Preprints
https://doi.org/10.5194/acp-2021-254
https://doi.org/10.5194/acp-2021-254

  31 May 2021

31 May 2021

Review status: this preprint is currently under review for the journal ACP.

Modelling the size distribution of aggregated volcanic ash and implications for operational atmospheric dispersion modelling

Frances Beckett1, Eduardo Rossi2, Benjamin Devenish1, Claire Witham1, and Costanza Bonadonna2 Frances Beckett et al.
  • 1Met Office, Exeter, UK
  • 2University of Geneva, Switzerland

Abstract. We have developed an aggregation scheme for use with the Lagrangian atmospheric transport and dispersion model NAME, which is used by the London Volcanic Ash Advisory Centre (VAAC) to provide advice and guidance on the location of volcanic ash clouds to the aviation industry. The aggregation scheme uses the fixed pivot technique to solve the Smoluchowski coagulation equations to simulate aggregation processes in an eruption column. This represents the first attempt at modelling explicitly the change in the grain size distribution (GSD) of the ash due to aggregation in a model which is used for operational response. To understand the sensitivity of the output aggregated grain size distribution (AGSD) to the model parameters we conducted a simple parametric study and scaling analysis. We find that the modelled AGSD is sensitive to the density distribution and grain size distribution assigned to the non-aggregated ash at the source. Our ability to accurately forecast the long-range transport of volcanic ash clouds is, therefore, still limited by real-time information on the physical characteristics of the ash. We assess the impact of using the AGSD on model simulations of the Eyjafjallajökull 2010 ash cloud, and consider the implications for operational forecasting. Using the time-evolving AGSD at the top of the eruption column to initialise dispersion model simulations had little impact on the modelled extent and mass loadings in the distal ash cloud. Our aggregation scheme does not account for the density of the aggregates; however, if we assume that the aggregates have the same density of single grains of equivalent size the modelled extent of the Eyjafjallajökull ash cloud with high concentrations of ash, significant for aviation, is reduced by ~3 %. If we assume that the aggregates have a lower density (500 kg m−3) than the single grains of which they are composed and make-up 75 % of the mass in the ash cloud the extent is 1.2 times larger.

Frances Beckett et al.

Status: open (until 12 Jul 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2021-254', Larry Mastin, 07 Jun 2021 reply
  • RC2: 'Comment on acp-2021-254', Anonymous Referee #3, 22 Jun 2021 reply

Frances Beckett et al.

Frances Beckett et al.

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Short summary
As volcanic ash is transported through the atmosphere it may collide and stick together to form aggregates. Neglecting the process of aggregation in atmospheric dispersion models could lead to inaccurate forecasts used by civil aviation for hazard assessment. We have developed an aggregation scheme for use with the model NAME, which is used by the London Volcanic Ash Advisory Centre. Using our scheme, we investigate the impact of aggregation on simulations of the Eyjafjallajökull 2010 ash cloud.
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