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Not Just Flash

Satellite data reveal how lightning influences climate
lightning bolt
Image: CHUCK CLARK
LIGHTNING BOLT is an indicator of heat being released into the atmosphere. By monitoring lightning, scientists can obtain more accurate estimates of the global "heat budget."
Lightning is beautiful, sometimes frightening and often destructive. But scientists are finding that those dramatic electrical discharges are far more than a sideshow in stormy skies. At the fall meeting of the American Geophysical Union last December, investigators reported that lightning is more common and widespread than previously believed and that it plays an important role in global climate.

The unlikely source of much of the recent information comes from data sent back to earth by a small satellite designed to detect clandestine nuclear weapons tests. Called FORTE, for Fast On-orbit Recording of Transient Events, the small satellite was developed jointly by Los Alamos and Sandia National Laboratories and was launched in August 1997. Its 30-foot-long antenna and sophisticated radio receiver samples a wide range of radio frequencies at a very high rate.

FORTE can detect the burst of electromagnetic energy from a nuclear blast but its scientific forte has proven to be detecting lightning: the spacecraft's sensitive instruments also pick up the brief radio impulses, lasting less than a thousandth of a second, from lightning flashes. FORTE also has two sensors that detect and measure the optical emissions. The satellite makes several passes each day over the lightning prone equatorial tropics.

FORTE

FORTE, a small satellite intended to detect clandestine nuclear detonations, has recorded more than 2 million lightning strikes since its 1997 launch.

So far, FORTE has recorded more than two million lightning strikes around the globe. The measurements are sufficiently detailed that the researchers can pick out signals from the separate processes that culminate in an atmospheric discharge. The satellite data is being combined with that collected by the National Lightning Detection Network and from a network of sensors that measure low-frequency electromagnetic emissions, or "sferics," and changes in the direction of the electric field. The combined data may help meteorologists to identify radio signatures of storm systems that spawn tornadoes or rain down damaging hail.

One of FORTE's most significant findings is that storms at sea generate more lightning than previously realized. "There's a saying that 'lightning loves land,' and that's true for discharges from clouds to the surface," says 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."

These lightning discharges are an indication of the way heat is being transported around the earth by the atmosphere. A portion of the sun's energy reaching Earth is captured and redistributed globally through latent heat carried in water vapor. The heat is released when the water vapor condenses and falls as rain. Storms with lightning tend to be more intense and release more heat. Tracking such systems may indicate where large amounts of heat are being released.

The flash of light from lightning within clouds is hard to detect and most monitoring stations are located on land. But the bursts of energy from lightning within clouds can be easily detected by FORTE. Argo used FORTE's observations and weather-satellite maps to pinpoint the location of storm systems; he confirmed about 200 storm systems that produced a lot of inter-cloud lightning and were located over the ocean as well as land masses.

One indicator of inter-cloud lightning was the distinctive radio signal of TIPPs--Trans-Ionospheric Pulse Pairs. These curious double bursts of energy a few tens of millionths of a second apart were discovered in 1993 during an experiment known as "Blackbeard" aboard an earlier and less sensitive anti-nuclear proliferation satellite called ALEXIS. FORTE data confirmed that TIPPS are generated by extremely intense bursts of lightning between clouds--the second pulse is produced by a reflection of the first pulse off Earth's surface.

Argo

cloud lightning
Image: GHCC
CLOUD BURSTS. Lightning often occurs within clouds, especially over the oceans. Until FORTE, the prevalence of inter-cloud discharges was underestimated.
suggests that researchers studying the global energy budget--which attempts to account for how the energy injected into the atmosphere through solar heating is redistributed--may need to reconsider the amount of energy released by storms over the ocean when predicting weather patterns and long-term climate effects. "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.

Other findings presented at the AGU meeting indicate that lightning also shapes atmospheric chemistry. In particular, it creates ozone that is transported over wide regions in the upper atmosphere. Louisa Emmons, a visiting scientist at the National Center for Atmospheric Research in Boulder, Colo., and her colleagues connected a region of elevated ozone levels in the eastern Indian Ocean with lightning produced in Africa.

The team used an atmospheric chemistry computer model called MOZART to analyze four years of ozone data collected between Japan and Antarctica. The ozone measurements, taken between 2 and 6 miles in altitude (3-10 kilometers) over a large part of the eastern Indian Ocean, were as high as 80 parts per billion-levels similar to a polluted day in a U.S. city and several times more than normal.

Lightning is known to produce nitrogen oxides (NOx) that may react with other gases in the presence of sunlight to produce ozone. Most previous studies have focused on the production of NOx in the immediate vicinity of storms. However, the ozone produced has a long lifetime in the upper troposphere and thus could be carried over long distances.

Ozone map

OZONE PLUME
The MOZART results indicate that the ozone did not descend from the stratosphere, where it is created by sunlight, thereby blocking out harmful ultraviolet radiation. Another possible source was the burning of forests and grasses upwind in Africa. However, when biomass burning was removed from the model calculations, ozone levels remained high.

When the researchers discounted the data about lightning flashes over Africa, the predicted ozone levels dropped significantly. The group concluded that ozone from storms across southern Africa is being transported by the subtropical jet stream to Australia. "Although there are uncertainties in the model results," says Emmons, "they indicate that lightning has a far-reaching and significant impact on tropospheric chemistry."

An obvious conclusion that can also be drawn from the spate of presentations on lightning at the AGU meeting is that a defense satellite has made a significant contribution to understanding global climate. But after faithfully sending back data on millions of lightning bolts, how many weapons tests has FORTE detected? No comment.

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