New finding could help in predicting hail, tornadoes
June 11, 1999: While lightning is one of the most dangerous aspects of severe storms, it appears that inside the storm lightning itself avoids the most violent regions. This finding has implications for tracking where tornadoes are likely to form and where large hail may fall.
"This can be a monitor of severe storm intensity, another tool to monitor when storms might produce tornadoes or hail," said James Dye, a researcher at the National Center for Atmospheric Research in Boulder, Colorado. Dye spoke this week at the International Conference on Atmospheric Electricity.
Lightning has long been associated with convection in storms. At a simple level, the charging mechanism is similar to shuffling across the carpet on a cold winter day, then touching a door knob. The friction between your feet and the rug builds a static electric charge that you carry until you touch something grounded. Inside thunderstorms, the mechanism which separates charges is collisions between growing snow pellets and numerous ice particles.
But storms still conceal a number of surprises in many elaborations and variations on that simple description. Dye said that scientists now are learning that the most energetic storms are prolific sources of intracloud lightning. Intracloud lightning, though obscured by the clouds, may provide another tool for monitoring severe storms.
Dye's findings come from the Stratospheric-Tropospheric Experiment: Radiation, Aerosols and Ozone (STERAO) Deep Convection experiment in northeast Colorado in June and July 1996. He used data from a French radio interferometer that collected five hours of uninterrupted observations of the locations of lightning within two major electrical storms, and from Doppler radar that shows wind direction and speed within a storm. Combining the two data sets showed where lightning was relative to updrafts and downdrafts.
"It seems that lightning channels themselves are not in the most intense updrafts," Dye explained, "but in the weaker updrafts and downdrafts." This would seem to go against expectation. But while the lightning avoided the updraft cores, it became more frequent around the cores as the storm grew stronger.
Dye said that storms with updrafts at speeds less than 5 meters/second (about 11 mph) produce little or no lightning. Storms with updrafts of 10 to 20 meters/second (22 to 44 mph) might have flash rates of 5 to 20 strikes per minute. At more than 40 meters/second (90 mph or more), things get busy and the flash rate goes to 1 per second or more.
Dye says no one knows sure why, but the answer probably lies in the microphysics of ice and hail formation, the separation of charges as these bodies grow and move past each other, and how they are transported inside the storms by vertical and horizontal winds.
The French interferometer is one of several systems that have been developed in recent years to give scientists an inside view of lightning in storms. Cloud-to-ground strikes across the United States can be recorded by the National Lightning Detection Network. But intracloud strikes in local storms can be recorded only by the Lightning Detection and Ranging system at Kennedy Space Center and the Lightning Mapping System at the New Mexico Institute of Mining and Technology. Also, when they are overhead at the right time the Lightning Imaging Sensor and the Optical Transient Detector, both in orbit, can count lightning from above the clouds.
An exciting addition to this new suite of research tools would be a satellite-borne Lightning Mapping Sensor, under study at the Global Hydrology and Climate Center, that would observe the Earth continually from geostationary orbit.
"This would provide much more information in terms of intracloud strikes," he said. "It could be an additional forecasting and nowcasting tool" for meteorologists watching severe storms.
Dye cautioned that his and other results are not conclusive yet, but "they're highly suggestive and promising, but we have more work to do."
One of a series of stories covering the quadrennial International Conference on Atmospheric Electricity, June 7-11, 1999, in Guntersville, Ala.