Particle Sedimentation in Numerical Modelling: A Case Study from the Puyehue-Cordón Caulle 2011 Eruption with the PLUME-MoM/HYSPLIT Models

Abstract

Numerical modelling of tephra fallout is a fast-developing research area in volcanology. Several models are currently available both to forecast the dispersion of volcanic particles in the atmosphere and to calculate the particles deposited at different locations on the ground. Data from these simulations can then be used both to manage volcanic crises (e.g., protect air traffic) or perform long-term hazard assessment studies (e.g., through hazard maps). Given the importance of these tasks, it is important that each model is thoroughly tested in order to assess advantages and limitations, and to provide useful information for quantifying the model uncertainty. In this study we tested the coupled PLUME-MoM/HYSPLIT models by applying them to the Puyehue–Cordon Caulle 2011 sub-Plinian eruption. More specifically, we tested new features recently introduced in these well-established models (ash aggregation, external water addition, and settling velocity models), we implemented a new inversion procedure, and we performed a parametric analysis. Our main results reaffirm the pivotal role played by mass eruption rate on the final deposit and show that some choices for the input parameters of the model can lead to the large overestimation in total deposited mass (which can be reduced with our inversion procedure). The parametric analysis suggests a most likely value of the mass eruption rate in the range 2.0–6.3 × 106 kg/s. More studies with a similar approach would be advisable in order to provide final users with useful indications about the parameters that should be carefully evaluated before being used as input for this kind of model.

Experimental Measurement of Enhanced and Hindered Particle Settling in Turbulent Gas‐Particle Suspensions, and Geophysical Implications

“…More generally in volcanic plumes and spreading umbrella clouds, enhanced (or hindered) settling can cause deposits with mass per unit area larger (or smaller) and grain sizes smaller (or larger) than expected for single size particles (Freret-Lorgeril et al, 2020;Gilchrist & Jellinek, 2021;Tadini et al, 2020Tadini et al, , 2022. Clustering in mixtures containing particles with Stokes number O(1) is an alternative to ash aggregation commonly assumed in simulations to explain enhanced settling (e.g., Tadini et al, 2022). The fact that bimodal fallout deposits, such as the one at Llaima (Figure 8), are relatively rare suggests effective particle size segregation occurs near the vent in most cases.…”

“…Experimental and numerical studies revealed that the particle settling velocity may increase (Dey et al., 2019; Wang & Maxey, 1993) or decrease (Dey et al., 2019; Fornari et al., 2016; Nielsen, 1993) depending on the degree of turbulence, and it can be lower (Marchetti et al., 2022) or higher (Del Bello et al., 2017) than predicted theoretically for a single particle depending on the particle size, shape, and concentration. Actual particle settling velocities different from theoretical velocities used in models may be a cause of discrepancies between observed and simulated volcanic fall deposits (Tadini et al., 2020, 2022) and this issue is critical in the context of hazard assessment.…”

“…2 of 16 (Del Bello et al, 2017) than predicted theoretically for a single particle depending on the particle size, shape, and concentration. Actual particle settling velocities different from theoretical velocities used in models may be a cause of discrepancies between observed and simulated volcanic fall deposits (Tadini et al, 2020(Tadini et al, , 2022 and this issue is critical in the context of hazard assessment. Dilute turbulent gas-particle mixtures are known to exhibit regions of higher particle concentrations called clusters, which affect the mixture dynamics and the particle settling velocity (Brandt & Coletti, 2022;Fullmer & Hrenya, 2017).…”