What Lies
Beneath a Hurricane
from Science NASA
Two orbiting NASA satellites are
giving scientists an unprecedented view of what goes
on beneath the obscuring cloud tops of great swirling
storms.
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to this story (requires RealPlayer)
Sept. 11, 2000 -- As this year's
Atlantic hurricane season reaches its peak, scientists
are using two orbiting research satellites to peer
into the hearts of storms in ways that were never
before possible. Unlike most weather satellites that
can only take pictures of a hurricane's cloud tops,
NASA's QuikScat and Tropical Rainfall Measurement
Mission (TRMM) satellites carry microwave sensors that
can "see" through the clouds and scrutinize
conditions -- including rainfall, wind and water
temperature -- at the ocean's surface. These new data
could allow researchers to detect tropical depressions
earlier and to predict where hurricanes are headed
with greater accuracy.
Right: This
satellite image of Hurricane Alberto churning across
the northern Atlantic Ocean was captured by the
OrbView-2 satellite on August 21, 2000. NASA's
QuikScat and TRMM missions are beginning to show
scientists what's underneath the obscuring clouds of
these great storms. [more
information]
"I think the rain and the wind
together is a very powerful tool to study
hurricanes," said Dr. Timothy Liu, project
scientist for the QuikScat mission at NASA's Jet
Propulsion Laboratory.
QuikScat, which was launched in June
1999, uses an instrument called a "radar
scatterometer" to measure both the speed and the
direction of surface winds over the world's oceans.
Other radar-based satellites can
measure wind speed, Liu said, but "the only thing
that can measure the wind vector -- that is, the speed
and the direction together -- is the scatterometer."
A scatterometer works by sending a
beam of microwave radiation toward the ocean surface
at an angle. The beam, which passes undisturbed
through clouds, gets scattered by the ocean surface,
and some of the microwaves bounce back toward the
satellite. A rougher ocean surface, which
indicates higher winds, will reflect more radiation
back toward the satellite than a smooth surface will.
Below: A snapshot of the speed
and direction of ocean surface winds taken by QuikScat. Pink
and yellow represent high velocity winds, and purple
and blue represent slower winds. The white lines
and arrows indicate direction. Courtesy JPL. [more
information]
Liu
and Dr. Kristina Katsaros of NOAA found that the 1999
wind data from QuikScat could be used to identify
potential hurricanes one to three days before
traditional methods.
Part of the reason for this, Liu
said, is that the satellite photographs used by the
National Hurricane Center show only the cloud tops of
forming hurricanes, which sometimes can be obscured
from view by higher clouds.
Another key to understanding and
predicting hurricanes is rainfall. Rainfall snapshots
are produced by the TRMM (Tropical Rainfall Measuring
Mission) satellite, which is a joint mission between
NASA and the National Space Development Agency (NASDA)
of Japan.
"The big impact that the
rainfall data can have is that the rainfall in these
tropical storms are signatures of the amount of latent
heat that's being released into the atmosphere,"
said Dr. Marshall Shepherd, a research meteorologist
at NASA's Goddard Space Flight Center.
Incorporating rainfall data from
TRMM into computer weather models "gives the
model a better handle on the energetics that are
required to drive the circulation, to drive the
hurricane and also affect its path," Shepherd
said.
Left: An
image of Typhoon Bilis, which recently struck Taiwan
and China, combining wind data from QuikScat and
rainfall data from TRMM. The background color
represents rainfall levels, and the red arrows
indicate wind speed and direction. An image like
this provides a useful snapshot of the typhoon for
scientists. Courtesy JPL.
The TRMM satellite can also use its
microwave sensors to measure ocean surface
temperatures beneath a hurricane.
"Hurricanes are intimately tied
to the sea surface temperature," Shepherd
said. "There's generally kind of a threshold
temperature (above which) hurricanes like to
form. If you have all of the other a priori
conditions in place, and if you have ample warm sea
surface temperature and moisture, then you can get a
hurricane that likes to grow," Shepherd said.
Higher sea surface temperatures mean
more evaporation of ocean water into the air. As
that moisture condenses into clouds, it releases heat
to the air that causes the air to rise. The
rising air creates a low pressure area beneath it that
pulls the surrounding air spiraling inward,
perpetuating the hurricane.
"It's that conversion of latent
heating that's carried from the water vapor when it
condenses to form the clouds in the hurricane --
that's really the fuel supply that powers the
hurricane engine,'" Shepherd said. "We
tend to think of hurricanes as big heat engines."
Low sea surface temperatures can
spell death for a hurricane, as in 1998 when the
"wake" of cold water behind Hurricane Bonnie
caused Hurricane Danielle, which was following close
behind, to dissipate.
Left: TRMM
Microwave Imager (TMI) sea-surface temperatures from
August 22 - Sept. 23, 1998. Blues represent cooler
water, greens and yellows are warmer water. A
translucent map of clouds photographed by a NOAA GOES
satellite is overlaid on the map to show Hurricane
Bonnie approaching the Carolina Coast (upper left) and
Hurricane Danielle following roughly in its path
(lower right). TMI is the first satellite microwave
sensor capable of accurately measuring sea surface
temperature through clouds.
Traditional weather satellites that
use infrared sensors can also measure sea surface
temperature, but "the big advantage that the TRMM
microwave imager has ... is that microwave instruments
can see through clouds, whereas infrared instruments
(on traditional weather satellites) can only give you
sea surface temperatures in clear regions,"
Shepherd said.
While the kind of rainfall and sea
surface temperature data produced by TRMM holds great
potential for improving hurricane forecasting, TRMM is
not primarily a hurricane-monitoring satellite.
"Things like hurricane
monitoring ... are extra benefits of the satellite,
but its main mission is to measure rainfall,"
Shepherd said. "TRMM is a research mission
-- it wasn't designed to be used in an operational
setting.
"But where (the data) can be
used, I'm sure it is ...."
Related Links:
Images
of Earth by QuikScat - A catalog of images
produced by QuikScat and TRMM, including short
descriptions.
TRMM
home page - The home page of the Tropical
Rainfall Measuring Mission.
TRMM
archive - An archive of images and movies
from the TRMM satellite.
Scatterometry
overview - A brief tutorial on scatterometry
and its use on QuikScat for measuring ocean surface
winds.
Seaflux
- Near real-time images of global ocean surface
winds from the QuikScat satellite.
