|
+ Play
Audio
|
+ Download Audio | +
Email to a friend | +
Join mailing list
April
13, 2007: Throw gasoline on a fire, and the flames
swell to a raging inferno. Throw dirt on a fire, and the flames
suffocate. But what happens when you throw dirt on a hurricane?
It's
a serious question.
Hurricanes
are born
in Atlantic waters just off the west coast of Africa. Thunderstorms
gather there and, sometimes, for reasons no one fully understands,
they merge into swirling monster storms that can cross the
ocean to hit the United States thousands of miles away.
The
place where hurricanes are born is very close to the Sahara
desert—a prodigious source of fine dirt and dust—and Sahara
dust storms can blow right into the hurricane genesis region.
What does all that dry, dusty air do to a baby hurricane?
This is a mystery of hurricane science.
Above:
Desert dust blows off the west coast of Africa and over the
Canary Islands in Nov. 2006. Credit: The MODIS instrument
on NASA's Terra satellite. [More]
"There
are at least two possibilities," notes Bill Lapenta,
an atmospheric scientist from NASA's Marshall Space Flight
Center. On one hand, dust might strengthen a hurricane. Dust
grains serve as nucleation points for clouds and raindrops.
This could cause a young storm to intensify because rain is
a key part of a hurricane's internal "heat engine."
On the other hand, dry, dusty air might have the opposite
effect, choking off a storm's development by altering atmospheric
circulation patterns normal to a growing storm.
Which
theory is true? Lapenta and colleagues recently gathered data
that brings them closer to the answer. They did it by flying
directly into a dusty hurricane.
Along
with dozens of other scientists, Lapenta spent last fall in
the Cape Verde Islands off the west coast of Africa. Their
mission: to catch hurricanes in the act of being born. The
name of the expedition was NASA African Monsoon Multidisciplinary
Analyses—or NAMMA for short. NAMMA researchers monitored the
ocean near Cape Verde for promising clusters of thunderstorms,
and when they saw a group gathering into a potential hurricane,
they sprang into action. NASA's DC-8 Airborne Laboratory flew
in and around the storms equipped with instruments to measure
wins, water vapor, moisture, atmospheric pressure and temperature.
NASA and NOAA satellites, weather balloons and ground-based
radar gathered even more data.
 Right:
On board NASA's DC-8 research aircraft, Philip Parker, an
NSSTC researcher from the University of Alabama in Huntsville,
monitors a hurricane. [Larger
image]
"We
sampled one particular storm two days in a row," recalls
Lapenta. "On the first day, our instruments detected
very little dust in the storm system. It was clean and pristine.
But the next day, using the same aircraft and the same instruments,
we detected lots of dust." From one day to the next,
the storm system had behaved like a dust mop, swooping up
tiny particles from the atmosphere and pulling them in.
What
happened next? The storm eventually went on to form a category
three hurricane, Helene, one of the strongest of the 2006
Atlantic hurricane season.
So
dust promotes hurricanes, right? Lapenta isn't ready to leap
to that conclusion. "It's a very complicated problem,"
he explains. "Dust is one factor in hurricane formation,
but there are many others, too." Atmospheric winds, humidity,
sea-surface temperature—they all play a role. The effect of
dust may be "situation dependent," meaning it depends
on what the rest of the atmosphere is doing when the dust
hits. "We're still analyzing our data to get the whole
picture," he says.
So
long after the aircraft has landed, the study continues. NAMMA
is a three-year mission, with the first year dedicated to
field research, followed by two years of data-analysis. Stay
tuned to Science@NASA for updates.
SEND
THIS STORY TO A FRIEND
Authors: Sherrie Super and Dr. Tony Phillips | Production
Editor: Dr.
Tony Phillips | Credit: Science@NASA
|
