Comparing three different RGBs after the eruption of Shiveluch
There are different RGBs available to monitor volcanic signatures within a cloud, and three common ones are shown above. The Dust RGB and the Ash RGB use identical channels/channel differences that are scaled differently. All three RGBs (here is the The SO2 RGB Quick Guide) include Band 11 information; Band 11 detects radiation in the part of the electromagnetic spectrum that is sensitive to absorption by SO2. For the Sheviluch eruption (described in blog posts here and here) that occurred just at the beginning of this animation above, the SO2 signal — bright yellow in the SO2 RGB — persists the longest. That, of course, will not be the case with every eruption; that’s why one must use more than one product to monitor an eruption.
Note that “Keep Out Zones” are apparent in the imagery above as regions of no data around 1440 UTC, when the Himawari-9 imager is turned off when it is pointing a little too closely towards the Sun.
Himawari-9 imagery in this blog post are courtesy of JMA. The Full-Disk HSD data were processed into RGB images using geo2grid.
By 14 April, much of the signal has shifted eastward out of Himawari-9’s field of view. The animation below, from GOES-18, shows the three RGBs from 0000 UTC on 14 April through 0000 UTC on 20 April. The signal of enhanced SO2 in particular has remarkable staying power.