Examination of VORTEX2 radar data uncovered a supercell mystery no one noticed when the data were being recorded.

When taking a second look at the data to trace the genesis of a tornado, scientists observed that there was an area of light rain and small raindrops inside the thunderstorm supercell that spawned the tornado. Normally supercells are filled with heavy rain and ice.

“We are so wind-centric that we still did not notice” the area at first, recalls Karen Kosiba of the Center for Severe Weather Research. She and two colleagues were partway through analyzing the radar data from a 2009 tornado and its parent supercell that occurred in Goshen County, Wyo., when they spotted the unusual area. It was in their office, well after the tornado, that they looked at precipitation data, not just the winds.

An oval marks an area of light rain and small raindrops inside a thunderstorm. (CENTER FOR SEVERE WEATHER RESEARCH)

They’re calling the area a “low reflectivity ribbon,” or LRR, because it reflects back much less radar energy than surrounding, rainier parts of the storm. Radar data also showed a similar LRR in the parent supercell of a tornado that hit near Booker, Tex., in 2010.

It’s unclear what the significance of the LRR might be and what role, if any, it might play in creating a tornado.

“We don’t know why it forms or whether it is important in tornado genesis,” said Josh Wurman, head of the research center. “It is probably present in lots of supercells, but, frankly, we won’t know until we look some more. We’re publishing our initial imagery of this newly discovered phenomenon in the hope that scientists will start pondering why it forms and whether it is important.”

The LRR apparently shows up only in close-up views captured by the more sophisticated radars used in VORTEX2 rather than in earlier field research.

In both cases noticed so far, the LRR region split the supercells in two from north to south and from the bottom to the top, more than 30,000 feet up. Both LRRs were roughly 2,000 feet thick.

“It was quite a surprise,” Wurman says. “The child part of me as a scientist thought, ‘This is way cool, nature is throwing us a curveball.’ ” He says that the radar images have been shown at scientific conferences and that no one has come up with a hypothesis to explain the LRR or whether it affects tornado formation.

Wurman says the question is: “Does this affect tornadoes, or is it like a rainbow, scientifically cool but with no effect on the weather?”

Jack Williams