Sulphur dioxide (SO2) plumes from the Kasatochi eruption circled the whole northern hemisphere over more than 20 days.
16 March 2021
14 August 2008
by HansPeter Roesli (EUMETSAT)
Kasatochi is a small island volcano located in the Aleutian Island chain just east of 180° longitude (see map ). According to the Alaska Volcano Observatory of the USGS, on 7 August 2008: "Satellite data show an ash plume to an altitude of at least 35,000 feet in the vicinity of Kasatochi Volcano 22:30 UTC (14:30 ADT). The plume is drifting to the south-southwest."
Courtesy of the same website an animated sequence of GOES-12 images (Credit: David Schneider) depicts the expanding ash cloud resulting from the eruption. More information on the Kasatochi volcano may be found at the Smithsonian's Global Volcanism Program.
The SO2 plumes trajectories have also been observed in the so-called Ash RGB composites of Meteosat-9, and with the GOME-2 and IASI instruments on Metop-A.
Both the amount of backscattered UV radiation from the sun (GOME-2) and the emitted IR radiation (SEVIRI, IASI) are sensitive to the presence of SO2. The hemispheric SO2 circulation is well depicted by an animated sequence of the GOME-2 SO2 product (daily composites, Credit: Univ. of Bremen). Limited to Meteosat-9's field-of-view, the Meteosat-9 Ash RGB sequence offers a smoother view of the trajectories of some of the light-green coloured SO2 streaks.
The situation on 21 August offers a good opportunity to compare the RGB with the SO2 product from GOME-2 and a simple SO2 scheme from IASI. A narrow SO2 streak over the eastern Atlantic moves slowly south-southeastwards crossing the British Isles and the Gulf of Biscay (see hourly Meteosat-9 Ash RGB sequence). A shorter RGB sequence (21 August, 08:00–14:00 UTC) overlaid with the GOME-2 product of the morning orbit matches reasonably well when taking into account the different observation techniques used and the considerable mismatch in spatial resolution.
IASI data are only available for the evening orbit. A simple qualitative scheme, namely the temperature difference on/off an SO2 absorption line (1284 cm-1 minus 1344 cm-1) and free of other trace gas absorption, is used to show the presence of SO2. There is an excellent match of the signals (left frame) between the light-green RGB streak (middle frame) and the dark IASI streak (right frame).
Compared to the Chaitén eruption in May 2008 (see link under See also), where the volcanic SO2 clouds followed the flow of smooth planetary waves around the southern hemisphere, Kasatochi's SO2 streaks trace a more wave-like upper tropospheric flow over the northern hemisphere — as one would expect.
Met-8 24-h Cloud Microphysics RGB animation (5-minute scan interval) (21 Aug 12:00 UTC–22 Aug 06 UTC)
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