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| Titel |
3D numerical simulations of dispersion of volcanic ash using a Lagrangian model |
| VerfasserIn |
Yujiro Suzuki, Takehiro Koyaguchi |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250094426
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| Publikation (Nr.) |
 EGU/EGU2014-10145.pdf |
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| Zusammenfassung |
| Dispersion of volcanic ash largely depends on the atmospheric wind speed and eruption
intensity. In general, when the atmospheric wind is weak and/or eruption intensity is strong
(i.e., magma discharge rate is small), the volcanic plume is characterized by the formation of
umbrella cloud and the particles (i.e., volcanic ashes) are transported by the gravity current of
umbrella cloud. On the other hand, if the wind is strong and/or eruption intensity is weak, the
volcanic plume tends to be distorted by wind and the particles are drifted mainly by the wind.
Because these effects of gravity current and wind also change depending on the
particle size, it is difficult to quantitatively predict the distributions of particles
suspended in the atmosphere and those deposited on the ground. In this study, we are
developing a 3-D numerical model which directly simulates the ash transport and
deposition.
The model is designed to simulate the injection of a mixture of solid pyroclasts and
volcanic gas from a circular vent above a flat surface in a stratified atmosphere, using a
combination of a pseudo-gas model for fluid motion and a Lagrangian model for particle
motion. During fluid dynamics calculations, we ignore the separation of solid pyroclasts from
the eruption cloud, treating an eruption cloud as a single gas with a density calculated using a
mixing ratio between ejected material and entrained air (Suzuki et al., 2005, JGR). In order to
calculate the location and movement of ash particles, we employ Lagrangian marker particles
of various sizes and densities. The marker particles are ejected from the vent with the
same velocity of the eruption cloud every 2 sec. The particles are accelerated or
decelerated by the drag force on the spheres and fall to the ground with their terminal
velocities.
We carried out a simulation of a small-scale eruption in the strong wind fields with the
magma discharge rate of 2.5 x 106 kg/s. The rising plume is largely distorted by wind and
shows a bent-over trajectory. Subsequently, the plume stops rising and a horizontally moving
cloud develops at 6-8 km asl. The simulation results of Lagrangian particles suggest three
classes in terms of dispersal behavior. The coarse particles separate from the rising plume
(class I); they have terminal velocity greater than the average rising velocity of the plume. On
the other hand, the fine particles are transported up to the top of the cloud and suspended into
the horizontal flow (class III); they have terminal velocities that are substantially
smaller than the velocities of the turbulence inside the cloud. Between these two
classes, class II particles are transported to the top of cloud but gradually separate
from the horizontally moving cloud; their terminal velocities are smaller than the
average rising velocity of the plume but greater than the velocities of the turbulence. |
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