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August 18, 2005

Hurricane season: New project hopes to improve forecasts of storms’ intensity

A collaborative research team headed in part by a UW atmospheric scientist this week began one of the largest hurricane research projects ever undertaken to better understand dramatic, rapid changes in tropical storm intensity that have baffled forecasters for years.

“Ideally, we’ll obtain a physical explanation of a hurricane’s intensity change in terms of the relationship between the inner and outer parts of the storm. These storms can jump up in intensity or drop a full category in a day, and the intensity changes are a big challenge,” said Robert Houze, a UW atmospheric sciences professor and co-principal investigator for the project.

The research is a collaboration that includes the UW, the University of Miami Rosenstiel School of Marine & Atmospheric Science, the National Center for Atmospheric Research, the National Oceanic and Atmospheric Administration and the U.S. Navy. The Hurricane Rainband and Intensity Change Experiment will study how the outer rainbands and inner eye of a hurricane interact to influence a storm’s intensity. The National Science Foundation provided $3 million to fund this study that may shed light on how and why a storm can change in strength in only a matter of hours.

“While great progress has been made in forecasting hurricane tracks, we need to improve in forecasting hurricane intensity,” said Steve Nelson, director of the science foundation’s physical and dynamic meteorology program.

“There are many factors that affect the intensity of hurricanes. This project will investigate one of those factors: the interactions between hurricane rainbands and the eyewall. We will better understand the impact of rainbands on a hurricane’s maximum winds.”

While researchers have studied the eye and outer rainbands of hurricanes extensively, “few, if any, experiments have ever examined these two components together and how their interaction might affect a storm’s strength,” said Shuyi Chen, an associate professor of meteorology and physical oceanography at the Rosenstiel School and a co-principal investigator for the project. “The outer bands of a hurricane often have strong winds and lots of rain, and that can actually affect the overall intensity of a hurricane.”

The project will study the interaction using data recorded from hurricane research flights. Starting Monday of this week and through the rest of this year’s Atlantic hurricane season, two P3 aircraft from the National Oceanic and Atmospheric Administration and a U.S. Navy P3 aircraft will fly simultaneously into hurricanes well before they threaten landfall. Flying in the hurricanes’ outer bands and punching into the eyewall on most flights, the aircraft will use sophisticated radar and other equipment to record wind speed and direction, temperature, humidity, atmospheric pressure and other critical data.

The UW and the National Center for Atmospheric Research in Boulder, Colo., will provide expertise in airborne Doppler radar analysis. Rosenstiel School will build a state-of-the-art hurricane model using the data collected from the research flights.

“The National Science Foundation funding allows the Navy aircraft to be added to the research. It will play a key role by observing the outer part of the storm while the other two aircraft observe the inner part,” Houze said.

Much of what scientists currently know about the interaction between the outer rainbands and the eyewall of a hurricane comes from the state-of-the-art numerical models developed for hurricane research and prediction, which can provide very detailed information but may not be completely accurate. But what researchers need is solid data to validate these models.

One of the breakthrough aspects of the project is the use of the three Doppler radar-equipped aircraft. Although eyewall flights are a routine part of hurricane research, this is the first field study to include simultaneous flights in and near rainbands.

Once the data are collected, the researchers will assimilate them into hurricane models to gain a better sense of whether the storm’s circulation speeds or slows as rainbands wrap around the hurricane. Additionally, the researchers will share this information with hurricane operational centers and national environmental prediction centers around the world.

Wen-Chau Lee from the National Center for Atmospheric Research is the lead scientist for the Naval Research Lab’s P-3 as it profiles rainbands. Sensors will measure temperature and wind as the instrument falls from the plane through a storm. On most flights, Doppler radar will collect data as a P-3 circles rainbands from six miles away, with occasional flights through a rainband, as needed.

“My main interest is in the rainband structure,” Lee said. “These flights can be turbulent, especially when we’re penetrating the rainbands. I think that’s the wild card — the challenge of the experiment — to capture internal rainband structure and its interactions with the eyewall in those conditions.”