Remote sensing of volcanic ash plumes from thermal infrared: a case study analysis from SEVIRI, MODIS and IASI instruments

Abstract. The Eyjafjallajökull eruption, which occurred during May 2010, is used as a case study to evaluate the consistency of the detection and characterization of volcanic ash plumes from different thermal infrared instruments. In this study, the well-known split window technique is used to retrieve the optical thickness and the effective particle size, and to estimate the mass concentration of volcanic particles from brightness temperatures measured in the infrared atmospheric window (8–12 μm). Retrievals are obtained for several mineral compositions whose optical properties are computed using Mie theory accounting for spectral variations of the refractive index. The impacts of errors in atmospheric parameters on the a posteriori uncertainties have been analysed. This analysis confirmed that major sources of errors are the layer altitude, the particle composition and, most of all, the size distribution for which uncertainties in retrievals can reach 50% in mass loading estimates. This retrieval algorithm is then applied to measurements acquired near-simultaneously from MODIS, SEVIRI and IASI space-borne instruments, using two channels around 11 μm and 12 μm. The retrievals are in close agreement when taking into account the different spatial and spectral configurations, and deviations between retrievals remain less than the uncertainties due to errors in atmospheric parameters. This analysis demonstrates the robustness of the retrieval method and the consistency of observations from these instruments for volcanic ash plume monitoring.