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.
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.
"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."
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 Nina, 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."
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.