HR: 08:30h
AN: V21E-03 INVITED    [Abstracts]
TI: 3-D simulations of eruption column and umbrella cloud development during explosive volcanic eruptions
AU: * Suzuki, Y J
EM: yujiros@jamstec.go.jp
AF: Japan Agency for Marine-Earth Science and Technology, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 235-0033, Japan
AU: Koyaguchi, T
EM: tak@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113- 0032, Japan
AB: During an explosive volcanic eruption, a mixture of solid pyroclasts and volcanic gas released from the volcanic vent buoyantly rises as a turbulent plume (i.e., eruption column) and laterally spreads at the neutral buoyancy level as a gravity current (i.e., umbrella cloud). Woods [1988] proposed a steady 1-D plume model which predicts the heights of eruption columns and umbrella clouds and the volume fluxes of the eruption clouds as a function of the mass-discharge rate at the vent. Sparks et al. [1997] proposed a gravity current model which predicts the spreading rate of umbrella clouds for given volume fluxes of the eruption clouds. These models allow us to quickly estimate the conditions at the vent from the observations on the eruption clouds, or, inversely, to predict the behavior of the eruption clouds for given vent conditions. However, these simplified models contain empirical constants (entrainment coefficient of turbulent plume, k, and Froude number of gravity current, Fr) that should be justified. We have developed a 3-D numerical model which simulates the dynamics of eruption clouds, and determined the values of the empirical constants in the simplified models. We apply a pseudo-gas model to describe the injection of a mixture of solid pyroclasts and volcanic gas from a circular vent in a stationary atmosphere; the nonlinear density change of the ejected materials and air with variable mixing ratios is calculated by changing the effective gas constant of the mixture in the equation of state. In order to reproduce the quantitative features of turbulent mixing correctly, a third-order accuracy scheme with fine grid sizes is applied [Suzuki et al., JGR, 2005]. Our model simulates the fundamental features of eruption clouds including eruption columns, pyroclastic flows, co-ignimbrite ash clouds and umbrella clouds, and has quantitatively reproduced the behavior of the eruption cloud in the Pinatubo 1991 eruption (e.g., the total column height, the altitude and the spreading rate of the umbrella cloud for the observed mass discharge rate). Systematic comparison between the 3-D simulations and the simplified models shows that the altitude and the spreading rate of the umbrella cloud in the 3-D simulations are approximated by the 1-D plume model with k=0.1 plus the gravity current model with Fr=0.2 for eruptions in the tropical regions and Fr=0.1 for those in the midlatitude regions. On the other hand, the total column height in the 3-D simulations highly oscillates even for a constant mass discharge rate and its time-average can be substantially greater than the estimate of the column height from the 1-D plume model with k=0.1 particularly for large scale eruption clouds in the midlatitude regions. The knowledge from the 3-D simulations is found to be useful for assessment and refinement of the 1-D plume and gravity current models.
DE: 8400 VOLCANOLOGY
DE: 8409 Atmospheric effects (0370)
DE: 8419 Volcano monitoring (7280)
DE: 8428 Explosive volcanism
DE: 8488 Volcanic hazards and risks
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting