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CSI Tornado: Decoding – and chasing – supercells with the experts

CSI Tornado: Chasing supercells, interviewing a homeowner sucked off his front porch in an Oklahoma tornado outbreak, and examining the path of a destructive funnel, an expert expedition shows how science is close to decoding the way a tornado works.

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Meanwhile, wind shear at mid-levels sets up a rolling pin-like circulation of air drawn into the updraft, imparting rotation to the core of the storm.

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All that happens well above the ground, Dr. Markowski says, but adds: "It can't ever explain why you would have spin right at the ground."

But maybe VORTEX 2, the largest tornado field study in history, can. During the 2009 and 2010 spring tornado seasons, more than 100 scientists crisscrossed the central US chasing supercells in hope of capturing the entire life cycle of tornadoes. Their tools: mobile radars, weather balloons, weather stations on wheels, and radio-controlled aircraft laden with instruments.

A picture of the final stages of tornado formation is still emerging from the data. But, says Markowski, spin at the ground appears to result from interplay between dense, rain-cooled air descending on the backside of a supercell's rainy core – a kind of exhaust system for the thunderhead – and a second source of shear near ground level that the storm itself can set up. In essence, it's a small-scale version of the process that forms the supercell in the first place. Typically, air in a thunderstorm's rear downdraft hits the ground, spreads, and dissipates. But under the right conditions, some of that air can get caught in small-scale updrafts in the rain-free base at the rear of the supercell.

If this cooler air has the right mix of temperature and moisture, the U-turn can result in a rotating "wall cloud" that descends from the rain-free base. If the storm's self-generated, low-level shear encounters the updraft, it is tugged upward, spins up, makes contact with the ground, and produces a tornado.

But are there early-warning signs of tornado formation? Enter the "blob." Researchers discovered that in some storms they intercepted, the downdraft seemed to flow in pulses that might be linked to discrete blobs of rain that the team's mobile radar picked up along the storm's rear flank. One of these blobs was present in a storm that generated a powerful tornado in Goshen County, Wyo., on June 5, 2009.

Some five to 10 minutes after the blob hit the ground, spin rapidly intensified and a tornado formed, says Markowski, who served on the project steering committee.

The radar units that picked up the blobs operate in the same way the NWS's weather radars will operate once the upgrades are complete. Known as dual-polarization Doppler radar, the upgraded radar will send out horizontal and vertical pulses that return detailed information on the size and distribution of particles in the clouds and clues to temperatures in the storm.

How to withstand vertical wind

When David Prevatt walked through residential areas in Joplin, Mo., and Tuscaloosa to survey damage after the tornadoes that hit each city last spring, he noticed an intriguing pattern: If a home wasn't obliterated, a house built before the 1940s tended to sustain less damage than homes built as recently as the mid-1990s.

Dr. Prevatt, an assistant professor of civil and coastal engineering at the University of Florida at Gainesville, noticed that the exterior walls, as well as the floors and roofs of the pre-1940s houses, typically were covered with 2-by-12-inch planks, set diagonally. More-heavily damaged postwar homes were sheathed in thinner, less-expensive plywood sheets.

It's "anecdotal evidence that may suggest that because we have changed the structural system" for homes, "we've changed the capacity and performance of these houses" to endure the stresses tornadic winds impose, he says.


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