Abstract
The spaceborne detection of volcanic ash clouds at infrared wavelengths helps to avoid regions with enhanced volcanic ash concentrations that pose a threat to aviation. Current volcanic ash data retrievals require detailed information on microphysical properties and the refractive index of volcanic ash, which are highly variable. Uncertainties in the latter currently limit the quality of volcanic ash nowcasts. Here, we introduce a novel method to calculate the complex refractive indices of volcanic ashes at wavelengths from 5 to 15 μm from measurements of their individual components based on generic petrological ash compositions. Thereby the refractive indices for volcanic glasses and bulk volcanic ashes of different chemical compositions are derived. The variability of the latter is mainly influenced by the silica content and the porosity and to a minor degree by the glass-to-crystals ratio. Calculating optical properties exhibits an equally large impact of bulk composition and grain size distribution, whereas particle shape is considered less important for particle sizes of the order 1 μm. Using these optical properties to determine brightness temperature differences between the 11 μm and 12 μm channels we show that the effect of ash composition is non-negligible for modern satellite instruments. Particularly, the dependence of the volcanic ash on the silica content (and to a much smaller extent on the glass-to-crystals ratio) is observable in its refractive index, its optical properties and the brightness temperature difference, indicating that composition might be retrievable to some degree by remote sensing methods.
Highlights
Volcanic ash emitted during eruptive activity poses a serious threat to critical infrastructure on the ground (Wilson et al, 2012) and to aircraft in operation, where contact with a volcanic ash plume or cloud can lead to damage and engine failure (Casadevall, 1994); here the term plume is used for volcanic emissions that are still attached to the vent, and cloud once they are detached
Monitoring and initializing of nowcastings for volcanic ash clouds is regularly performed using satellite-borne passive infrared imagers. As these retrievals often rely on radiative transfer calculations, a good knowledge of the microphysical properties and the complex refractive index of volcanic ash is necessary
In this work we describe a method to calculate the complex refractive index of volcanic ash in the infrared (5 to 15 μm)
Summary
Volcanic ash emitted during eruptive activity poses a serious threat to critical infrastructure on the ground (Wilson et al, 2012) and to aircraft in operation, where contact with a volcanic ash plume or cloud can lead to damage and engine failure (Casadevall, 1994); here the term plume is used for volcanic emissions that are still attached to the vent, and cloud once they are detached. In order to reduce the impact of future eruptions, satellite remote sensing methods have been developed and applied to monitor volcanic radiative transfer calculations, which in turn require the microphysical properties and the complex refractive index of the volcanic ash, Eq (1), for the accurate simulation of brightness temperatures as a function of optical, micro- and macrophysical properties The volcanic ash properties are quite variable (Langmann, 2013), depending e.g. on the geographic location (Rogers, 2015) and the eruption style of the volcano (Polacci et al, 2019). Atmospheric processes further change the properties of the volcanic ash plumes and clouds (Langmann, 2013). In this study we focus on fine, distal ash, i.e. particles of a few micrometers in size which can travel within volcanic ash clouds thousands of kilometers. The corresponding typical properties are reviewed in the methods section of this paper
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
