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Blasts and bifurcation: modeling tephra dispersal using 3D solutions

Tephra from explosive volcanic eruptions poses many hazards to humans and infrastructure, from shutting down jet engines to collapsing roofs. As such, it is important to predict tephra dispersal and deposition in advance of explosive events. Most models used to do this or to interpret prehistoric fall deposits are limited by simplifying assumptions that reduce the eruption column to a simple line or point in the sky from which tephra is emitted. While these models work well enough for tracking ash cloud motion at great distances from the vent, they fall apart close to the source. The Active Tracer High-resolution Atmospheric Model with Lagrangian Particle Module (ATHAM-LPM) allows us to model tephra dispersal and deposition in realistic eruption simulations. The interaction between the wind and the eruption column becomes significant, particularly for the weak plumes that make up >90% of the annual explosive eruptions around the world annually. Wind-plume interaction captured using this method includes plume bifurcation, wind-driven stability thresholds, and locally enhanced re-entrainment of sedimenting pyroclasts back into the plume. Model output is remarkably consistent with reported field data for the Fogo A and Pululagua BF fall deposits, and work in continuing to use ATHAM-LPM to recalculate standard approaches to deposit inversions.