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by Valerie Kimble

See pdf image by Harald Edens

SOCORRO, N.M., March 24, 2008 – New Mexico Tech Professor Paul Krehbiel has long looked upward as part of his research in atmospheric physics, and an online article in this week’s Nature Geoscience reports on a recent collaborative discovery.

Krehbiel and team identified how lightning can exit the top of a thunderstorm to produce discharges that propagate upward into the atmosphere.

Also credited in the study are Professors Ronald Thomas and William Rison, doctoral student Harald Edens and alumnus Mark Stanley, all of New Mexico Tech; along with graduate student Jeremy Riousset and Professor Victor Pasko, both of Pennsylvania State University.

Krehbiel and Pasko met at a scientific conference several years ago, and recently embarked on a formal collaboration to combine the detailed pictures of lightning obtained with New Mexico Tech’s Lightning Mapping Array of “bolt-from-the-blue” lightning flashes with computer simulations of the discharges, to better understand lightning discharge processes.

As Krehbiel explained, the upward discharges take the form of gigantic jets and blue jets, and have been reported for many years by commercial airline pilots, but only observed scientifically since the early 1990s.

While the various types of in-cloud and cloud-to-ground lightning are reasonably well understood, the cause and nature of upward discharges has remained somewhat of a mystery.

Both types of jet discharges are infrequent, but the Tech researchers recently discovered several examples of blue jets in their lightning mapping measurements. They combined this with an earlier observation of a gigantic jet over Puerto Rico, obtained in 2001 by Pasko and Stanley; and, with computer simulations of the storm electrodynamics and of the lightning discharges themselves, showed how both types of upward jets are produced by storms.

In the study, an important clue to understanding how gigantic jets occur has been observations with New Mexico Tech’s Lightning Mapping Array of “bolt-from-the-blue” lightning flashes.

Bolt-from-the-blue (BFB) discharges start off as normal upward lightning inside the cloud, but continue horizontally, exit the cloud top, and turn downward to ground, striking far from the storm core.

In addition to being a particularly dangerous and surprisingly common form of lightning, BFBs reveal a deficit of positive charge in the upper storm that sometimes allows in-cloud discharges to continue to travel out out from the storm and upward toward the ionosphere, as a gigantic jet.

These gigantic jets are thus the upward analog of normal, downward cloud-to-ground lightning, according to the scientists.

Last summer, while working at New Mexico Tech’s Langmuir Laboratory, a mountaintop research facility west of Socorro, doctoral candidate Edens obtained a spectacular photograph of a BFB that started to develop upward above the cloud top before being coerced to turn downward and eventually to ground.

The photo was quickly included as part of the paper being drafted at the time.

Krehbiel and colleagues suggest that the smaller blue jets result from electrical breakdown at the cloud top; whereas the energetic gigantic jets (extending upwards into the atmosphere for tens of kilometers) and bolts-from-the-blue (curving back down to Earth), originate from the dominant negative charge at mid-level in the cloud.

Krehbiel has retired from the classroom, but continues the research he started at Langmuir Laboratory, which sits atop 10,400-foot-high South Baldy Peak in the Magdalena Mountains, back in 1966 as a freshly minted research engineer at Tech.

He recently was named a Fellow of the American Meteorological Society.

“Lightning studies at Tech began in the late 1940s when E. J. Workman moved from UNM to the then-New Mexico School of Mines to become President”, Krehbiel said.

“The studies grew substantially under the inspired and dedicated efforts of physics researchers such as Steve Reynolds, Marx Brook, Charles Holmes and Charles Moore,” he said.

“We have been able to take advantage of the major technological developments since those early years to continue advancing our understanding of lightning,” Krehbiel concluded.

Krehbiel can be contacted via the Langmuir Laboratory Office (575 835-5423) or by email at krehbiel@ibis.nmt.edu.

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