Grain size distribution uncertainty quantification in volcanic ash dispersal and deposition from weak plumes

Abstract

We present the results of uncertainty quantification and sensitivity analysis applied to volcanic ash dispersal from weak plumes with focus on the uncertainties associated to the original grain size distribution of the mixture. The Lagrangian particle model Lagrangian Particles Advection Code is used to simulate the transport of inertial particles under the action of realistic atmospheric conditions. The particle motion equations are derived by expressing the particle acceleration as the sum of forces acting along its trajectory, with the drag force calculated as a function of particle diameter, density, shape, and Reynolds number. Simulations are representative of a weak plume event of Mount Etna (Italy) and aimed at quantifying the effect on the dispersal process of the uncertainty in the mean and standard deviation of a lognormal function describing the initial grain size distribution and in particle sphericity. In order to analyze the sensitivity of particle dispersal to these uncertain variables with a reasonable number of simulations, response surfaces in the parameter space are built by using the generalized polynomial chaos expansion technique. The mean diameter and standard deviation of particle size distribution, and their probability density functions, at various distances from the source, both airborne and on ground, are quantified. Results highlight that uncertainty ranges in these quantities are drastically reduced with distance from source, making them largely dependent just on the location. Moreover, at a given distance from source, the distribution is mostly controlled by particle sphericity, particularly on the ground, whereas in air also mean diameter and sorting play a main role.

Effect of particle volume fraction on the settling velocity of volcanic ash particles: insights from joint experimental and numerical simulations

“…The range of particle size and volume fraction used in the models and experiments match well those expected, for instance, inside recent eruption plumes from Mt. Etna (Italy) within 20 km from the eruptive vent30394041. In such a case, an ash accumulation rate on the ground of about one centimeter per hour would translate, assuming particles settling at 1 m/s, to a ϕ p of about 4 × 10 −6 , with four-way coupling effects starting to be important.…”

Sensitivity of Volcanic Ash Dispersion Modelling to Input Grain Size Distribution Based on Hydromagmatic and Magmatic Deposits

“…Comparison of the results for Eyjafjallajökull 2010 using PSDs defined by (1) the TGSD based on ground-only, and (2) ground-plus-satellite data shows that using ground-based TGSDs produces lower peak ash concentrations and reduces the spatial extent of the plume. This accords with results of other studies highlighting the importance of compiling TGSDs from both ground and airborne measurements (e.g., [84,85]). However, satellite retrieval data can also be problematic as the particles are generally assumed to be dense spheres and the retrieval algorithm is set to interpret brightness temperature difference as coming from particles with effective radius <~16 µm.…”

Reconstructing eruptive source parameters from tephra deposit: a numerical study of medium-sized explosive eruptions at Etna volcano

“…The reliability of the output from such tephra dispersal models depends strongly on the reliability and uncertainty of ESPs. In recent years, the application of inversion techniques to tephra dispersal models, based on advection-diffusion sedimentation equations, has shown promise for determining EPSs (Connor and Connor 2006;Burden et al 2011;Bursik et al 2012;Pardini et al 2016;Scollo et al 2008;Bonasia et al 2010;Volentik et al 2010;Fontijn et al 2011;Johnston et al 2012;Klawonn et al 2012;Magill et al 2015).…”