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Devenish, Benjamin J
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- PublicationOpen AccessThe Transition From Eruption Column to Umbrella CloudWe present a coflowing integral plume model for the transition from an eruption column to an umbrella cloud. This transition occurs above the level of neutral buoyancy where the rising plume is surrounded by a descending annulus. We model this transition by extending the coflowing integral plume model of Bloomfield and Kerr (2000, https://doi.org/10.1029/2018JB015841), which was originally developed for Boussinesq fountains, to volcanic plumes. In addition to the transition region, the new model includes the part of the eruption column below the level of neutral buoyancy. The eruption column and the transition to an umbrella cloud are treated as a continuous process from the vent upward. Equations for the variation with height of the mass, momentum, enthalpy, and moisture fluxes are presented for both the upward and downward plumes. The interaction between the upward and downward plumes is accounted for by two entrainment relations: from the upward to the downward plume and vice versa; entrainment from the environment into the downward plume (or the upward plume in the absence of a downward plume) is also accounted for. The model is applied to the two eruptions considered by Costa et al. (2016, https://doi.org/10.1016/j.jvolgeores.2016.01.017) for the volcanic-plume intercomparison study. Profiles of the mass and momentum fluxes are compared with those from an equivalent large-eddy simulation. The new model captures the order of magnitude of the fluxes, the relative magnitudes of the upward and downward fluxes and aspects of the profiles’ shape. In particular, the upward plume reaches a maximum before decreasing toward the top of the plume consistent with the large-eddy simulation plume.
64 12 - PublicationRestrictedResults of the eruptive column model inter-comparison study(2016)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ;; ;; ; ; ; ;; ;; ; ;; ; ; ; ; ; ; ; ; ;This study compares and evaluates one-dimensional (1D) and three-dimensional (3D) numerical models of volcanic eruption columns in a set of different inter-comparison exercises. The exercises were designed as a blind test in which a set of common input parameters was given for two reference eruptions, representing a strong and a weak eruption column under different meteorological conditions. Comparing the results of the different models allows us to evaluate their capabilities and target areas for future improvement. Despite their different formulations, the 1D and 3D models provide reasonably consistent predictions of some of the key global descriptors of the volcanic plumes. Variability in plume height, estimated from the standard deviation of model predictions, is within ~20% for the weak plume and ~10% for the strong plume. Predictions of neutral buoyancy level are also in reasonably good agreement among the different models, with a standard deviation ranging from 9 to 19% (the latter for the weak plume in a windy atmosphere). Overall, these discrepancies are in the range of observational uncertainty of column height. However, there are important differences amongst models in terms of local properties along the plume axis, particularly for the strong plume. Our analysis suggests that the simpli- fied treatment of entrainment in 1D models is adequate to resolve the general behaviour of the weak plume. However, it is inadequate to capture complex features of the strong plume, such as large vortices, partial column collapse, or gravitational fountaining that strongly enhance entrainment in the lower atmosphere. We conclude that there is a need to more accurately quantify entrainment rates, improve the representation of plume radius, and incorporate the effects of column instability in future versions of 1D volcanic plume models.314 43 - PublicationOpen AccessA Lagrangian stochastic model of a volcanic eruption columnWe develop a Lagrangian stochastic model (LSM) of a volcanic plume in which the mean flow is provided by an integral plume model of the eruption column and fluctuations in the vertical velocity are modelled by a suitably constructed stochastic differential equation. The LSM is applied to the two eruptions considered by Costa et al. (2016) for the volcanic-plume intercomparison study. Vertical profiles of the mass concentration computed from the LSM are compared with equivalent results from a large-eddy simulation (LES) for the case of no ambient wind. The LSM captures the order of magnitude of the LES mass concentrations and some aspects of their profiles. In contrast with a standard integral plume model, i.e. without fluctuations, the mass concentration computed from the LSM decays (to zero) towards the top of the plume which is consistent with the LES plumes. In the lower part of the plume, we show that the presence of ash leads to a peak in the mass concentration at the level at which there is a transition from a negatively buoyant jet to a positively buoyant plume. The model can also account for the ambient wind and moisture.
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