2.1 The Volcanic Ash RGB
Figure 11: Meteosat-11 Ash RGB, Mount Etna eruption on 7 March 2021, 06:00 - 14:30 UTC. © EUMETSAT
To better monitor volcanic ash clouds and gases continuously day and night, specific infrared channels are combined to form the Ash RGB. The same channel combination is used for the 24-hour Microphysical and the Dust RGB, but the Ash RGB product is tuned to detect SO2 and ash clouds typical of volcanic eruptions.
All three RGBs use the same infrared channels but differ in the applied temperature thresholds and image enhancements (i.e., gamma correction). Although the Volcanic Ash RGB has been tuned to highlight volcanic ash clouds and SO2 gases, the other two RGB types can also be used to monitor volcanic eruptions, albeit with less overall efficiency.
The Volcanic Ash RGB exploits the fact that thin ash clouds tend to have a positive brightness temperature difference (BTD) between SEVIRI channels IR12.0 and IR10.8, while water/ice clouds (and thin ice clouds in particular) have a negative BTD (Figure 12). The same radiation pattern is used in the Dust RGB to detect suspended mineral dust particles. Thus, in thin ash clouds the red color beam features prominently (Table 1), whilst water/ice clouds have less red, and thin ice clouds in particular appear very dark. Depending on the height of the ash cloud and hence on its temperature, ash clouds are depicted in red when the cloud is colder and in magenta (= red + blue) when the cloud is warmer.
Figure 12: Transmission through thin ash and thin ice clouds, © COMET
Mineral ash particles can act as ice condensation nuclei and form thin cirrus clouds. Hence, they are depicted in black in the Volcanic Ash RGB (Figure 13).
In addition, the Ash RGB exploits the SO2 absorption band at around 8.6 µm (SEVIRI IR8.7 channel). The BTD [IR10.8 - IR8.7] is positive when SO2 is present in the atmosphere, resulting in a strong contribution of the green color beam (Table 1 and 2).
If both constituents are present, the mixed ash cloud appears yellow (= red + green) (Figure 13).
Table 1: SEVIRI channels used for the Volcanic Ash RGB.
Table 2: Temperature thresholds used for the SEVIRI channels of the Volcanic Ash RGB.
Under certain conditions, the 24h Microphysical RGB or the Dust RGB may perform even better than the Volcanic Ash RGB, especially when the viewing angle is strongly inclined, the ash concentration very high or the cloud temperature very cold. Nevertheless, the Volcanic Ash RGB is a good all-rounder for depicting most volcanic plumes.
Figure 13: Suomi-NPP Volcanic Ash RGB, 19 February 2018, 06:06 UTC. © NASA
More about the Volcanic Ash RGB:
- Quick Guide
- CAL module on the Volcanic Ash RGB
Quiz
Mount Etna eruption on 24 December 2016 as seen in the Ash RGB at 14:00 UTC.
Order the terms in the left column so that they match the terms in the right column!