Mount Etna's effusive eruption
18 December 2022 00:00-23:45 UTC
For a number of days Mount Etna has been actively releasing lava through a fissure vent that is surrounded by a snow at around 2800m high.
03 October 2023
20 December 2022
By Ivan Smiljanic
The thermal signal from the lava region is visible through the SEVIRI IR3.9 channel (Figure 1).
Under such a setup, a lava flow is melting (i.e. boiling) the surrounding snow, producing the elongated cloud of steam that stretches towards the east (with the predominant NE flow of the jet stream at tropopause level — see nearby radiosounding measurement at 12 UTC, Trapani, Italy).
The emanating plume is only seen at the higher spatial resolution and shorter wavelengths of OLCI True Colour RGB (Figure 2), also indicating a presence of smaller smoke particles that are vaguely seen at longer near-infrared wavelengths (see a comparison with Cloud Phase RGB in Figure 3).
The presence of smaller aerosol particles is evident through comparison between the RGB product that utilises shorter, visible wavelengths - the True Colour RGB) and the product that mainly uses channels from the longer near-IR spectral region - the Cloud Phase RGB. Due to the smaller size of the aerosol particles, this elongated (blue) plume is vaguely seen in the latter (Figure 3).
Image comparison
Figure 3: Comparison between Sentinel-2 True Colour RGB and Cloud Phase RGB, 18 December
It is interesting to observe the thermal signal in the near-infrared region. This is possible through the two near-infrared channels of the Cloud Phase RGB (namely, NIR1.6 and NIR2.25 for red and green RGB component, blue is the VIS0.6 channel) which are sensitive to high temperature scenes. Usually the Cloud Phase RGB shows green shades for hot objects like lava (Figure 5 in this case study), but this time it is more cyan to white, due to the strong reflectivity of snow in the blue component, and a small contribution from the red component (white pixels).
Snow has a strong blue component in the Cloud Phase RGB because very low reflectivity in the red and green component (big ice/snow particles reflect very little). Blue shades can also indicate high/icy clouds, or a smoke plume, as we have seen — ash cloud, with bigger aerosol particles, would show brown to green shades, due to more reflectance in the near-infrared spectral region.
