Simulation of SEVIRI infrared channels: a case study from the Eyjafjallajökull April/May 2010 eruption

[摘要] Infrared satellite images are widely and successfully used to detect andfollow atmospheric ash from erupting volcanoes. We describe a new radiativetransfer model framework for the simulation of infrared radiances, which canbe compared directly with satellite images. This can be helpful to getinsight into the processes that affect the satellite retrievals. As input tothe radiative transfer model, the distribution of ash is provided bysimulations with the FLEXPART Lagrangian particle dispersion model,meteorological cloud information is adopted from the ECMWF analysis and theradiative transfer modelling is performed with the MYSTIC 3-D radiativetransfer model. The model framework is used to study an episode during theEyjafjallajökull eruption in 2010. It is found that to detect ash by thereverse absorption retrieval technique, accurate representation of the ashparticle size distribution is required. Detailed investigation of individualpixels displays the radiative effects of various combinations of ash, liquidwater and ice clouds. In order to be clearly detectable, the ash clouds needto be located at some distance above other clouds. If ash clouds are mixedwith water clouds or are located only slightly above water clouds, detectionof the ash becomes difficult. Simulations were also made using the so-calledindependent pixel approximation (IPA) instead of the fully 3-D radiativetransfer modelling. In the two simulations, different clouds (or differentparts of the clouds) or the ground are effectively emitting radiation towardsthe instrument, thus causing differences in the brightness temperature of upto ± 25 K. The presented model framework is useful for further studiesof the processes that affect satellite imagery and may be used to test bothnew and existing ash retrieval algorithms.

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