Lightning Over Oceans More Plentiful Than Expected

Energy from the sun can be captured and redistributed globally through the latent heat carried in water vapor. Storm clouds with lightning tend to be more intense and provide a relatively greater release of latent heat through rainfall -- condensation of the water vapor. Remote tracking of such systems may tell researchers where substantial amounts of latent heat are being released over the ocean and thus better understand energy distribution through the atmosphere.

The new observations, reported today at the annual fall meeting of the American Geophysical Union, were gleaned from measurements of radio emissions from more than 200 storms observed by the FORTE satellite.

FORTE -- for Fast On-orbit Recording of Transient Events -- is a joint project of the Department of Energy's Los Alamos and Sandia national laboratories. The small satellite was launched August 1997 and carries radio and optical detection systems.

"There's a saying that 'lightning loves land,' and that's true for discharges from clouds to the surface," said Los Alamos physicist Paul Argo, who presented the results at the AGU meeting. "FORTE has shown that storms over the oceans produce a lot of lightning within the clouds themselves."

The optical flash from lightning within clouds is hard to detect and thus easily overlooked; furthermore, continuous lightning monitoring stations are located on the continents and not at sea. Lightning's burst of radio energy, however, penetrates the cloud and can be detected by FORTE.

FORTE's circular, low-Earth orbit is inclined 70 degrees to the equator, so it makes several passes per day over lightning-prone tropical regions.

Argo used FORTE's radio-based observations and weather-satellite maps to pinpoint the location of storm systems, and in doing so confirmed about 200 storm systems that produced a lot of intracloud lightning and were located over the ocean as well as land masses.

The appearance of TIPPs -- Trans-Ionospheric Pulse Pairs -- clearly signified the intracloud discharges. TIPPs consist of two sharp bursts of energy a few tens of millionths of a second apart. The second pulse in a TIPP is a reflection off Earth's surface, which means the discharge must have occurred at sufficient height in the atmosphere to have originated within the clouds.

A radio experiment called "Blackbeard" aboard Los Alamos' ALEXIS satellite produced the first scientific reports of TIPPs in 1994, but it took FORTE's more sophisticated radio equipment to demonstrate conclusively that TIPPs arise from energetic lightning discharges. Blackbeard and FORTE were both developed to demonstrate advanced technologies for detecting clandestine nuclear weapon detonations, but have returned copious data on natural phenomena as a bonus.

"Observations based on optical emissions indicate that lightning occurs preferentially over land masses, but we've found that is only part of the picture," Argo said.

Lightning is a key tracer for where energy resides in the atmosphere, and thus is useful for studies of the "global energy budget." The global energy budget tries to account for how the energy injected into the atmosphere through solar heating is redistributed through storm systems and other atmospheric dynamics. This, in turn, is important for understanding weather patterns and long-term climate effects.

The new FORTE findings suggest researchers studying the global energy budget may need to reconsider the amount of energy released by storms over the ocean.

Los Alamos National Laboratory is operated by the University of California for the U.S. Department of Energy.