NCAR Radar Probes Hurricane Rainbands
Hurricane field program could improve intensity forecasts
August 8, 2005
BOULDER—A collaborative research team will soon begin one of the largest hurricane research projects ever undertaken. Its goal is to better understand dramatic, rapid changes in tropical storm intensity that have baffled forecasters for years.
The team includes scientists from the University of Miami Rosenstiel School of Marine and Atmospheric Science, the University of Washington, the National Center for Atmospheric Research (NCAR), the National Oceanic and Atmospheric Administration (NOAA), and the U.S. Navy.
The project, called the Hurricane Rainband and Intensity Change Experiment (RAINEX), will study how the outer rainbands and inner eye of a hurricane interact to influence the storm's intensity. The National Science Foundation (NSF) provided $3 million to fund the study, which 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 forecasting hurricane intensity," says Steve Nelson, director of NSF's physical and dynamic meteorology program.
Many factors affect the intensity of hurricanes. RAINEX will investigate one of those factors: the interactions between a hurricane's rainbands and its eyewall. "From RAINEX, we will better understand the impact of rainbands on a hurricane's maximum winds," Nelson says.
Although 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, according to Shuyi Chen, an associate professor of meteorology and physical oceanography at Rosenstiel School and a RAINEX principal investigator. 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 RAINEX team will study this interaction using data recorded during hurricane research flights. Beginning August 15 and running through the remainder of this year's Atlantic hurricane season, two NOAA P-3 aircraft and a U.S. Navy P-3 will fly simultaneously into hurricanes well before the storms threaten landfall. Flying in the hurricane's outer bands and punching into the eyewall on most flights, the aircraft will use sophisticated Doppler radar and GPS dropsondes to record wind speed and direction, temperature, humidity, atmospheric pressure, and other critical data.
Researchers at the University of Washington and NCAR will provide expertise in airborne Doppler radar analysis, while researchers at Rosenstiel School will construct a state-of-the-art hurricane model using the data collected during the research flights.
"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," says Robert A. Houze, Jr., a professor in atmospheric sciences at the University of Washington and one of the project's principal investigators. "These storms can jump up in intensity or drop a full category in a day, and the intensity changes are a big challenge.
Much of what scientists currently know about the interactions between the outer rainbands and the eyewall of a hurricane comes from numerical models developed for hurricane research and prediction, which can provide very detailed information but may not be completely accurate. Researchers need solid data to validate these models.
One of the breakthrough aspects of RAINEX is the use of the three aircraft equipped with Doppler radar. Although eyewall flights are a routine part of hurricane research, this is the first field study to include simultaneous flights in and near rainbands.
NCAR's Wen-Chau Lee will be the lead scientist for the Naval Research Lab's P-3 as it profiles rainbands. Dropsonde sensors will measure temperature and wind as the instruments fall from the plane through storms. On most flights, the ELDORA Doppler radar will collect data as the P-3 circles rainbands from six miles away, with occasional flights through a rainband as needed.
"These flights can be turbulent, especially when we're penetrating the rainbands," Lee says. "I think this is the wild card—the challenge of the experiment—to capture the internal rainband structure and its interactions with the eyewall in these conditions."
Once the data are collected, the researchers will assimilate them into hurricane models to gain a better sense of whether a storm's circulation speeds up or slows down as rainbands wrap around the hurricane. The researchers will share this information with hurricane operational centers and national environmental prediction centers around the world.
"Having the Navy P-3 fly with the NOAA P-3 aircraft will expand the area covered by airborne Doppler radar to include the rainbands as well as the inner core," says Robert Rogers, field program director for NOAA's Hurricane Research Division. "This data will improve our understanding of intensity change and contribute to the development and evaluation of the next generation operational hurricane model."
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