A comprehensive merging of satellite and ground-based lightning data has painted a new picture of how infrequently lightning reaches the ground in a thunderstorm, a measure that could ultimately help forecast the possibility of tornadoes in a given storm.
Much of the electrical energy of a thunderstorm is known to dissipate within the clouds, in what researchers call cloud-to-cloud lightning. This type of lighting is often associated with fast-rising air, a condition that tends to spawn tornadoes.
The new data confirm that in the United States, cloud-to-cloud lighting strikes achieve the highest ratio (as compared with cloud-to-ground) in a swath of the Midwest known as Tornado Alley.
About lightning
The rapid rise and fall of air currents cause thunderstorms. The friction from this moving air creates electrical charges within a cloud. Water droplets and ice pellets fall, carrying charged electrons to the lower portion of the cloud, where a negative charge builds. A positive charge builds up near the top of a cloud.
Studying lightning is no easy task: Every minute there are a thousand thunderstorms going on around the Earth, causing some 6,000 ephemeral flashes of lightning. Researchers admit there is much to be learned about the behavior of these colossal discharges of electricity, which typically release some 30 million volts.
New data
The Optical Transient Detector satellite, launched in 1995, records the locations of lightning flashes in clouds below, and it has mapped the ratio across the entire United States.
"Strong updrafts, which cause lightning, correlate to stronger storms," said Dennis Boccippio of the Global Hydrology and Climate Center, affiliated with NASA's Marshall Space Flight Center in Huntsville, Alabama. "(With the new data) 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."
Researchers were surprised to learn that much of Oregon and northwest California experiences the second highest ratio of cloud-to-cloud lightning versus those bolts reaching the ground. They say this might be explained by the fact that more lightning in that region occurs in winter storms, which behave differently than summer time thunderstorms. It might also reflect poor data because of fewer ground monitoring stations.
Another interesting finding involved areas where cloud-to-cloud and cloud-to-ground lightning occurred in roughly equal amounts (blue areas in the accompanying image), including the Appalachians, the Rockies and Sierra Nevada mountains.
"The conventional wisdom is that the ground is closer to the main negative charge layer," Boccippio said. With less air between the negatively charged clouds and the positively charged ground below, lightning might more easily jump the gap.
"Also, the topography may keep storms from organizing supercells," he said.
Boccippio and his colleagues combined four years of satellite and ground-based data in their study, which was presented at a recent meeting of the American Geophysical Union.
Image at top of page: Night storm clouds illuminated by cloud-to-cloud lightning. Credit: NOAA
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