Abstract
Abstract. The Eyjafjallajökull volcano in Iceland emitted a cloud of ash into the atmosphere during April and May 2010. Over the UK the ash cloud was observed by the FAAM BAe-146 Atmospheric Research Aircraft which was equipped with in-situ probes measuring the concentration of volcanic ash carried by particles of varying sizes. The UK Met Office Numerical Atmospheric-dispersion Modelling Environment (NAME) has been used to simulate the evolution of the ash cloud emitted by the Eyjafjallajökull volcano during the period 4–18 May 2010. In the NAME simulations the processes controlling the evolution of the concentration and particle size distribution include sedimentation and deposition of particles, horizontal dispersion and vertical wind shear. For travel times between 24 and 72 h, a 1/t relationship describes the evolution of the concentration at the centre of the ash cloud and the particle size distribution remains fairly constant. Although NAME does not represent the effects of microphysical processes, it can capture the observed decrease in concentration with travel time in this period. This suggests that, for this eruption, microphysical processes play a small role in determining the evolution of the distal ash cloud. Quantitative comparison with observations shows that NAME can simulate the observed column-integrated mass if around 4% of the total emitted mass is assumed to be transported as far as the UK by small particles (< 30 μm diameter). NAME can also simulate the observed particle size distribution if a distal particle size distribution that contains a large fraction of < 10 μm diameter particles is used, consistent with the idea that phraetomagmatic volcanoes, such as Eyjafjallajökull, emit very fine particles.
Highlights
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Methods exist for estimating the total mass eruption rate and particle size distSriboultiidon.EFaorretxhample, as there is no direct method of measuring the mass eruption rate of erupting volcanoes, most volcanic ash dispersion models use Published by Copernicus Publications on behalf of the European Geosciences Union
In a study combining ash fallout data observed up to 100 km from the volcano, for a range of volcanic eruptions, Horwell (2007) found that there was a strong linear relationship between 4 μm and 10 μm ash particles but a weaker non-linear relationship between 4 μm and 63 μm fractions. Both these results show that the coarse mode in the particle size distributions (PSD) shifts to finer sizes with distance from the volcano in the near-source region
Summary
NAME is a Lagrangian particle trajectory model designed for many dispersion applications, including the prediction of the dispersion and deposition of volcanic ash in the atmosphere (Jones et al, 2007). This distribution is based on an average of measurements made in the plumes from explosive eruptions of Mount Redoubt on 8 January 1990, Mount St. Helens on 18 May 1980, and St. Augustine on 8 February 1976 (Hobbs et al, 1991; Leadbetter and Hort, 2011). The ash density in NAME is assumed to be 2300 kg m−3
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