Lightningposition in storm may circle strongest updrafts

Lightning position in storm
may circle strongest updrafts

New finding could help in predicting hail, tornadoes

One of a series of stories covering the quadrennial International Conference on Atmospheric Electricity, June 7-11, 1999, in Guntersville, Ala.

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.

Above: Time-lapse photography captures multiple cloud-to-ground lightning strokes During a night thunderstorm over Norman, Oklahoma, March, 1978. (C. Clark, NOAA)

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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, mayÂ provide another tool forÂ 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.

Left, Above:The main reflector for a Doppler radar, one of the principal tools for peering inside severe storms. (NSSL/NOAA)

"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.

Web Links

Human Voltage (June 18, 1999) Scientists discuss biology, safety, and statistics of lightning strikes.
News shorts from Atmospheric Electricity Conference (June 16, 1999) Poster papers on hurricanes and tornadoes summarized.
Soaking in atmospheric electricity (June 15, 1999) 'Fair weather' measurements important to understanding thunderstorms.
Lightning position in storm may circle strongest updrafts (June 11, 1999) New finding could help in predicting hail, tornadoes
Lightning follows the Sun (June 10, 1999) Space imaging team discovers unexpected preferences
Spirits of another sort (June 10, 1999) Thunderstorms generate elusive and mysterious sprites.
Getting a solid view of lightning (June 9, 1999): New Mexico team develops system to depict lightning in three dimensions.
Learning how to diagnose bad flying weather (June 8, 1999): Scientists discuss what they know about lightning's effects on spacecraft and aircraft.
Three bolts from the blue (June 8, 1999): Fundamental questions about atmospheric electricity posed at conference this week.
Lightning Leaders Converge in Alabama (May 24, 1999): Preview of the 11th International Conference on Atmospheric Electricity.
What Comes Out of the Top of a Thunderstorm? (May 26, 1999): Gamma-rays (sometimes).
Lightning research at NASA/Marshall and the Global Hydrology and Climate Center.

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."

More web links

45th Weather Squadron at Patrick AFB, lightning reference page.
National Severe Storms Laboratory, Norman, OK
Numerical Modeling at NSSL
The New Mexico Tech 3D Lightning Mapping System
Lightning Detection and Ranging project at Kennedy Space Center.
National Severe Storms Laboratory Photo Library, where we got a lot of the neat pictures in these stories.

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Author: Dave Dooling
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