Volcano Taal, on Luzon Island in the Philippines, erupted for some hours on 12 and 13 January 2020.
By HansPeter Roesli (Switzerland)
Taal is a complex volcanic system, peaking up to a couple of hundred metres and flooded by a large lake. The eruptions produced a massive high-rising ash plume that rapidly spread northwards over Manila, and beyond.
In the animated sequence of 10-minute Ash and SO2 RGB images from Himawari-8, 12 January 07:00 UTC–13 January 15:00 UTC, (Figure 2) the plume did not show the typical colours of ash (red), SO2 (green) or a mixture of them (yellow). Instead it showed an ash-contaminated water cloud that frayed into thin cirrus on the fringes (see pointers in Figure 1, top right, click to expand), a sign that the plume evolved in very humid air (lake water steam) with the volcanic material acting as condensation nuclei.
Figure 2: Himawari-8 Volcanic Ash RGB animation, 12 Jan 07:00 UTC–13 Jan 15:00 UTC
In fact, the patterns shown in the visible high-resolution band VIS0.64 (AHI band 3) at 08:00 UTC (Figure 3, left) are the white shades typical for water and ice cloud. This panel also identifies two stages of the eruption, one that spread at lower levels, and a second which fanned out, probably under the tropopause. The second plume cast shadows on the top of the lower plume.
Another interesting feature in this image is the towering cloud (a kind of above-anvil cloud) that had emerged at the position of Taal. The evening sun cast a long shadow on the plume below. Although in lower spatial resolution, the cloud tower also stood out in the IR window band IR10.4 (AHI band 13) (Figure 3, right) as a very cold spot (yellow, ~192 K) surrounded by a relatively warm ring (dark red, ~212 K). This cloud formation is similar to a cumulus flammagenitus (pyrocumulus), but of volcanic origin.
In the winter evening the sun brought out a beautiful display of gravity waves rippling the top of the expanding plume, only slightly marred by thin cirrus streaks advected from the south east. Figure 4 shows the area at sunset.
A sequence of highly enhanced images from the VIS0.64 band, paired with the IR10.4 band, between 08:00 UTC and 09:30 UTC on 12 January (Figure 5), shows the evolution of the wave pattern. Although at a lower spatial resolution, there was sufficient temperature contrast in the IR10.4 band to also reveal the wavy structure.
Figure 5: Himawari-8 VIS0.64 (left) and IR10.4 (right) animation, 12 Jan 06:50–09:30 UTC
While in the Ash and SO2 RGB animation (Figure 2) there were very weak hints of green SO2 signals during the initial phase of the massive ash outbreak, by 18:00 UTC on 12 January the ash release had started to weaken and a green SO2 signal emerged from the ash-water clouds moving on a more easterly trajectory than the thick plume. Figure 6 zooms in on the situation on 13 January at 00:30 UTC.
On the VIS0.64 band (left) a thin plume is visible, fanning out west-southwest from Taal. The IR bands (AHI bands 11, 13, and 15) used in the Ash and SO2 RGB (right) detected no activity at the volcano, but revealed the presence of SO2 as a strong patch and weaker veils of green east-northeast of Taal’s location.
Sentinel-3’s SLSTR captured the eruption during the night of 12/13 January, and in daylight the following day (13 January).
As already noted with the Himawari-8 imagery, the massive plume persisted during the night. The IR10.8 band (S8) showed the temperature structure of its top (Figure 7, left). There were still indications of gravity waves expressed as temperature variations smaller than 5 K across the plume.
The split-window difference IR10.8–IR12.0 (Figure 7, right) brought out very small negative temperature differences (<0.5 K, blue shades), indicative of the presence of ash close to the volcano (green dot).
Comparing the daylight scene on 13 January with a scene prior to the eruption (on 9 January) using Natural Colour RGBs (Figure 8), reveals the presence of delicate ash veils in the area after the eruption, which blur the scene somewhat, in particular just north of Taal (red dot). Also, a tiny plume extended to the south west.
On local day 22 January an ISS astronaut took picture ISS061-E with a digital camera. The photograph shows light-brown ash clouds downwind of Taal's crater, which were described by NASA EO as re-suspended ash, deposited during the eruption. Himawari-8 imagery (VIS0.64 band and True Colour RGBs) confirmed the presence of suspended ash as a long streak reflecting in the morning sun (Figure 9).
Figure 9: Himawari-8 VIS0.64 (left) and True Colour (right) animation, 21 Jan 23:30– 22 Jan 01:00 UTC