H i g h l i g h t s
S c i e n c e
T e r a G r i d
TG
10
Titanic
Twisters
University of Oklahoma researchers use TeraGrid
supercomputers to form a new hypothesis on how
tornadoes form
“There are various theories that try to explain the
cause of rotation in tornadoes,” says Ming Xue, professor of meteorology at the University of Oklahoma, and
director of the Center of Analysis and Prediction of
Storms. “But the true causes are still not well understood.”
Using the Ranger supercomputer at the Texas
Advanced Computing Center (TACC), Xue’s Ph.D. student, Daniel Dawson, obtained an accurate prediction
of the May 1999 tornado (Fig. 1). The prediction
required the assimilation of radar observations and a
prediction model with a sophisticated treatment of
cloud microphysics processes.
Another of Xue’s projects used the TeraGrid systems at
the Pittsburgh Supercomputing Center (PSC) to obtain
the most realistic simulation to date of a theoretical
tornado developing with a supercell thunderstorm.
Image courtesy: Daniel Dawson and Ming Xue, University of Oklahoma
With next-generation, multi-petascale supercomputers
on the way, Xue believes it will soon be possible to
resolve supercell thunderstorms in real-time with a
resolution of about 250 meters—not tornado-resolving
capacity, but enough to give a good indication of where
and when a tornado may occur.
“Having access to large supercomputers on the
TeraGrid allows us to do simulations that were not
possible before and analyze huge volumes of data
much faster,” according to Xue. “They make such
advanced research possible.”
TG
11
Using 2,000 processors and several days’ computing
time, he obtained a tornado with 12.5 meter horizontal
resolution throughout the simulation that matched
most of the observed characteristics of real twisters,
including the tornado condensation funnel (Fig. 2).
Xue’s research prompted a new hypothesis about why
tornadoes form. According to his theory, the cloud
microphysics affect how the mid-level updraft and
rotation are positioned relative to the low-level rotation.
Fig. 1. Surface wind swaths obtained in the simulation with a
250 m horizontal grid spacing (left panel), as compared to
observed ground damage track (right panel) for the May 3, 1999,
Oklahoma City F5 tornado.
“This relative position is a key factor affecting whether
a thunderstorm can produce a tornado or not,” Xue
says. “It explains why some thunderstorms produce
tornadoes, and others, though they may look very
similar, don’t.”
Te r a G r i d 2 0 0 9
About a thousand tornadoes strike the United States
each year, leading to hundreds of millions of dollars in
damages, many dozens of deaths, and thousands of
injuries annually. Despite the horrendous costs in lives
and property, how tornadoes form still remains something of a mystery.
One important principle behind the tornado’s rotation
is the conservation of angular momentum—the same
force that makes ice-skaters spin faster when they
embrace their stretched arms. Tornadoes form when
strong vertical motion causes a concentration of air
towards a convergence center. However, the source of
the initial rotation, and the forcing that causes strong
low-level convergence, is unclear for many tornadoes.
Titanic Twisters
O
n May 3, 1999, violent thunderstorms swept through
Oklahoma and Kansas
spawning 66 tornadoes and
claiming 48 lives. Most severe among
them was the tornado that swept
Ming Xue, University
through Moore, Oklahoma, carrying
of Oklahoma
the most powerful winds ever recorded
on earth—more than 300 miles per hour.
Fig. 2
3D volume rendering of simulated cloud water content
showing the tornado condensation funnel reaching the
ground.
Image courtesy of Greg Foss, Pittsburgh Supercomputing Center and Ming
Xue, University of Oklahoma
NSF grants: ATM-0530814, ATM-0802888.
TeraGrid grant: MCA95T006