmospheric and related sciences. For more information and images: http://www.gsfc.nasa.gov/topstory/20020531cyclone.html