The dispersion and atmospheric concentrations of co-PDC ash clouds

Hagenbourger, Marie and Jones, Thomas (2025) The dispersion and atmospheric concentrations of co-PDC ash clouds. Masters thesis, Lancaster University.

Text (2025HagenbourgerMRes.pdf)
2025HagenbourgerMRes.pdf - Published Version
Download (41MB)

Abstract

Pyroclastic density currents (PDCs) are gravity currents that frequently form during explosive volcanic eruptions. These ground-hugging density currents consist of high-temperature mixtures of pyroclasts (e.g., ash, pumice), lithics, and gas. These flows have the potential to generate co-PDC plumes, which detach from the underlying PDC as they buoyantly rise into the atmosphere. These co-PDC plumes, composed of fine-grained ash particles and hot gas, can reach heights of tens of kilometres, potentially dispersing large volumes of ash over continental-scale areas, impacting the environment, and posing a risk to aviation. Owing to their formation mechanism, co-PDCs have unique characteristics, such as a fine particle size (e.g., < 90 μm) and a high-aspect ratio, irregular-shaped, source geometry. Here, I consider how the release of ash into the atmosphere from a co-PDC plume may differ from that of a typical Plinian eruption column. I also assess the implications of any differences for operational modelling of the resulting ash cloud. I use the atmospheric dispersion model, NAME, which is used by the London Volcanic Ash Advisory Centre and perform a sensitivity analysis to determine which co-PDC source parameters are important for modelling the associated ash clouds. I find that variations in the source geometry, i.e., the total area and aspect ratio, show only a minor impact on the modelled ash cloud location and total mass in the atmosphere. However, different plume heights and associated mass eruption rates show a significant impact on the area, location, and concentration of the predicted downwind ash cloud. The newly introduced concentration thresholds for quantitative volcanic ash forecasts (QVA) are also considered within this study. When applying these thresholds, most clouds are very high-concentrated, often above 10 mg m−3.