Lightning Strikes an Odd Pattern
Over the Plains

Satellite sensor points to most violent storm activities

Jan 4, 2000: Where there's lightning, many people worry that tornadoes may follow. Lightning is associated with energetic storms since it takes large upward movements of air - plus water in various forms including raindrops and ice crystals - to produce a large electric potential.

Right: This map depicts the ratio of cloud-to-cloud lightning to cloud-to-ground lightning over the continental United States. The red areas show where the ratio is as high as 10 cloud-to-cloud strikes for every ground strike. Blue areas indicate ratios as low as 1:1. Links to 911x650-pixel, 300KB JPG. Or, click here for 212KB Acrobat PDF version that can be edited in Illustrator. Credit: Dennis Boccippio, NASA/Marshall.

When the potential becomes great enough, electricity punches its way through air that normal insulates and builds a narrow bridge of electrified gas or plasma. The current burrows its way in search of an oppositely charged region where the imbalance can be relieved. When the two are joined, current flows freely and ionizes even more air on its path, thus creating the glowing hydra that we see as a lightning bolt. The heated air expands and, when the discharge is suddenly stopped, it slams back together to produce the thunderclap.

The feeling that many people have about lightning and tornadoes is gradually getting scientific support. The latest find is that storms with far more cloud-to-cloud lightning than cloud-to-ground are more energetic and are more likely to produce violent storms.

"The really interesting area is the Great Plains region where the ratio goes as high as 10-to-1," said Dr. Dennis Boccippio of the Global Hydrology and Climate Center, affiliated with NASA's Marshall Space Flight Center in Huntsville, Ala.

Boccippio, working with Drs. Hugh Christian and Steve Goodman of the GHCC and Dr. Cummins of Global Atmospherics in Tucson, Ariz., combined four years of satellite and ground-based data to estimate the ratio of intercloud (IC) and cloud-to-ground (CG) lightning. He presented their findings to the American Geophysical Union's fall meeting in San Francisco on Thursday.

The space segment of Boccippio's data came from two similar instruments developed by the GHCC and NASA/Marshall. The Optical Transient Detector, launched in 1995, records the locations of lightning flashes in clouds beneath the satellite.

"After four years of observations, the OTD has enough data to merge with the National Lightning Detection Network," or NLDN, Boccippio explained. The NLDN is ground-based and detects where lightning strikes the ground. This information is vital to utilities and communications companies so they can tell which facilities are at increased risk as a weather system moves through an area.

But the NLDN can only measure strikes that reach the ground, and those are only part of the story.

Right: Storm clouds from nighttime thunderstorm Illuminated by cloud-to-cloud lightning. Credit: NOAA Photo Library, NOAA Central Library

Scientists have known for some years that storms have a significant amount of energy tied up in lightning that never reaches the ground. While the lightning itself poses no direct threat to people on the ground, it is a strong indicator of vertical motion in a storm.

"That ratio has never been known very well because it's very hard to measure," Boccippio said. Much of what was known was based on small data sets collected by ground observers and instruments carried by balloons.

The two satellite sensors let improve our understanding with a great deal of confidence.

"This is the first time we've been able to map the ratio across the entire United States," he continued.

Two areas on the map stand out right away. First is Tornado Alley, an area covering much of Kansas and Nebraska. Second is most of Oregon and part of northwest California.

While one might react with, "Well, I could have told them that," it's one thing to "know" something, and quite another to assign numerical values that tell a scientific story that eventually might help predict the weather.

"Strong updrafts, which cause lightning, correlate to stronger storms," Boccippio said. "The absolute flash rate value from satellites is useful. Combined with the ground rate, we can break the two types of lightning out, and be more confident in saying that a given storm is going to be a supercell and have hail or tornadoes."

Oregon, though, is not known for tornadoes. One possibility is that the area is not as densely populated with sensors as the rest of the United States, so the ratio is artificially high. Also, more lightning in that region occurs in winter storms, which behave differently than summertime thunderstorms.

Also striking is the number of blue areas, indicating very low ratios, almost 1:1, of intercloud and cloud-ground lightning. These correspond with the Appalachian Mountains in the East, and the Rockies and Sierra Nevada mountains in the West.

Two factors may literally short-circuit lightning there, he suggested.

"The conventional wisdom is that the ground is closer to the main negative charge layer," he said. With less insulating air between opposite charges, lightning will discharge more easily.

"Also, the topography may keep storms from organizing supercells," he added.

With further study, Boccippio expects that understanding the lightning ratio can benefit people on the ground and in the air. The two sensors can only be used for limited research because they are in low orbits and see only a tiny fraction of the Earth at any given time.

A Lightning Mapping Sensor in geostationary orbit, though, would see an entire hemisphere, and would have optics to monitor danger areas as they brewed.

This would help severe storm forecasters provide quicker, more precise warnings of storms that are likely to unleash damaging hail, high winds, and tornadoes. They could also reduce false alarm rates for severe weather hazards.

It would also help airlines route aircraft around storm centers where intercloud lightning poses a significant hazard to aircraft electronics.

The long range component of the North American Lightning Detection Network has been providing experimental data products since July 1996, offering cloud-to-ground lightning coverage throughout the Atlantic and Western Pacific oceans, as well as south to the Intertropical Convergence Zone. The network experiences a strong decrease in detection efficiency with range, which is also significantly modulated by differential propagation under day, night and terminator-crossing conditions. A climatological comparison of total lightning data observed by NASA's Optical Transient Detector (OTD) and CG lightning observed by the long range network is conducted, with strict quality control and allowance for differential network performance before and after the activation of the Canadian Lightning Detection Network. This yields a first-order geographic estimate of long range network detection efficiency and its spatial variability. Intercomparisons are also performed over the continental US, allowing large scale estimates of the midlatitude climatological IC:CG ratio. We find significant spatial variability in this parameter, little latitude dependence, some indication of elevation dependence, and a strong anomaly possibly associated with enhanced MCS activity in the upper midwest.

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Author: Dave Dooling
Curator: Linda Porter
NASA Official: M. Frank Rose