Lightningfollows the Sun

Lightning follows the Sun

Space imaging team discovers unexpected preferences

One of a series of stories covering the quadrennial International Conference on Atmospheric Electricity, June 7-11, 1999, in Guntersville, Ala.

June 10, 1999: Lightning not only likes land, it follows the sun, and may even change its schedule to follow the El Niño/Southern Oscillation phenomenon.

"We've been watching the global distribution and established a picture of how it changes as a function of time of day, season, and even from year to year," said Dr. Hugh Christian of the Global Hydrology and Climate Center in Huntsville, Alabama. Christian is the principal investigator for the Lightning Imaging Sensor (LIS) on the Tropical Rainfall Measuring Mission, and its predecessor, the Optical Transient Detector (OTD) on Microlab 1.

Above: Lightning likes land: Data from the Lightning Imaging Sensor shows that most lightning strikes occur over land where the ground can warm the air more effectively. This map covers the latitudes 35 deg. N to 35 deg S overflown by the Tropical Rainfall Measuring Mission carrying the LIS. Links to 937x224-pixel, 37KB GIF. (NASA/GHCC)

They provide a view of lightning from above the cloud tops, thus revealing the large percentage of cloud-to-cloud and intracloud flashes that cannot be seen from the ground. LIS and OTD both register the time of a lightning flash and its location within the instrument's field of view. This is then overlaid with weather pictures and other data so scientists can track the locations and frequency of lightning and how it corresponds with other weather phenomena.

Since their launches - OTD four years ago and LIS 18 months ago - Christian and his team have generated several maps showing global lightning patterns.

Left: At the Global Hydrology and Climate Center in Huntsville, Ala., Hugh Christian (foreground), Steve Goodman and Richard Blakeslee (background) monitor data from the Lightning Imaging Sensor aboard the Tropical Rainfall Measuring Mission. (NASA)

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"Can we use lighting to monitor global change?" Christian said the team asked itself. "It does look like lightning is sensitive to changing weather patterns that evolve from year to year."

The first of these patterns to emerge was the discovery that lightning is more common in storms over land than over oceans. "It's probably a consequence of enhanced convection from increased warming over land." The LIS team announced its initial findings in 1998. Today, Christian will present expanded findings that buttress their claim.

The team has also found that lightning prefers afternoons.

"Over land, we see tremendous diurnal changes, a strong peak in lightning in the afternoon over land," Christian continued. "Over water we see very little variation. We believe it's due to the land absorbing heat and causing strong convection. On the other hand, water can store a lot more heat, and releases it slowly."

Lightning patterns also vary from one season to the next.

"We see tremendous variations in extratropical regions," meaning areas north or south of the tropics of Cancer and Capricorn. "You see lightning activity truly following the sun. As summer in the northern hemisphere progresses, you see lightning moving farther north," Christian continued. "You see a similar pattern in the southern hemisphere, but not so pronounced because there isn't as much land outside the tropics."

Web Links

Human Voltage (June 18, 1999) Scientists discuss biology, safety, and statistics of lightning strikes.
News shorts from Atmospheric Electricity Conference (June 16, 1999) Poster papers on hurricanes and tornadoes summarized.
Soaking in atmospheric electricity (June 15, 1999) 'Fair weather' measurements important to understanding thunderstorms.
Lightning position in storm may circle strongest updrafts (June 11, 1999) New finding could help in predicting hail, tornadoes
Lightning follows the Sun (June 10, 1999) Space imaging team discovers unexpected preferences
Spirits of another sort (June 10, 1999) Thunderstorms generate elusive and mysterious sprites.
Getting a solid view of lightning (June 9, 1999): New Mexico team develops system to depict lightning in three dimensions.
Learning how to diagnose bad flying weather (June 8, 1999): Scientists discuss what they know about lightning's effects on spacecraft and aircraft.
Three bolts from the blue (June 8, 1999): Fundamental questions about atmospheric electricity posed at conference this week.
Lightning Leaders Converge in Alabama (May 24, 1999): Preview of the 11th International Conference on Atmospheric Electricity.
What Comes Out of the Top of a Thunderstorm? (May 26, 1999): Gamma-rays (sometimes).
Lightning research at NASA/Marshall and the Global Hydrology and Climate Center.

Variations show up even from one year to the next. Christian said that data still being analyzed show hints that lightning patterns are influenced by El Niño and La Niña, a complementary variation in sea surface temperatures in the Pacific Ocean west of Peru. While this might be expected since El Niño can cause droughts and monsoon-like conditions, Christian said it also shows that global lightning patterns may be one way to take the pulse of the planet's weather trends.

It most certainly can be used on a small-scale, short-term basis to monitor the progress of storms.

"We can use lightning to monitor and study storms, including severe thunderstorms," Christian explained, since the lightning can only be generated by convection within a cloud system. "It's tightly coupled with the dynamics and physics of the storm. We use it to monitor its evolution and life."

Right: Lightning. (NOAA)

As successful as OTD and LIS have been - and are expected to be over the next 1 year and 6 years (respectively) that they are expected to continue operating - they can only be used in research. Their view is limited to a small area directly under their satellites, so global or even regional monitoring is impossible.

To fill that role, Christian and his team are studying designs for a Lightning Mapping Sensor that would be placed aboard geostationary weather satellites. From 35,680 km (22,300 mi) up, the sensor could track severe activity and enhance meteorologists' warning capabilities.