From needles to plates: The origin of extreme volcanic ash shapes and implications for dispersion modelling and retrieval algorithms

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The fall velocity, and consequently the distance volcanic ash travels through the atmosphere, depends on the size, shape and density of the particles. To forecast the transport of ash requires that ash dispersion models include appropriate particle characteristics. The programme used by the Met Office to forecast ash dispersal, however, assumes spherical particles. Yet there are volcanoes that repeatedly produce ash with extreme shapes including needles, typical of silicic eruptions at Katla also some eruptions of Vulcano (Italy), cuspate-shaped particles such as those from the 2011 eruption of Grimsvotn (Iceland) that were deposited in the UK, and large flat shards characteristic of very large silicic eruptions. Particles with extreme shapes will have a significantly lower fall velocity, remain in the atmosphere for longer and travel further from the volcano than spherical particles of the same density. To evaluate the origins of specific extreme ash shapes, the student will quantify shapes of ash from various eruptions using samples in the University of Bristol (UoB) collection and new samples of needle-shaped ash from Katla and Vulcano collected by the student, and place these within the context of published descriptions of ash morphology. The shapes of particles will be quantified in 2D (both projected shapes and slices through ash) by optical microscopy and scanning electron microscopy (SEM). 3D shapes will be constructed with MeX software from multiple SEM images taken at different angles, and selected larger particles will be imaged by X-ray tomography. The velocities and orientations of falling individual ash particles will be observed in a laboratory at UoB with high-speed video. Settling properties of bulk samples will be studied by the mass accumulated with time and analysis of size and shape of the ash as a function of height in the deposit. This will be complemented by settling idealized particles in water with particles made with a 3D printer so that shape can be systematically modified. In collaboration with the Met Office, the results of the experiments will be developed into a scheme that will then be incorporated into NAME to account for the fall velocity of non-spherical particles. The student will run sensitivity tests of the effects of ash shape on NAME forecasts and run simulations of likely scenarios for eruptions of Katla volcano. The spherical ash assumption is also standard in measurements of ash size distributions both in the lab and in the air during eruptions. The student will measure the apparent size distributions of ash samples with known (limited)