Measuring volcanic plume and ash properties from space
R. G. Grainger, D. M. Peters, G. E. Thomas, A. J. A. Smith, R. Siddans, E. Carboni, A. Dudhia, 2013. "Measuring volcanic plume and ash properties from space", Remote Sensing of Volcanoes and Volcanic Processes: Integrating Observation and Modelling, D. M. Pyle, T. A. Mather, J. Biggs
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The remote sensing of volcanic ash plumes from space can provide a warning of an aviation hazard and knowledge on eruption processes and radiative effects. In this paper new algorithms are presented to provide volcanic plume properties from measurements by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), the Advanced Along Track Scanning Radiometer (AATSR) and the Spinning Enhanced Visible and Infrared Imager (SEVIRI). A challenge of remote sensing is to provide near-real-time methods to identify, and so warn of, the presence of volcanic ash. To achieve this, a singular vector decomposition method has been developed for the MIPAS instrument on board the Environmental Satellite. This method was applied to observations of the ash clouds from the eruptions of Nabro and the Puyehue–Cordón Caulle in 2011 and led to a sensitive volcanic signal flag which was capable of tracking changes in the volcanic signal spectra as the plume evolved. A second challenge for remote sensing is to identify the ash plume height. This is a critical parameter for the initialization of algorithms that numerically model the evolution and transport of a volcanic plume. As MIPAS is a limb sounder, the identification of ash also provides an estimate of height provided the plume is above about 6 km. This is complemented by a new algorithm, Stereo Ash Plume Height Retrieval Algorithm, that identifies plume height using the parallax between images provided by Along Track Scanning Radiometer-type instruments. The algorithm was tested on an image taken at 14:01 GMT on 6 June 2011 of the Puyehue–Cordón Caulle eruption plume and gave a height of 11.9±1.4 km, which agreed with the value derived from the location of the plume shadow (12.7±1.8 km). This plume height was similar to the height observed by MIPAS (12 ± 1.5 km) at 02:56 GMT on 6 June. The quantitative use of satellite imagery and the full exploitation of high-resolution spectral measurements of ash depends upon knowing the optical properties of the observed ash. Laboratory measurements of ash from the 1993 eruption of Mt Aso, Japan have been used to determine the refractive indices from 1 to 20 µm. These preliminary measurements have spectral features similar to ash values that have been used to date, albeit with slightly different positions and strengths of the absorption bands. The refractive indices have been used to retrieve ash properties (plume height, optical depth and ash effective radius) from AATSR and SEVIRI instruments using two versions of Oxford-RAL Retrieval of Aerosol and Cloud (ORAC) algorithm. For AATSR a new ash cloud type was used in ORAC for the analysis of the plume from the 2011 Eyjafjallajökull eruption. For the first c. 500 km of the plume ORAC gave values for plume height of 2.5–6.5 km, optical depth 1–2.5 and effective radius 3–7 µm, which are in agreement with other observations. A weakness of the algorithm occurs when underlying cloud invalidates the assumption of a single cloud layer. This is rectified in a modified version of ORAC applied to SEVIRI measurements. In this case an extra model of a cloud underlying the ash plume was included in the range of applied models. In cases where the plume overlay cloud, this new model worked well, showing good agreement with correlative Cloud–Aerosol Lidar with Orthogonal Polarization observations.
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Remote Sensing of Volcanoes and Volcanic Processes: Integrating Observation and Modelling
Volcanoes have played a profound role in shaping our planet, and volcanic activity is a major hazard locally, regionally and globally. Many volcanoes are, however, poorly accessible and sparsely monitored. Consequently, remote sensing is playing an increasingly important role in tracking volcano behaviour, while synoptic remote sensing techniques have begun to make major contributions to volcanological science. Volcanology is driven in part by the operational concerns of volcano monitoring and hazard management, but the goal of volcanological science is to understand the processes that underlie volcanic activity. This volume shows how we may reach a deeper understanding by integrating remote sensing measurements with modelling approaches and, if available, ground-based observations. It includes reviews and papers that report technical advances and document key case studies. They span a range of remote sensing applications to volcanoes, from volcano deformation, thermal anomalies and gas fluxes, to the tracking of eruptive ash and gas plumes. The result is a state-of-the-art overview of the ever-growing importance of remote sensing to volcanology.