August 6, 2011

Research shakes up tornado theories

Vortex2, a two-year field study of tornadoes in the nation's midsection, is prompting researchers to reconsider theories on how twisters develop, a leader of the project said Friday.

Vortex2, a two-year field study of tornadoes in the nation's midsection, is prompting researchers to reconsider theories on how twisters develop, a leader of the project said Friday.

Analysis of the data collected in the springs of 2009 and 2010 is still in its early stages, said Don Burgess, a research scientist at the Cooperative Institute for Mesoscale Meteorological Studies at the University of Oklahoma, who spoke at the High Plains weather conference at the Wichita Marriott.

But two events during the field study are getting special emphasis, he said: the tornado that touched down in Goshen County, Wyo., on June 5, 2009; and a storm system that produced several tornadoes in central and eastern Oklahoma on May 10, 2010.

A small yet distinct storm feature called a descending reflectivity core — a column of precipitation visible on radar but not always evident to the naked eye — lowered to the ground just before a large tornado formed in rural Wyoming two years ago.

"Would the tornado form without the DRC?" Burgess asked during an interview after his presentation. "Maybe that was a necessary step."

Perhaps the storm was so strong a tornado would have developed without the core descending first, he said.

Researchers had dismissed the core as significant because "we've seen so many of those" in thunderstorms "it can't be important," Burgess said.

But the timing of the core's development may be key, he said, by destabilizing the atmosphere beneath the wall cloud enough to allow a tornado to develop.

The data clearly showed the storm's forward flank was a significant contributor to a tornado's development, Burgess said.

"We'd seen it before, but it's re-emerging in importance," he said.

Vortex2's data seems to suggest that factors closer to the earth's surface play more important roles in whether a tornado forms than previously realized, he said.

But researchers were unable to collect that detail during the field study because the tornadic storms didn't form within the geographic area for which low-level aircraft flights were approved.

The data gleaned from the Oklahoma tornadoes showed not all hook echoes — the classic radar signature indicating the presence of a tornado — are alike, he said.

And supercell thunderstorms produce multiple tornadoes differently than previously thought.

The prevailing idea had been that one tornado dies as a second developing twister steals energy from it. But Vortex2 data suggests the collapse of one tornado helps create the conditions allowing the next tornado to develop, Burgess said.

The cycle can repeat itself many times, he said. The Oklahoma storm produced 20 tornadoes over a four-hour period.

"We were kind of zeroing in on one mechanism" that generates tornadoes, he said. "Now we're opening the door to multiple mechanisms."

Vortex2 documented 15 supercell thunderstorms that produced tornadoes and 29 that did not produce tornadoes over the course of 82 days in Tornado Alley.

2009 was noted for its lack of tornado activity, hampering efforts to collect data.

But it would be wrong to presume this year's record-setting tornado season would have provided a mother lode of data for Vortex2, Burgess said.

Researchers would not have been able to gather much data this year because the storms struck in areas filled with trees and hills — features that would have interfered with the beams from the project's portable radars.

Adequately crunching Vortex2's data will last until at least 2014, Burgess said. But researchers are already talking about how to approach the next field study — whenever that takes place years from now.

Rather than amassing an armada of vehicles that often proved slow to respond to changing storm conditions, he said, the better approach next time might be smaller groups that can move and set up more quickly.

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