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Field observations and laboratory experiments have shown that ashsedimentation can be significantly affected by collective settling mechanismsthat promote premature ash deposition, with important implications forassociated impacts. Among these mechanisms, settling-driven gravitationalinstabilities result from the formation of a gravitationally-unstable particleboundary layer (PBL) that grows between volcanic ash clouds and the underlyingatmosphere. The PBL destabilises once it reaches a critical thickness,triggering the formation of rapid, downward-moving ash fingers that remainpoorly characterised. We simulate this process by coupling a Lattice Boltzmannmodel, which solves the Navier-Stokes equations for the fluid phase, with aWeighted Essentially Non Oscillatory (WENO) finite difference scheme whichsolves the advection-diffusion-settling equation describing particle transport.Since the physical problem is advection dominated, the use of the WENO schemereduces numerical diffusivity and ensures accurate tracking of the temporalevolution of the interface between the layers. We have validated the new modelby showing that the simulated early-time growth rate of the instability is invery good agreement with that predicted by linear stability analysis, whilstthe modelled late-stage behaviour also successfully reproduces quantitativeresults from published laboratory experiments.

Modelling Settling-Driven Gravitational Instabilities at the Base of Volcanic Clouds Using the Lattice Boltzmann Method