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March
19, 2008: Did you know that there's a new breakfast
food that helps meteorologists predict severe storms? Down
South they call it "GrITs."
GrITs
stands for Gravity wave Interactions with Tornadoes. "It's
a computer model I developed to study how atmospheric gravity
waves interact with severe storms," says research meteorologist
Tim Coleman of the National Space Science and Technology Center
in Huntsville, Alabama.
According
to Coleman, wave-storm interactions are very important. If
a gravity wave hits a rotating thunderstorm, it can sometimes
spin that storm up into a tornado.
Above:
Click on the image to watch a gravity wave roll over Tama,
Iowa, on May 7, 2006. Credit: Iowa Environmental Mesonet Webcam.
What
is an atmospheric gravity wave? Coleman explains: "They
are similar to waves on the surface of the ocean, but they
roll through the air instead of the water. Gravity is what
keeps them going. If you push water up and then it plops back
down, it creates waves. It's the same with air."
Coleman
left his job as a TV weather anchor in Birmingham to work
on his Ph.D. in Atmospheric Science at the University of Alabama
in Huntsville. "I'm having fun," he says, but his
smile and enthusiasm already gave that away.
"You
can see gravity waves everywhere," he continues. "When
I drove in to work this morning, I saw some waves in the clouds.
I even think about wave dynamics on the water when I go fishing
now."
Gravity
waves get started when an impulse disturbs the atmosphere.
An impulse could be, for instance, a wind shear, a thunderstorm
updraft, or a sudden change in the jet stream. Gravity waves
go billowing out from these disturbances like ripples around
a rock thrown in a pond.
When
a gravity wave bears down on a rotating thunderstorm, it compresses
the storm. This, in turn, causes the storm to spin faster.
To understand why, Coleman describes an ice skater spinning
with her arms held straight out. "Her spin increases
when she pulls her arms inward." Ditto for spinning storms:
When they are compressed by gravity waves, they spin faster
to conserve angular momentum.
"There
is also wind shear in a gravity wave, and the storm can take
that wind shear and tilt it and make even more spin. All of
these factors may increase storm rotation, making it more
powerful and more likely to produce a tornado."
"We've
also seen at least one case of a tornado already on the ground
(in Birmingham, Alabama, on April 8, 1998) which may have
become more intense as it interacted with a gravity wave."
Above:
Click on the graphic to play an actual Doppler radar movie
of a gravity wave interacting with a rotating thunderstorm
and making it stronger in northwest Alabama on Jan. 22, 1999.
Credit: NOAA.
Coleman
also points out that gravity waves sometimes come in sets,
and with each passing wave, sometimes the tornado or rotating
storm will grow stronger.
Tim
and his boss, Dr. Kevin Knupp, are beginning the process of
training National Weather Service and TV meteorologists to
look for gravity waves in real-time, and to use the theories
behind the GrITs model to modify forecasts accordingly.
Who
would have thought grits could predict bad weather? "Just
us meteorologists in Alabama," laughs Coleman. Seriously,
though, Gravity wave Interactions with Tornadoes could be
the next big thing in severe storm forecasting.
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Editor's
note: The gravity
waves of this story should not be confused with the gravitational
waves of astrophysics. One is an ordinary wave of water
or air; the other is a ripple in the fabric of spacetime itself.
Author: Dauna
Coulter | Editor:
Dr. Tony Phillips | Credit: Science@NASA
| more
information |
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Giant
Atmospheric Waves over Iowa -- (Science@NASA)
Below:
This GrITS (Gravity wave Interactions with Tornadoes)
computer model output shows how the vorticity of a rotating
thunderstorm increases as a gravity wave passes through
it. Credit: Tim Coleman.
A
"model" is a computer simulation based on
mathematical equations that describe atmospheric processes.
The researchers run the model many times for many scenarios
to get a general picture of how the process under investigation
works. For GrITs, they make sure the model reflects
patterns of how waves affect mesocyclones/tornadoes
and indicate the factors that amplify those effects.
Every
storm is different, and the researchers show the forecasters
what to look for, in general. Forecasters do not directly
use the GrITs model, but will instead use the general
results the researchers get from the model and share
with them. The researchers may share charts with the
forecasters to show how each parameter (angle between
wave and storm inflow, wave amplitude, wave speed, storm
intensity, etc.) changes the effect a wave will have
on a mesocyclone.
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