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Entrainment parameterization for volcanic plumes with pulsating source parameters

Johan T. Gilchrist 1, Cyril Mergny2, Franck Donnadieu3, Mark Jellinek4, Eric C. P. Breard5, Colin Rowell4, Valentin Freret-Lorgeril3, Josef Dufek1, Frederic Peyrin6, Thierry Latchimy6

  • Affiliations: 1University of Oregon; 2Université Paris-Saclay; 3Université Clermont-Ferrand; 4University of British Columbia; 5 Univesity of Edinburgh; 6Observatoire Physique du Globe de Clermont-Ferrand

  • Presentation type: Talk

  • Presentation time: Thursday 09:45 - 10:00, Room S160

  • Programme No: 3.12.6

  • Theme 3 > Session 12

Abstract

Eruption columns occur in gravitationally stable (buoyant plume) or unstable (collapsing) mass transport regimes. Eruption hazards can be predicted with known eruption source parameters (ESPs) and rates of ingestion of atmosphere (entrainment) where they are relatively constant (steady) over time. However, most eruptions have unsteady ESPs, which change faster than erupted mixture rise time, and have poorly understood entrainment and rise dynamics. We use a multimethod approach to test a source Pulsation number that predicts entrainment into unsteady volcanic plumes with pulsating ESPs. We conduct analog experiments on pulsating buoyant flows of fresh water injected into a density-stratified saltwater layer, and model them using three-dimensional numerical simulations, to test the Pulsation number-entrainment relationship and characterize the turbulent kinetic energy spectrum. Based on the entrainment rate predicted by the Pulsation number, we classify experimental and simulated plumes into the steady plume, connected thermal, and discrete thermal regimes. We use Doppler radar and thermal camera measurements of unsteady ESPs at Sabancaya volcano to show that the Pulsation number can predict the effective entrainment rate of air into unsteady eruption plumes. Measurements of the vertical speed and outer plume temperature variations taken above the eruption plume source reveal a turbulent kinetic energy dissipation rate linked to the formation of large coherent vortices that are efficient at entraining ambient air. We combine our empirical Pulsation number-entrainment relationship with the semi-empirical Richardson number-entrainment formulation of previous studies to provide a way forward towards a generalized entrainment parameterization for use in 1D eruption column models.