Comparison with other RGB types
Ash RGBs are created from the same channels (channel differences) as Dust RBGs. Only the enhancement is different. For Dust RGB the main aim is to separate dust clouds from other features. For Ash RGB the main aim is to separate ash and SO2 plumes from each other and from water clouds, ice clouds and cloud-free surface
Fig. 9 shows Dust and Ash RGB images of a volcanic ash plume southwest of Iceland. The colours are similar, but in the Ash RGB the water and ice clouds appear in paler tones, so the ash plume has a better colour contrast against the water and ice clouds.
| Figure 9: Volcanic eruption on Iceland. Dust RGB (left) and Ash RGB (right) for 23 May 2011 at 12:10 UTC |
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Usually the Ash RGB is more appropriate for monitoring the movement of ash and SO2 plumes. However, in some cases the Dust RGB is better. For example, at high satellite viewing angles SO2 plumes are easier to detect in Dust RGBs. In Fig. 10 the Dust RGB provides better colour contrast between a SO2 plume and the lower level clouds. On the far edges of MSG images, one should use the Dust RGB rather than the Ash RGB for ash and SO2 detection. However, for all other areas, the Ash RGB gives better contrast for ash/SO2 detection.
| Figure 10: Volcanic eruption on Iceland, SO2 gas plume. Dust RGB (left), Ash RGB (right) for 22 May 2011 at 06:55 UTC |
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Note that higher level SO2 plumes are detected by Airmass RGB as well (in reddish colours). SO2 has an absorption band around 7.3 micrometers (see Fig. 2), which channel is used in the Airmass RGB. In certain situations (e.g. over desert surfaces) the Airmass RGB is even better for SO2 detection than the Ash RGB. Fig. 11 shows a SO2 gas plume from the Nabro Volcano eruption both in Airmass and Ash RGBs (day and nighttime). Its colour contrast against the desert surface is low in Ash RGB and much better in Airmass RGB. (The SO2 gas plume is reddish in Airmass RGB and greenish in Ash RGB.)
| Figure 11: Volcanic eruption on Iceland, SO2 gas plume. Dust RGB (left), Ash RGB (right) for 22 May 2011 at 06:55 UTC |
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