Brightness Temperature Differences
IR12.0 - IR10.8
Volcanic ash clouds consist of silicate particles, aluminium dioxide, ferrous sulphates and other trace elements. After the eruption of Eyjafjallajökull the Nordic Volcanological Center performed a comprehensive analysis of the components of the volcanic ash cloud. Results showed a remarkable high concentration of silicate particles of about 58 per cent (Nordic Volcanological Center). Therefore channels sensitive to this characteristic should be used for tracking the ash cloud. Volcanic ash clouds which contain a high concentration of silicate particles can be detected using the brightness temperature difference between IR12.0 and IR10.8. This difference arises as an effect of the lower emissivity of silicate particles at 10.8 μm than at 12.0 μm. In consequence the resulting brightness temperature difference of volcanic ash clouds will be positive. In contrast the brightness temperature difference for ice clouds will be negative due to the lower emissivity at 12.0 μm. Therefore this method enables the discrimination among volcanic ash clouds and ice clouds.
IR10.8 - IR8.7
Apart from producing ash clouds volcanic eruptions release enormous amounts of gases. Volcanic plumes with high concentration of sulfur dioxide can be detected using the IR8.7 channel. Generally S02 clouds are more transparent at IR10.8 than at IR8.7 due to the higher absorption at spectral ranges about 8.7μm. As a consequence the brightness temperature difference IR10.8-IR8.7 for SO2-clouds is positive. In contrast ice clouds are more transparent at IR8.7 than at IR10.8. As a result observing ice clouds the brightness temperature difference will be negative. This fact enables a clear discrimination between S02 and iceclouds, and this information is finally also used in the RGB-composites. Be careful in case of temperature inversions, since brightness temperature differences can be negative then.
Figure 3.4: loop 20100414 21UTC - 20100415 18UTC
As the analysis of the eruption of Eyjafjallajökull demonstrates, this information is only useful if you know the exact location where the eruption took place. On this basis the corresponding ash plumes can be analysed in regard to positive brightness temperature differences. Since positive brightness temperature differences can also arise due to different reasons (for example you can detect positive brightness temperatures in the loop over Norway) one has to be careful with further assumptions. This method again only gives a hint where SO2 signals can be found. Neither single satellite images nor one specific brigthness temperature difference enable the clear discrimination of ash clouds. This highly recommends to use combined products (see next section 3.3).