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March
1, 2007 Data collected in
2005 from
Hurricane Rita is providing the first documented evidence that rapid
intensity
changes can be caused by clouds outside the wall of a hurricane's eye
coming
together to form a new eyewall. Hurricanes can gain
or lose
intensity with startling quickness, a phenomenon never more obvious
than during
the historic 2005 hurricane season that spawned the remarkably
destructive
Katrina and Rita. Researchers flew
through Rita,
Katrina and other 2005 storms trying to unlock the key to intensity
changes.
Now, data from Rita is providing the first documented evidence that
such
intensity changes can be caused by clouds outside the wall of a
hurricane's eye
coming together to form a new eyewall. "The comparison
between
Katrina and Rita will be interesting because we got excellent data from
both
storms. Rita was the one that showed the eyewall replacement," said
Robert
Houze Jr., a "The implication of
our
findings is that some new approaches to hurricane forecasting might be
possible," he said. Houze and Shuyi
Chen, an
associate professor of meteorology and physical oceanography at the
University
of Miami Rosenstiel School of Marine and Atmospheric Science, lead a
scientific
collaboration called the Hurricane Rainband and Intensity Change
Experiment.
The project is designed to reveal how the outer rainbands interact with
a
hurricane's eye to influence the storm's intensity. Chen is a co-author
of the
Science paper, as are Bradley Smull of the UW and Wen-Chau Lee and
Michael Bell
of the The project is the
first to use
three Doppler radar-equipped aircraft flying simultaneously in and near
hurricane rainbands. The project also uses a unique computer model
developed by
Chen's group at the "The model provided
an
exceptionally accurate forecast of eyewall replacement, which was key
to
guiding the aircraft to collect the radar data," Chen said. A hurricane's
strongest winds
occur in the wall of clouds surrounding the calm eye. The researchers
found
that as the storm swirled into a tighter spin, a band of dry air
developed
around the eyewall, like a moat around a castle. But while a moat
protects a
castle, the hurricane's moat eventually will destroy the existing
eyewall,
Houze said. Meanwhile, outer rainbands form a new eyewall and the moat
merges
with the original eye and the storm widens, so the spin is reduced and
winds around
the eye are slowed temporarily, something like what happens as a figure
skater's arms are extended. But the storm soon intensifies again as the
new
eyewall takes shape. "The exciting thing
about
the data from Rita is that they show that the moat is a very dynamic
region
that cuts off the old eye and establishes a wider eye," Houze said.
"It's not just a passive region that's caught in between two
eyewalls." Hurricane
forecasters in recent
years have developed remarkable accuracy in figuring out hours, even
days,
ahead of time what path a storm is most likely to follow. But they have
been
unable to say with much certainty how strong the storm will be when it
hits
land. This work could provide the tools they need to understand when a
storm is
going to change intensity and how strong it will become. Scientists already
knew that
intensity can change greatly in a short time -- in the case of Rita the
storm
grew from a category 1, the least powerful hurricane, to a category 5,
the most
powerful, in less than a day. Aircraft observation of the moat allowed
scientists to see Rita's rapid loss of intensity during eyewall
replacement,
which was followed by rapid intensification. "Future aircraft
observations focused in the same way should make it possible to
identify other
small-scale areas in a storm where the processes that affect intensity
are
going on, then that data can be fed into high-resolution models to
forecast
storm intensity changes," Houze said. That understanding
could prove
valuable for coastal residents deciding whether a storm is powerful
enough to
warrant their seeking safety farther inland. Rita and Katrina, among
the six
most intense Atlantic hurricanes ever recorded in terms of the
barometric
pressure within the core of the storm, struck just three weeks apart in
August
and September 2005, together resulting in some 2,000 fatalities and
more than
$90 billion in damage along the The National Oceanic
and
Atmospheric Administration provided two research aircraft for the
project and
the third was provided by the U.S. Navy and funded by the National
Science
Foundation. The planes flew
several novel
flight paths, including a circular track in Rita's moat, to gather
information
from the edges of rainbands and other structures in the hurricane. "We used a
ground-control system
with a lot of data at our fingertips to focus the aircraft into places
in the
storm where there were processes happening related to intensity
changes,"
Houze said.
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