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November 2005

RAINEX: Bad weather is good news

Wen-Chau Lee. (Photo by Carlye Calvin, UCAR.)

NCAR researchers capture new data during Gulf storms

One thing that can be said about RAINEX, the recent field project in Florida to study hurricanes, is that the weather certainly cooperated. The project ran from mid-August through the end of September, encompassing two Category 5 hurricanes and one Category 1 hurricane. Although the storms had terrible consequences on the ground, from a scientific perspective they presented a dazzling natural laboratory to the researchers who flew aircraft inside them.

"It was just amazing," says EOL's Wen-Chau Lee, one of the project's co–principal investigators. "Observing two major hurricanes within a month and sampling different stages of these hurricanes from tropical depression through Category 5 was unexpected when we planned RAINEX."

The goal of RAINEX (the Hurricane Rainband and Intensity Change Experiment) was to better understand interactions between a hurricane's eyewall and its outer rainbands. These interactions can cause a storm to change dramatically in strength in mere hours.
Hurricanes are composed of bands of thunderstorms that spiral around a calm center, or eye. As the bands get closer to the eye, they have progressively stronger winds. The innermost band that snakes tightly around the eye, called the eyewall, is home to the storm's heaviest rains, strongest winds, and tallest clouds. Bands of heavy showers that spiral outside the eyewall and closer to the edge of the storm, known as rainbands, are less intense but can still affect the overall strength of the storm.

Although researchers have studied the eyewall and rainbands separately, few experiments have ever looked at the two components together to determine how different interactions between them might change a storm's strength.

RAINEX broke ground by not only taking data on the eyewall and rainbands simultaneously, but also by witnessing a process called eyewall replacement that has puzzled scientists for years. Eyewall replacement occurs when a hurricane's eyewall contracts to a smaller size. The outer rainbands form a second eyewall that eventually squeezes out the original one. The storm weakens during this process, which lasts about a day, but it can be even stronger after the replacement is complete.

Wen-Chau and colleagues aboard a Navy P-3 aircraft observed Hurricane Rita's eyewall replacement as the plane crisscrossed the storm during RAINEX. "Everybody on board was so excited because we knew this data set had never been collected before," Wen-Chau says.

Understanding hurricanes. RAINEX, by collecting data on both the powerful eyewall and the rainbands of a hurricane, will help scientists learn more about these intense storms. This graphic, from COMET's Hurricane Strike! module, is an artist's depiction of how a hurricane would look if it were cut in half vertically and the interior exposed. The eye, eyewall, and rainbands are visible, and arrows show how warm, moist air flows into the eyewall from the ocean's surface and rises, spiraling counterclockwise upward to the top of the hurricane. Some of this air in the upper levels of the hurricane then sinks into the eye, drying and warming as it descends and creating the mild conditions in the eye at the surface. The rainbands, curved storm formations that form spiraling rings around a hurricane, can be between 3 and 30 miles (about 5 to 50 kilometers) wide and up to 300 miles (480 kilometers) long. Typically, the closer a rainband is to the center of the hurricane, the bigger and stronger it is. (Courtesy The COMET Program. View more COMET graphics of hurricanes.)

Flying into a tempest

About 20 staffers, mostly from EOL, participated in RAINEX as part of a larger team of researchers from NOAA, the U.S. Navy, the University of Miami, and the University of Washington. The team flew three P-3 aircraft simultaneously into hurricanes Katrina, Rita, and Ophelia before the storms made landfall. The aircraft—two from NOAA and a third from the Naval Research Laboratory that carried ELDORA, NCAR's airborne Doppler radar—flew into the outer rainbands and penetrated the eyewall on most flights.

The flights were surprisingly less turbulent than one might expect, according to the researchers. "On the Navy P-3, we actually didn't hit much turbulence because we flew between the rainbands. But in very intense storms, if you do make a wrong turn, you can get into trouble in the eyewall or where rainbands pinch together," says EOL's Michael Bell, a flight scientist who works with ELDORA.

A view of Rita. This high-resolution radar image, taken by ELDORA inside Hurricane Rita on September 22, shows the process of eyewall replacement. The Navy P-3 aircraft (dashed line indicates track) flew through the "moat" between the deep convection of the primary and secondary eyewalls. Darker colors in this reflectivity image indicate heavier rainfall. (Courtesy Michael Bell and Wen-Chau Lee.)

Pavel Romashkin, a project manager in the EOL's Research Aviation Facility, credits Carl Newman, a NOAA pilot, for his skill in navigating the Navy P-3 through the storms. As he points out, pilots are trained to fly away from, not into, severe weather. "Carl's experience allowed him to work closely with us, understand radar data from ELDORA and the belly radar, and guide the airplane safely to maximize scientific output without risking too much," Pavel says. "He deserves a lot of credit for the success of RAINEX."

The aircraft were equipped as airborne weather labs in which crews used radar, dropsondes, and other instruments to record wind speed and direction, temperature, humidity, atmospheric pressure, and other conditions. With a 1,150-foot (350-meter) resolution, ELDORA produced the crispest picture of rainbands to date, including images of Hurricane Rita's double eyewall structure during the storm's replacement process on September 22.

"The high resolution of ELDORA is going to reveal new secrets of the hurricane," Michael says. "This is the first time we've flown three airborne Doppler radars into a hurricane, especially one with ELDORA's resolution. That is itself one of the major parts of RAINEX that is going to shine because we got so many different looks at the storm."

Eric Loew, EOL's project engineer for ELDORA, says that for him the biggest challenge during RAINEX was to keep the radar running reliably. Built in the early 1990s, ELDORA has reached the point where its signal processor and receiver need an upgrade. "The radar met all the objectives and the scientists collected a data set they're very happy with," he says. "But I would like to see 100% reliability so we don't miss any data, regardless of whether it happens at critical collection moments or not."

The radar is especially important because it offers investigators a tool for examining atmospheric conditions in a way that isn't possible with ground-based radar systems. "You don't have to wait for the storms to come to you because you're going out to them," Eric says.

Keeping everyone on the same page

Another area where RAINEX broke ground was in field project logistics with its sophisticated communications system. Radar data from the two NOAA P-3 aircraft were transmitted via satellite in real time to the RAINEX ground operations center at the National Hurricane Center in Miami. In a matter of minutes, the same data were then transmitted up to the Navy P-3. The Navy aircraft, unlike the NOAA planes, lacked a belly-mounted Doppler radar for navigation to help steer its crew to the most important parts of the storm.

"The whole package worked especially well," says Chris Burghart, a software engineer in EOL who generated the images that were sent to the Navy P-3. "It was recognized beforehand that getting this sort of information up to the Navy P-3 was an important piece."

Now that the field campaign is over, the next step for the RAINEX team is to run quality control tests on the data, a process that takes about three months. After that, scientists at NCAR and in the greater atmospheric sciences community can start analyzing results. Most of what they currently know about the interactions between the eyewall and outer rainbands comes from computer models that may not be completely accurate.

When data from RAINEX become available, scientists will incorporate them into models to determine whether a storm's circulation speeds up or slows down as rainbands wrap around it. In doing so, they'll get a better understanding of hurricane intensity that will eventually improve storm models and forecasts.

• by Nicole Gordon

On the Web

For more about RAINEX

Winds from Hurricane Rita damaged structures along much of the Texas and Louisiana coasts, including this gas station in Port Arthur, Texas. (Photo by Bob McMillan, FEMA.)

Also in this issue...

HIRDLS comes through

Planning a national supercomputing center

RAINEX: Bad weather is good news

UCAR policy on classified research

Random Profile: Meg McClellan

COMET project wins recognition

Delphi Questions

Just One Look: Super Science Saturday


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