Author: Carolyn Gramling / Source: Science News
WASHINGTON — Tornadoes may form from the ground up, rather than the top down.
That could sound counterintuitive. Many people may picture a funnel cloud emerging from the bottom of a dark mass of thunderstorms and then extending to the ground, atmospheric scientist Jana Houser said December 13 in a news conference at the American Geophysical Union meeting.
Scientists have long debated where the wind rotations that lead to twisters in these thunderstorms begin. Now Houser, of Ohio University in Athens, and her colleagues have new data that upend this “top-down” idea of tornadogenesis.
The supercell thunderstorms that can spawn tornadoes form where a powerful updraft of warm, moist air — such as air moving northward from the Gulf of Mexico — gets trapped beneath a layer of colder, drier air — such as air moving southward from Canada. That’s an inherently unstable condition, but the other necessary ingredient for tornadoes to form is wind shear: fast-moving winds that move the air masses, causing them 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. Tornadoes can form within just 30 to 90 seconds, so research on how they start is often limited by not quite getting to the right place at the right time to watch a twister actually being born — and her team was no exception, Houser said.
But using a rapid-scanning Doppler radar mounted at the back of a truck, the team managed to capture the full evolution of four tornadoes, including two powerful twisters that struck near El Reno, Okla., one on May…
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