When the Eyjafjallajökull volcano in Iceland erupted in 2010, satellite imagery proved to be instrumental in helping track the movement of the large ash plume.
Twelve months on and another volcano in Iceland erupted — Grímsvötn. Once again, satellite imagery played a major role in helping scientists understand where the ash cloud was and where it was likely to go.
Download full resolution RGB image of Grímsvötn eruption, 21 May, 11.49 UTC
by Kenneth Holmlund, Marianne Koenig and Sancha Lancaster (EUMETSAT)
During a volcanic eruption, large amounts of ash and trace gases, especially sulphur dioxide (SO2), are ejected into the atmosphere. Because eruptions can happen at any time, having a global, constant monitoring system available is vital.
Knowing where a plume is helps forecasters at the Volcanic Ash Advisory Centres (VAACs), who issue the volcanic ash advisories. The warnings issued by the VAACs are vital for air travel.
While the impacts of volcanic emissions on aircraft are still not fully resolved and currently under investigation, there have been incidence when planes’ engines have stopped working after travelling through volcanic clouds.
Research following the Eyjafjallajökull eruption shows that VAACs rely heavily on access to real-time satellite imagery and products to identify and locate volcanic clouds. The primary types of data used are images — visible and infrared. Animating the images also proves to be very useful.
Interpreting these data requires a high degree of meteorological skill and training. Experts are able to use context and experience to quickly identify and interpret volcanic features within satellite images.
Locations of volcanic features within images are compared with the output of atmospheric dispersion models and an estimate of the extent and location of the volcanic hazard is made.
Our Meteosat Second Generation (MSG) geostationary satellites are the only ones in orbit that can be used to derive near real-time imagery (every 15 minutes) of the movement of volcanic clouds over Europe.
The SEVIRI imaging radiometer on Meteosat-9 has 12 channels (at visible and infrared wavelengths), experts are able to construct red-green-blue (RGB) images which have proved very useful for identifying volcanic clouds. The SEVIRI data allow the retrieval of quantitative ash properties like the total column loading of ash.
In addition to SEVIRI on MSG, volcanic ash can also be detected by the hyperspectral Infrared Atmospheric Sounding Interferometer (IASI) instrument on Metop. The concentration of particles, which is important data for air traffic control, can be determined by combining satellites products with other available information or models.
EUMETSAT produces a Volcanic Ash Detection product for VAACs and National Meteorological Services. The input data used for the product includes the reflectances in the VIS0.6 and the IR3.9 channels; the brightness temperatures in the IR3.9, IR8.7, IR10.8 and IR12.0 channels, and the total column loading of ash.
Monitoring sulphur dioxide
Volcanoes are also a major source of sulphur dioxide (SO2), which may or may not always travel together with the ash. SO2 detection is useful because, often, SO2 emissions precede eruptions.
The GOME-2 (Global Ozone Monitoring Experiment-2) scanning spectrometer on Metop can monitor SO2 using the specific absorption features of this gas in the ultraviolet spectral region.
Also measurements by IASI on Metop-A are sensitive to SO2. The concentration is derived from the variations observed in the measured infrared radiation, derived as brightness temperature levels.
Combining all the data from both Metop and MSG satellites gives experts a fuller picture of exactly what is happening in atmosphere following a volcanic eruption.
Monitoring in the future
Although satellite images can’t be used in isolation for monitoring volcanic emissions, the last two Icelandic eruptions have shown how useful the data can be.
In May and June 2012 Metop-B and MSG-3 were launched, ensuring continuity of the monitoring ash plumes from space. But the biggest step will be when the Meteosat Third Generation (MTG) satellites are launched. In addition to improved imagery at 10-minute repeat cycles, the provision of data from the MTG infrared and ultraviolet/visible sounding missions will be crucial for volcanic ash modelling.