The European Space Agency said its EarthCARE satellite captured vertical profiles of the volcanic plume from Indonesia's Anak Krakatau, which began erupting on 4 September, and that the data was used to support aviation advice. ESA said the observations also offer new insights into how volcanic eruptions affect the climate system.
What EarthCARE observed
ESA said Anak Krakatau, in the Sunda Strait between Sumatra and Java, started erupting on 4 September, "leading to thousands of flights being grounded and hundreds of thousands of passengers stranded".
According to ESA, the eruption fell under the remit of the Darwin Volcanic Ash Advisory Centre, operated by the Australian Bureau of Meteorology, which added data from EarthCARE's atmospheric lidar, ATLID, to its suite of satellite monitoring tools. ESA said a global network of nine Volcanic Ash Advisory Centres provides aviation advice, each covering a specific part of the globe.
ESA published an image combining a 5 September capture from Copernicus Sentinel-3 with data from EarthCARE's multispectral imager (MSI), ATLID and cloud profiling radar (CPR). In the MSI swath, purple areas highlight ash and bright green indicates sulphur dioxide gas, the agency said.
Technical details
ATLID is one of EarthCARE's four instruments, alongside the CPR, MSI and broadband radiometer, which ESA said are used in synergy to measure clouds and aerosols. ESA described ATLID as a laser instrument that sends pulses of ultraviolet light towards Earth and measures the light scattered back, giving vertical profiles of atmospheric aerosols.
With advanced algorithms, ATLID can also be used to estimate aerosol types such as ash, smoke, sulphate, sea salt and desert dust, ESA said. The vertical curtain in the released image shows layers of optically thin and thick sulphate, fine ash, and what is thought to be coarse ash.
A grey shaded region marks where ATLID's beam cannot penetrate the optically thick sulphate layer above and where the CPR lacks sensitivity to fine ash, according to ESA.
What the scientists say
Robin Hogan of the European Centre for Medium-Range Weather Forecasts said that normally particles in a plume 200 km from the source would be too small for radar, but that thanks to the 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 of the University of Reading said that, if confirmed, this would be evidence that large ash particles can stay aloft longer and travel farther than often assumed, offering an opportunity to constrain ash sedimentation rates and long-range transport in dispersion models.
Andy Prata of the Australian Bureau of Meteorology 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." He added that ATLID was crucial in helping forecasters verify the altitude of the upper-level component of the plume.
Shannon Mason of ECMWF said the timely observation allowed the team to grasp the vertical structure of the event in near-real time to inform aviation advice. ESA's Alex Hoffmann said understanding the injection altitude of volcanic material and its optical properties is important for assessing climate impacts, noting that sulphate aerosols from the 1883 Krakatoa eruption persisted for more than a year.
What to do
- ESA said that while the contents of the obscured grey region require further study, the message to aviation authorities is simple: do not fly aircraft there.
- Aviation forecasters can use ATLID vertical profiles to verify the altitude of upper-level plume components, as the Darwin VAAC did, according to ESA and the Australian Bureau of Meteorology.
Key facts and where they come from
- Anak Krakatau began erupting on 4 September, grounding thousands of flights.
started erupting on 4 September, leading to thousands of flights being grounded and hundreds of thousands of passengers stranded
- The Darwin VAAC, run by Australia's Bureau of Meteorology, used EarthCARE ATLID data.
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
- Nine Volcanic Ash Advisory Centres worldwide provide aviation advice.
a global network of nine Volcanic Ash Advisory Centres (VAAC) is responsible for providing aviation advice
- EarthCARE carries four instruments: ATLID, CPR, MSI and BBR.
the cloud profiling radar (CPR), the multispectral imager (MSI) and the broadband radiometer (BBR), ATLID is one of EarthCARE's four instruments
- The CPR detected a feature from the surface to about 6 km, thought to be large ash aggregates.
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
- Forecasters verified a westward-moving plume at FL500, about 15 km altitude.
we were able to verify the forecast guidance of the westward moving plume at FL500, which is at the altitude of around 15 km
- Volcanic ash can stall aircraft engines and sulphur dioxide can contaminate cabin air.
Volcanic ash can damage or even cause aircraft engines to stall, while also reducing pilots' visibility by scratching cockpit windows.