An elongated signature of altered cloud microphysical structure was captured by different satellites above the Geldingadalir volcanic eruptions in April 2021.
20 April 2021
15 April 2021
By Ivan Smiljanic (CGI) and Lewis Grasso (Colorado State University)
On 19 March, an effusive eruption (steady lava flow) started at approximately 8:45 pm local time in Geldingadalir, on the Reykjanes Peninsula in southwest Iceland. By 14 April lava was still erupting from the area. Although none were explosive eruptions, when magma is violently spewed from a volcano, the affects on the atmosphere could still be seen in satellite products.
The smoke coming out of multiple small craters entered the layer of stratiform clouds aloft, introducing many new condensation nuclei into the clouds. This process changed the microphysical structure of the clouds, so they have a larger number of smaller cloud droplets.
The difference in the particle size of the clouds can be detected, during night hours, through a different emission signature of IR window channels — more specifically, the difference between spectral channels in the regions around 3.9 and 10.5 microns, the green beam of the Night Microphysics RGB (Figure 1).
With the advection of clouds above the eruptive fissures (in this case clouds field moved north eastwards), this microphysical signature became an elongated plume consisting of small cloud particles, easily tracked by satellites, especially when animated (Figures 2 and 3).
Comparing the data from two different satellites/instruments, Meteosat-11/SEVIRI and GOES-16/ABI (Figure 4), over the same region, with same product and the same projection, at the same time, we can observe:
- better contrast for smaller particles (slightly different spectral response functions of ABI instrument in the IR regions around 3.8 and 10.5 micron seem to be better suited for particle size detection);
- similar pixel size (ABI infra-red pixel is nominally smaller, 2x2 km v 3x3 km for SEVIRI, but Meteosat-11 pixels are less 'stretched' since the satellite has less slant view on the region);
- different pixel shape (with very slant GOES view, Iceland is at the very edge of the ABI scanning disc), and
- different scanning times match better (due to a different scanning mechanisms, and revisit times, of ABI and SEVIRI satellite, at these high latitudes, better match is found for ABI scans that are acquired 10 min later than SEVIRI scan).
At certain times, a volcanic crater could be seen, as indicated by a dark purple pixels in the VIIRS Night Microphysics RGB (Figure 1). At this time, the clouds aloft were thinner, or volcanic activity was stronger, and the 3.9-10.5 micron difference became very negative (no green contribution). This is due to higher sensitivity of the 3.9 channel to a very high temperatures (lava ejections).
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