Numerical simulation of the tephra fallout and plume evolution of the eruptions of the L

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Numerical simulation of the tephra fallout and
plume evolution of the eruptions of the Láscar
volcano in April 1993 and July 2000

• Angelo Castruccio¹ Alvaro Amigo¹ ² Laura
  Gallardo²
• ¹ Departamento de Geología, Universidad de
  Chile, Plaza Ercilla 803, Casilla 13518-Correo
  21, Santiago
• ² Centro de Modelamiento Matemático (CMM),
  Universidad de Chile (UMR-CNRS 2071), Casilla
  203, Santiago

Introduction The numerical simulation of
volcanic tephra fallout from an eruptive column
is an important issue in volcanology, both to
understand the eruption dynamics and as a tool to
make hazard prediction and maps. In this work, we
present preliminary results of the simulation of
tephra fallout and plume tracking, for two
eruptions of the Láscar volcano in Northern
Chile the subplinian one of April 19th-20th 1993
and the vulcanian one of July 20th 2000. We apply
a three-dimensional chemical, transport and
deposition model, and reanalysis winds. Emissions
were taken from reported estimates based on
satellite observations and field data.
Photo by Dr. P. W. Francis
Láscar volcano The Láscar volcano (5592m,
2322S, 6744W) is the most active volcano of
the Andes of Northern Chile (Gardeweg and Medina,
1994). It is an ESE-WNW elongated composite
stratocone (Gardeweg et al, 1998). Activity since
1984 displays cycles of lava dome formation in
the summit crater, lava dome subsidence with
crater collapse, vulcanian to plinian explosive
eruptions culminating in the major explosive
eruption of 19/20 April 1993 (Gardeweg et al,
1998). Here we address the dispersion of tephra
in connection with the subplinian eruption
observed in April 1993 and the vulcanian eruption
of July 20 2000.
Observed (red) and simulated (blue) limits of
the ash fall deposit, using ECMWF data.
Evolution of the ash plume for the 20 July 2000
eruption
Model The MATCH model solves the continuity
equation for atmospheric tracers in a 3-D
Eulerian framework where ci represents
the mass mixing ratio of the trace species of
interest, v is the 3-D wind, K is the turbulent
pseudo-difussion tensor and Qi and Si represent
internal sources and sinks (Robertson et al,
1999). To simulate the tephra fallout, we
considered 10 size categories of particles (15 um
- 1.6 cm in radius), for which a size and height
dependent removal can be applied As a
first approximation, we assumed a constante Vs
equal to the mean velocity of a particle falling
from a 10-20km plume heigth (Bonadonna and
Philipps, 2003). Further, only the sedimentation
from the turbulent umbrella cloud and particles
smaller than 1.6cm are considered. We use two
sets of meteorological data ECMWF reanaysis data
linearly interpolated to 0.5, and HIRLAM fields
that correspond to dynamilcally interpolated
reanalyses of 0.1 horizontal resolution.
Simulated
Column heigth, mass flux and total mass erupted
Observed
Satellite images of the eruptive plume of the
20/07/2000 eruption of Láscar volcano.
Volcanic input parameters Grain size
distribution Since the grain size distribution of
the April 1993 eruption is not available, we
adjusted it by trial and error taking into
account that 1.4 of the total emitted tephra
corresponds to fine ash (1-12?m) (Rose et
al.,2000).
Conclusions Despite simplifications (e.g.,
constant settling velocity, spreading current
only, etc.) the results presented are consistent
with available observations, especially the
proximal to medial deposition (size, shape and
position), and also, at the regional scale, the
limits of the ash deposit for the April 1993
subplinian eruption. The tracking of the
plume of the July 2000 eruption is also in good
agreement with observations, especially the shape
of the plume. The temporal evolution is less well
captured, possibly due to shortcomings in the
representation of the grain size distribution and
the assumed constant settling velocity.
Future work considers the implementation of
vertically varying settling velocities, and the
simulation of other eruptions and Andean
volcanoes. We expect this to result in a reliable
tool for diagnostic and forecast studies and
hazard assessments.
Results
Deposit of the 19/20 April 1993 eruption
References Bonadonna, C., Phillips, J, 2003
Sedimentation from strong volcanic plumes.
Journal of Geophysical Research. V.108 n B7,
2340 Gardeweg, M., Medina, E., 1994 La erupción
subpliniana del 19-20 de Abril de 1993 del volcán
Láscar, N de Chile, 7 Congreso Geológico
Chileno, Actas volumen I, p 299-304. Gardeweg M
C, Sparks R S J, Matthews S J, 1998. Evolution of
Lascar volcano, northern Chile.
J Geol Soc London, 155 89-104 Robertson,
L., Langner, J., Engardt, M. (1999). An
Eulerian limited area transport model. J. Appl.
Met., Vol 38, No 2, 190-210. Rose, WI, Bluth,
GJS, Ernst, GGJ (2000) Integrating retrievals of
volcanic cloud characteristics from satellite
remote sensors A summary. Phil Trans R Soc Lond
A358 1585-1606 Sparks, R.S.J., Bursik, M.J.,
Carey, S.N., Gilbert, J.S., Glaze, L.S.,
Sigurdsson, H. Woods, A.W.
(1997) Volcanic Plumes, John Wiley Sons.
Isopachs in cm for the observed (dashed red
contours) and simulated deposits (continous blue
contours), using HIRLAM fields. Also shown
observed and modeled thickness vs distance and
isopach area (Httpp)
The mass flux for the April 1993 eruption was
obtained using H
1.83Q0.259 This is based on Sparks (1997), and it
considers that the total mass erupted was 345Mt
(Rose et al., 2000) and the column height from
Gardeweg and Medina (1994). The same formula was
applied for the July 2000 eruption, assuming a
cloumn height in a range between 5 (coarse ash)
and 7 km (fine ash)
Acknowledgements. We are grateful for the support
provided by the staff at the Swedish Meteorology
and Hydrology Institute(SMHI), FONDECYT Grant
1030809. This work was partially financed by the
Center for Mathematical Modeling, University of
Chile(CMM).