Mount Etna eruption in winter 2015

By Ivan Smiljanic (DHMZ ), Martin Setvak (CHMI ), Fred Prata (NILU ) and Jochen Kerkmann (EUMETSAT)

Following an earlier 'pre-eruption' emission, at around 02:00 UTC Mount Etna, an active stratovolcano on the east coast of Sicily, Italy, erupted. It was a short, but spectacular paroxysm, which could be clearly seen in satellite imagery.

In particular, the eruption and the resulting plume can be tracked using Meteosat's Airmass and Dust RGB imagery.

The Dust RGB loop starts at 02:00 UTC. In this RGB combination the SO₂ plume, that the volcano has pumped all the way to the tropopause, appears in green shades. The origin of this colour can be easily depicted by the 'RGB Colour Interpretation Tool ', developed by the EUMeTrain project (Figure 1). The 'X' sign on this figure shows the sampling position of this tool.

The main contribution here comes naturally from the green beam which reveals relatively big differences between signals from SEVIRI channels IR10.8 and IR8.7, while the the other two contributions are rather small.

The Airmass RGB loop also starts at the 02:00 UTC and in the beginning of the eruption gives the strong white signal over the wider volcano area.

Normally with this RGB combination, high-reaching thick clouds appear in white shades, but only very high clouds give such a bright signal. In later moments, when the clouds gets thinner and made of only SO₂, the colour changes to specific 'peach'

The initial phase of the main eruption was not captured by polar orbiting satellite instruments due to a mismatch between the timing of the satellite passes and the onset of the eruption.

However, there was a Suomi-NPP overpass at 1:21 UTC that captured the precursor-initial (low level) plume, see the yellow-green plume on the VIIRS Ash RGB image (Figure 3).

This low level, initial plume can also be seen on the Meteoesat-9 rapid scan loops of the 24-hour Microphysics RGB , 3 December 00:00–12:50 UTC and the zoomed-in 24-hour Microphysics loop , 3 December 00:00–13:20 UTC.

The SEVIRI data suggests that in the early phase of the eruption the ice cloud masked the SO₂ signature, either because the radiative effects of the ice particles prohibited to see the SO₂ or because much of the SO₂ gas was absorbed onto the ice particles, only to sublimate at a later stage as the ice evaporated.

The change of the character of the volcanic cloud, from ice cloud (masking the SO₂) in the initial phase to a more pure SO₂ cloud at a later stage (from 04:00 UTC onwards), can be seen by the change in the colour of the cloud.

In the Dust RGB imagery (Figure 1 animation), initially the volcanic cloud appears with a red colour (indicating a thick ice cloud) with dark blue borders (indicating a thin ice cloud). Later, at about 06:00 UTC on 3 December, the volcanic cloud appears like a mixed cloud: thin ice (black) and SO₂ (green) mixed. After about 09:00 UTC the entire cloud becomes green (SO₂), and the cloud is split into a northern branch (high level, around 10 km height) and a southern branch, travelling at a lower level (about 6.5 km).

Later analysis revealed there were least two clouds at different levels, drifting into two slightly different directions. By 4 December the leading edge of the higher plume was over Iraq.