EARTH & ENVIRONMENT
Tornado funnels may rise from below
Scientists capture the first few seconds of a twister’s life
BY CAROLYN GRAMLING
Tornadoes may form from the ground
up, rather than the top down.
That could sound counterintuitive.
Many people picture a funnel cloud
emerging from a dark mass of storms
and then extending to the ground, atmospheric scientist Jana Houser of Ohio
University in Athens said December 13
at the American Geophysical Union
meeting. But Houser and her colleagues
have new data that upend this “
top-down” idea of tornado formation.
The supercell thunderstorms that can
spawn tornadoes form where a powerful
updraft of warm, moist air gets trapped
beneath a layer of colder, drier air. The
ATOM & COSMOS
Hints of water
found on asteroid
OSIRIS-REx will return sample
of the space rock to Earth
BY LISA GROSSMAN
As the asteroid Bennu comes into sharper
focus, planetary scientists are seeing
signs of water locked up in the asteroid’s
rocks, scientists announced December 10.
“It’s one of the things we were hoping
to find,” Amy Simon of NASA’s Goddard
Space Flight Center in Greenbelt, Md.,
said in a news conference at the American
Geophysical Union meeting. “This is
evidence of liquid water in Bennu’s past.
This is really big news.”
NASA’s OSIRIS-REx spacecraft, which
arrived at the roughly 500-meter-wide
Bennu on December 3, made the dis-
covery. Over the next year, scientists
will look for a spot for the spacecraft to
collect asteroid dust and then return it
to Earth (see Page 20).
OSIRIS-REx’s spectrometers, which
measure the chemical signatures of
minerals based on the wavelengths of
light they emit and absorb, saw signs of
hydrated minerals about a month before
the spacecraft reached the asteroid. The
signal remained strong all over the surface as the probe approached, Simon said.
Those minerals form only in the presence
of liquid water and suggest that Bennu
once had a hydrothermal system.
Bennu’s surface also has more boulders
and craters than expected. Observations
from Earth indicated there would be at
most only a few large boulders, about
10 meters wide. But Bennu has hundreds, some more than 50 meters wide.
That rough surface can reveal details
of the asteroid’s internal structure. If
the asteroid were one solid mass, for
instance, a major impact could crack or
shatter its entire surface. The fact that
the asteroid has large craters means it
has survived impacts intact. Therefore,
Bennu may be a pile of rubble loosely
held together by its own gravity.
Bennu’s low density supports that idea.
OSIRIS-REx data indicate that the den-
sity is about 1,200 kilograms per cubic
meter, said principal investigator Dante
Lauretta of the University of Arizona
in Tucson. The average Earth rock is
about 3,000 kg/m3. Water is less dense
than rock, so the hydrated minerals help
lower the density. But up to 40 percent of
Bennu may be full of caves and voids, fur-
ther lowering the density, Lauretta said.
The mission’s next big task is to find a
suitable place to sample. OSIRIS-REx’s
sampling arm can’t pick up grains larger
than 2 centimeters. “I am confident that
we’ll find some fine-grained regions,”
Lauretta said. The challenge will be finding an area wide enough that the probe’s
navigation system can steer to it. s
other necessary ingredient is wind shear,
when fast-moving winds cause the air
masses to rotate horizontally. Air then
rising through the supercell can tip the
rotation from horizontal to vertical —
creating conditions ripe for a tornado.
But the very moment of twister birth
remains largely elusive because tornadoes form so fast, sometimes within
30 to 90 seconds. By using a rapid-scanning Doppler radar mounted on a
truck, Houser’s team managed to capture the full evolution of four tornadoes,
including two powerful twisters that
struck near El Reno, Okla., one on May 24,
2011, and another on May 31, 2013.
The radar, which collects data every
16 seconds, showed that rotating winds
for the 2011 storm appeared to start at
multiple levels in the atmosphere at once,
rather than starting high up and moving
downward. For the 2013 storm, tornado-
strength rotation began at the lowest
measured elevation, about 20 meters
above ground. A minute later, the
rotation was at a height of about 3. 5 kilo-
meters. Crowdsourced images of the
storm also revealed a funnel near the
ground, even before the radar captured it.
The discovery that the 2013 storm had
a funnel long before the radar saw rota-
tion is “a pretty compelling finding,” says
Leigh Orf, an atmospheric scientist at the
University of Wisconsin–Madison. But,
he says, that twister was such a bizarre
storm that he hesitates to use it as an indi-
cator of typical tornado behavior.
“There are likely different modes of
tornadogenesis,” Orf says. s
Craters on Bennu, such as this 20-meter-wide
one, are a clue that the asteroid is a pile of
rubble held together by its own gravity.