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May 14, 2007
NASA'S CLOSE-UP LOOK AT A HURRICANE'S
EYE REVEALS A NEW 'FUEL'
SOURCE
In
the
eye of a furious hurricane, the weather is often quite calm and sunny.
But new
NASA research is providing clues about how the seemingly subtle
movement of air
within and around this region provides energy to keep this central
"powerhouse" functioning.
Using
computer simulations and observations of 1998's Hurricane Bonnie in
southern
North Carolina, scientists were able to get a detailed view of pockets
of
swirling, warm humid air moving from the eye of the storm to the ring
of strong
thunderstorms in the eyewall that contributed to the intensification of
the
hurricane.
The findings suggest that the flow of air parcels between the eye and
eye wall
- largely believed trivial in the past - is a key element in hurricane
intensity and that there's more to consider than just the classic
"in-up-and-out" flow pattern. The classic pattern says as air parcels
flow "in" to the hurricane's circulation, they rise "up,"
form precipitating clouds and transport warm air to the upper
atmosphere before
moving "out" into surrounding environmental air.
"Our
results improve understanding of the mechanisms that play significant
roles in
hurricane intensity," said Scott Braun, research meteorologist at
NASA's
Goddard Space Flight Center, Greenbelt, Md. "The
spinning flow of
air parcels - or vortices - in the eye can carry very warm, moist eye
air into
the eyewall that acts as a turbocharger for the hurricane heat engine."
The research appears in the June 2007 issue of the American
Meteorological
Society's Journal of the Atmospheric Sciences.
"While the 'in-up-and out' pattern has been the prevailing paradigm for
the past five decades, when you closely examine intense hurricanes it's
apparent that a second family of moist air parcels often travels from
the
border of the eyewall to the eye, where it picks up moisture from the
ocean
surface," said co-author Michael Montgomery, professor of meteorology
at
the U.S. Naval Postgraduate School, Monterey, Calif. "These
moisture-enriched air parcels then rather quickly return to the main
eyewall
and collectively raise the heat content of the lower eyewall cloud,
similar to
increasing the octane level in auto fuel."
The researchers analyzed thousands of virtual particles to track the
movement
of air between the eye and eyewall, and between the eyewall and its
outside
environment. To uncover the impact of these particles on storm
intensity, they
used a simulation of Hurricane Bonnie from a sophisticated computer
model and
data gathered during the NASA Convection and Moisture Experiment
(CAMEX).
The
simulation has also helped to explain the formation of deep
“hot towers” observed
in Bonnie and many other hurricanes by NASA’s Tropical
Rainfall Measuring
Mission (TRMM) satellite. TRMM carries the first and only space-based
precipitation radar that allows researchers to peer through clouds and
get a
3-D view of storm structure. It captured a particularly deep hot tower
in
Bonnie as the storm intensified several days before striking North Carolina.
Hot towers are deep, thick clouds that reach to the top of the
troposphere, the
lowest layer of the atmosphere, usually about ten miles high in the
tropics.
The updrafts within these "towers" act like express elevators,
accelerating the movement of energy that boosts hurricane strength, and
are
called “hot” because of the large amount of latent
heat they release as water
vapor is condensed into cloud droplets. Deep hot towers in the eyewall
are
usually associated with a strengthening storm.
In previous research, Braun, Montgomery, and
Zhaoxia Pu of the University
of Utah, Salt Lake City,
found a direct relationship between these
deep hot towers and the intense vortices inside the eye. "The vortices
were shown to be especially crucial in providing the focus and lift
needed for
hot tower formation and add insight into when and where hot towers will
develop
in storms," said Braun. The study was published in the January 2006
CAMEX
special issue of the Journal of the Atmospheric Sciences.
Vortices are created in response to the rapid change in wind speed from
the
fierce eyewall to the calm eye. Near the surface, air spiraling inward
collides
with these vortices to force air up, forming updrafts. Strong updrafts
in the
eyewall carry moisture much higher than normal and help create hot
towers.
The current study suggests that in addition to providing lift, these
vortices
also feed high energy air from the low-level eye into the eyewall,
boosting the
strength of the updrafts. This transfer of energy allows the storm to
remain
stronger than expected, particularly when encountering weakening
influences,
including cooler ocean water temperatures and wind shear, the change in
the
direction and speed of winds with altitude.
"This discovery may help explain why strong storms can remain intense
for
several hours or longer after encountering conditions that usually
bring
weakening," said Montgomery.
"Ongoing research will add to our understanding of the dynamics
associated
with storm intensity so that we can pinpoint the variables and
processes that
must be represented in numerical models to improve intensity forecasts."
When hurricane Bonnie finally began to lose strength a couple days
before
landfall, a significant amount of air in the eyewall was traced back -
not to
the eye - but to the middle levels of the atmosphere away from the
storm. This
inflow was caused by wind shear and brought much cooler, drier
environmental
air into Bonnie’s circulation, acting like an anti-fuel to
reduce energy in the
storm and weaken its strong winds.
Despite these and other recent advances in understanding the internal
workings
of hurricanes, forecasting their intensity is still a significant
challenge.
"Most of today's computer models that aid forecasters cannot
sufficiently
account for the extremely complex processes within hurricanes, and
model
performance is strongly dependent on the information they are given on
the
structure of a storm," said Braun. "We also typically only see small
parts of a storm at a given time. That is why it is important to
combine data
from field experiments such as CAMEX with data from TRMM and other
satellites.
As observing technologies and models improve, so too will forecasts."
For more
information, images
and animations, visit:
http://www.nasa.gov/mission_pages/hurricanes/archives/2007/eye_fuelsource.html
http://www.nasa.gov/vision/earth/lookingatearth/eye_fuelsource.html
http://svs.gsfc.nasa.gov/vis/a000000/a003400/a003413/index.html
NASA's
Hurricane Resource Website:
http://www.nasa.gov/mission_pages/hurricanes/main/index.html
Writer:
Mike Bettwy, NASA Goddard Space Flight Center
##
Contact:
Lynn Chandler
NASA
Goddard Space Flight Center
301-286-2806
This text is
derived from:
http://www.nasa.gov/mission_pages/hurricanes/archives/2007/eye_fuelsource.html
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