Catching a bolt from beyond the blue
The Meteosat satellites in geostationary orbit are important for nowcasting – where forecasters predict what will happen over the next few hours. The satellites help meteorologists to spot strong and severe thunderstorms, which can disrupt airline traffic, and can bring hazardous conditions on the ground such as strong winds, large hail, and lightning.
The next generation of Meteosat satellites will deliver extra information about these storms, as they will carry a new instrument to image lightning. Records of lightning strikes on the ground are collected by a variety of organisations and can be seen online in almost real time, as each lightning discharge in the atmosphere is picked up. Photographs and videos of lightning from the International Space Station, around 400km up, are dramatic and beautiful.
This video from NOAA shows the average global lightning strike rate using data from NASA’s Optical Transient Detector, from 1995-2000 and NASA’s Lightning Imaging Sensor from 1998 – 2005. The average lightning strikes are much higher over the tropics than at higher latitudes. Very little lightning is recorded at sea compared with strikes over land.
Both of these lightning detectors are on satellites in low Earth orbit, so they can only see swathes of the planet’s surface as they pass over. For uninterrupted data for weather forecasting, it’s the constant view from geostationary orbit, around 36,000 km high that is needed.
“Being in geostationary orbit is good because you can see a large part of the Earth all the time,” says Marianne Koenig, the Atmospheric and Imagery Applications Manager at EUMETSAT, “But it makes the detection of lightning a lot more difficult, because you are so much farther away.”
“The Lightning Imager will detect the bright bursts lightning makes. Like many things, the concept is fairly simple, but the devil is in the detail,” she says.
This detail includes making sure that the lightning imager detects lightning and only lighting. The Meteosat Third Generation team is working to make sure the imager will not be fooled by other bright signals from Earth.
“Clouds are quite bright during the daylight hours,” says Stephan Kox, who is developing the computer software to simulate lightning, the instrument, and the filtering, “So we need to be able to distinguish the bright flash of lightning from the bright clouds around.
“Spacecraft also move a little bit, what we call ‘jitter’, so we have to be certain that an increase in brightness is not just an artefact of that movement.”
Thunderstorms usually have a mixture of cloud-to-ground lightning, which can be seen as bolts connecting with the ground, and intracloud – or cloud-to-cloud lightning. The Lightning Imager will be able to detect intracloud lightning that might otherwise go unnoticed.
“Intracloud lightning is often not observable,” says Koenig, “It might not even be visible to a person close to the storm, but an increase in this kind of lightning can be linked with severe conditions including tornadoes.”
Knowing when there is an increase in intracloud lightning will help forecasters to identify very severe thunderstorms and issue precise warnings to people who may be in the path of the storm. This is particularly useful for places where there is no radar coverage or other ground observations.
The data that the Lightning Imager will gather is likely to have lots of other uses for scientists studying weather, since it will provide a record of long-term patterns. The imager will also help to show whether other satellite products designed to detect severe weather are working properly.
Although big storms tend to come and go quickly, the role of lightning in long-term patterns is also being studied. Lightning helps to produce nitrogen oxides (NOx) in the atmosphere, which can cause acid rain and add to the production of the greenhouse gas, ozone.
The Lightning Imager will give atmospheric scientists more information about these processes, and when combined with the equivalent instruments on American NOAA and NASA satellites, will contribute to coverage that stretches around most of the world.
The first Meteosat Third Generation satellite will launch around 2018.