EarthCARE’s view of volcanic plume boosts air safety

Anak Krakatau, which means Child of Krakatoa, has been active sporadically since it emerged from the sea at the beginning of the last century in the caldera formed after the 1883 eruption of Mount Krakatoa.

While Anak Krakatau did not explode with the terrifying power of Krakatoa, which caused a devastating tsunami, today even relatively minor volcanic eruptions can have a major impact on air travel.

Located in the Sunda Strait between Sumatra and Java in Indonesia, Anak Krakatau started erupting on 4 September, leading to thousands of flights being grounded and hundreds of thousands of passengers stranded.

Volcanic ash can damage or even cause aircraft engines to stall, while also reducing pilots’ visibility by scratching cockpit windows. Toxic gases, such as sulphur dioxide, can further contaminate the cabin air supply. To mitigate these risks, a global network of nine Volcanic Ash Advisory Centres (VAAC) is responsible for providing aviation advice, each with the responsibility for a specific part of the globe.

The Anak Krakatau eruption came under the remit of the Darwin VAAC, operated by the Australian Bureau of Meteorology (BOM), who were quick to add EarthCARE’s unique atmospheric lidar (ATLID) data to their suite of satellite monitoring tools.

Along with the cloud profiling radar (CPR), the multispectral imager (MSI) and the broadband radiometer (BBR), ATLID is one of EarthCARE’s four instruments – which are all used in in synergy to measure clouds and aerosols to deepen our understanding of how they regulate Earth’s energy balance.

ATLID is a highly specialised laser instrument. It sends pulses of ultraviolet light towards Earth and measures the light that scatters back to the satellite, providing a unique view of the tops of clouds and vertical profiles of atmospheric aerosols.

Not only does ATLID yield important information about the altitude and concentration of aerosols, but it can also be used with advanced algorithms to estimate types of aerosol – such as ash, smoke, sulphate, sea salt and desert dust.

The image below uses a capture from Copernicus Sentinel-3 on 5 September overlaid with data from EarthCARE’s MSI, ATLID and CPR instruments. The image also notes aircraft flight levels: most aircraft cruise around FL350, which means 35,000 ft (about 10,700 m).

The swath from the MSI instrument shows the volcanic plume in false colour, with purple areas highlighting ash, and bright green indicating sulphur dioxide gas.

Above the MSI swath, the vertical curtain shows a range of different types of aerosols measured by ATLID and by the CPR. These aerosol types are spread in layers of optically thin and thick sulphate, fine ash, and a distinct feature of what is thought to be coarse ash (dark brown).

The area shaded in grey is a region where ATLID’s laser beam cannot penetrate through the optically thick layer of sulphate particles (orange) above, and where CPR lacks sensitivity to fine ash.

Robin Hogan, from the European Centre for Medium-Range Weather Forecasts (ECMWF), explained, “The CPR has a very different sensitivity to atmospheric particles than ATLID: its much longer wavelength means that it is much more sensitive to large particles and much less sensitive to small particles. Normally we would expect the particles in a volcanic plume 200 km from the source to be too small to be detected by a radar.

“However, thanks to the unprecedentedly high sensitivity of the CPR, it detects a feature extending from the surface up to around 6 km (dark brown), which is believed to correspond to larger ash aggregates that are settling out of the plume.”

Helen Dacre, from the University of Reading in the UK, added, “If confirmed, this would provide useful evidence that large ash particles can remain aloft for longer, and travel farther, than is often assumed. The observation, therefore, offers a valuable opportunity to constrain both ash sedimentation rates and long-range transport processes in volcanic ash dispersion models.”

While there remains speculation as to what’s actually in the grey area and this requires further study – the message is simple to the aviation authorities: do not fly aircraft here.

Andy Prata of BOM said, “Together with the VAAC forecasters, we were able to verify the forecast guidance of the westward moving plume at FL500, which is at the altitude of around 15 km. ATLID was crucial to help forecasters verify the altitude of the upper-level component of the plume.”

Shannon Mason also from ECWMF said, “The separate, layered and interacting plumes of ash, clouds, gases and aerosols from volcanic eruptions are complex and evolve rapidly.

“With a timely observation from EarthCARE we were able to grasp the vertical structure of this event in near-real time to inform aviation advice – and scientists will continue using EarthCARE’s measurements to better understand how ash and other volcanic aerosols behave in the hours and days after an eruption.”

ESA’s Alex Hoffmann added, “While immediately challenging for aviation, the impacts of major volcanic eruptions can be much longer lasting. Sulphate aerosols injected into the stratosphere by the 1883 Krakatoa eruption persisted for more than a year, reflecting incoming sunlight and causing temporary global cooling.

“Understanding the injection altitude of volcanic material and its optical properties is particularly important to understand climate impacts – information that EarthCARE’s lidar is well positioned to provide.”

With EarthCARE, scientists are able to track the evolution of plumes such as those of Anak Krakatau and to observe subsequent global aerosol transport, as demonstrated in 2024 when ATLID data contributed to understanding global transport of stratospheric volcanic aerosols from the Mount Ruang eruption, also in Indonesia.

In synergy, EarthCARE’s instrument package provides global insights into the effects of eruptions on Earth’s radiation balance.