Modelling the transport and dispersion of volcanic co-PDC ash clouds using NAME: an evaluation of source geometry and mass eruption rate

Marie Hagenbourger, Frances Beckett, Thomas J. Jones, Samantha Engwell

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. They have the potential to generate co-PDC plumes, which detach from the underlying PDC as they buoyantly rise into the atmosphere. 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 fine particle sizes (e.g., < 90 μm) and a high-aspect ratio, irregular-shaped, source geometry. Here, we consider how the release of ash into the atmosphere from a co-PDC plume may differ to that from a typical Plinian eruption column, and the implications for operational modelling of the resulting ash cloud for the provision of advice to the aviation industry. We use the atmospheric dispersion model, NAME, which is used by the London Volcanic Ash Advisory Centre. We performed a sensitivity analysis to determine which co-PDC source parameters are important for modelling the associated ash clouds. We show that variations in the source geometry, i.e., the total area and aspect ratio, have a minor impact after the first ~ 6 hours in the atmosphere.