Close-up look at a hurricane's eye reveals a new 'fuel' source
Date: Monday, May 14, 2007 @ 23:53:13 GMT
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.
Picture Credit: NASA GSFC Scientific Visualization Studio
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
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
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
"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."
Source: by Mike Bettwy, Goddard Space Flight Center