Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
Atmos. Chem. Phys., 16, 9399-9420, 2016
https://doi.org/10.5194/acp-16-9399-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
28 Jul 2016
Adjusting particle-size distributions to account for aggregation in tephra-deposit model forecasts
Larry G. Mastin1, Alexa R. Van Eaton1, and Adam J. Durant2,3 1U.S. Geological Survey, Cascades Volcano Observatory, 1300 SE Cardinal Court, Bldg. 10, Suite 100, Vancouver, Washington, USA
2Section for Meteorology and Oceanography, Department of Geosciences, University of Oslo, Blindern, 0316 Oslo, Norway
3Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, USA
Abstract. Volcanic ash transport and dispersion (VATD) models are used to forecast tephra deposition during volcanic eruptions. Model accuracy is limited by the fact that fine-ash aggregates (clumps into clusters), thus altering patterns of deposition. In most models this is accounted for by ad hoc changes to model input, representing fine ash as aggregates with density ρagg, and a log-normal size distribution with median μagg and standard deviation σagg. Optimal values may vary between eruptions. To test the variance, we used the Ash3d tephra model to simulate four deposits: 18 May 1980 Mount St. Helens; 16–17 September 1992 Crater Peak (Mount Spurr); 17 June 1996 Ruapehu; and 23 March 2009 Mount Redoubt. In 192 simulations, we systematically varied μagg and σagg, holding ρagg constant at 600 kg m−3. We evaluated the fit using three indices that compare modeled versus measured (1) mass load at sample locations; (2) mass load versus distance along the dispersal axis; and (3) isomass area. For all deposits, under these inputs, the best-fit value of μagg ranged narrowly between  ∼  2.3 and 2.7φ (0.20–0.15 mm), despite large variations in erupted mass (0.25–50 Tg), plume height (8.5–25 km), mass fraction of fine ( <  0.063 mm) ash (3–59 %), atmospheric temperature, and water content between these eruptions. This close agreement suggests that aggregation may be treated as a discrete process that is insensitive to eruptive style or magnitude. This result offers the potential for a simple, computationally efficient parameterization scheme for use in operational model forecasts. Further research may indicate whether this narrow range also reflects physical constraints on processes in the evolving cloud.

Citation: Mastin, L. G., Van Eaton, A. R., and Durant, A. J.: Adjusting particle-size distributions to account for aggregation in tephra-deposit model forecasts, Atmos. Chem. Phys., 16, 9399-9420, https://doi.org/10.5194/acp-16-9399-2016, 2016.
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Short summary
During volcanic eruptions, fine ash settles out of the atmosphere to form deposits. Particle aggregation makes it difficult for models to calculate where fine ash will fall. In this study we show that the Ash3d dispersion model can accurately predict where fine ash will land if one assumes a Gaussian size distribution of aggregates, ~ 0.18–0.23 mm in diameter and 600 kg m−3 in density. This aggregation scheme has optimally reproduced deposits for four well-documented eruptions.
During volcanic eruptions, fine ash settles out of the atmosphere to form deposits. Particle...
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