In a typical year, 1,200 tornadoes cause 70 fatalities and 1,500
injuries nationwide. Most of the damage, deaths, and injury are
due to a very small percentage of these tornadoes, the so-called
“supertwisters” whose winds of more than 200 miles per
hour put them at the extreme end of the Fujita Scale of Tornado
Intensity (the so-called F4 and F5 tornadoes). On average, there
are 12 or 13 of these tornadoes in the United States each year.
Ideally, forecasters would be able to provide enough warning that
people could protect themselves from these killer storms. While
they have successfully identified atmospheric conditions that are
favorable for supercell formation, accurately predicting which storms
will produce tornadoes and at what time is a feat that continues
to challenge forecasters. Researchers from the University of Illinois
at Urbana-Champaign collaborated with visualization experts at NCSA
in an effort to shed light on how the most violent tornadoes form
and to create animations that reveal the inner behavior of tornado
Their work was showcased this March in an episode of the PBS TV
series NOVA called "Hunt for the Supertwister."
Scientists know that the strongest tornadoes are generated by a
particular type of rotating thunderstorm called a supercell. The
swirling winds of a supercell can produce tornadoes. But not all
supercells lead to tornadoes, and not all tornadoes become supertwisters.
In fact, only about 20 percent to 25 percent of supercells produce
tornadoes. Why some storms spawn tornadoes while others don't --
and why some tornadoes become extraordinarily strong supertwisters
-- is not yet well understood.
Supercells form in an unstable and adequately deep atmospheric
layer that has sufficient moisture and significant change in the
horizontal wind speed. An environment that favors the formation
of tornadoes also requires high relative surface humidity, considerable
low-level horizontal wind, and steep low-level lapse rates (meaning
the temperature drops rapidly at greater atmospheric heights).
In an effort to pinpoint what triggers tornadoes, researchers --
including NCSA research scientist Robert Wilhelmson -- create computer
simulations of evolving storms. Just as physicians use X-rays and
CAT scans to diagnose disease, these storm researchers use simulations
and visualization to analyze tornado formation.
"The big problem in storm science is that with the instrumentation
we have we can't sense all the things that we need to know,"
explains Lou Wicker, a scientist at the National Severe Storms Laboratory
who frequently collaborates with Wilhelmson. "From the field,
we can't figure out completely what's going on, but we think the
computer model is a reasonable approximation of what's going on,
and with the model we can capture the entire story."
Wicker developed a model called NCOMMAS (NSSL Collaborative Model
for Multiscale Atmospheric Simulation) to computationally simulate
thunderstorms and their associated tornadoes. NCOMMAS is based upon
an earlier model developed by Wilhelmson.
The simulation begins with data describing the pre-tornado weather
conditions -- wind speed, atmospheric pressure, humidity, etc. --
at discrete points separated by distances ranging from 20 meters
to three kilometers. Starting with these initial variables, partial
differential equations that describe changes in the atmospheric
flow are solved. The numerical solution of these equations proceeds
in small time intervals for two to three storm hours as the supercell
forms and produces a tornado. A virtual storm is born.
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Access Online | Posted 11-8-2004