Abstract
The size distribution of volcanic ash is rarely measured in real time and Volcanic Ash Advisory Centres (VAACs) often rely on a default particle size distribution (PSD) to initialise their dispersion models when forecasting the movement of ash clouds. We conducted a sensitivity study to investigate the impact of PSD on model output and consider how best to apply default PSDs in operational dispersion modelling. Compiled grain size data confirm that, when considering particles likely to be in the distal ash cloud (< 125 µm diameter), magma composition and eruption size are the dominant controls on grain size distribution. Constraining the PSD is challenging but we find that the grain size of deposits from large hydromagmatic eruptions remains relatively constant with distance, suggesting that total (whole-deposit) grain size distributions (TGSDs) for these eruptions could be estimated from a few samples. We investigated the sensitivity of modelled ash mass loadings (in the air and on the ground) to input PSDs based on coarse to fine TGSDs from our dataset. We found clear differences between modelled mass loadings and the extent of the plume. Comparing TGSDs based on ground-only and ground-plus-satellite data for the Eyjafjallajökull 2010 eruption, we found that basing input PSDs on TGSDs from deposits alone (likely missing the finest particles) led to lower modelled peak ash concentrations and a smaller plume.
Highlights
Ash clouds from explosive volcanic eruptions can travel huge distances and cause severe disruption to air traffic (e.g., [1])
We have focused on Iceland, as it is within the London Volcanic Ash Advisory Centres (VAACs)’s area of responsibility
VAACs often rely on using a default particle size distribution (PSD) to initialise model runs but it has not been clear how best to constrain these
Summary
Ash clouds from explosive volcanic eruptions can travel huge distances and cause severe disruption to air traffic (e.g., [1]). Grain size is typically the dominant particle parameter in controlling the ash settling velocity and whether the ash residence time is significantly affected by turbulence [6,7,8,9,10]. It is, important to select an appropriate particle size distribution (PSD) to initialise model runs, but data on the size distribution of ash are rarely available in near real time and so most VAACs use default PSDs (e.g., [3,11,12])
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