EVALUACIÓN DEL MODELO FALL3D-6.2 PARA TRES CASOS DE ...

EVALUATION OF THE FALL3D-6.2 MODEL FOR THREE CASES OF STUDY: HUDSON,
CHAITÉN AND CORDÓN DEL CAULLE VOLCANOES
Abstract
Based on the large amount of active volcanoes located in Chile and Argentina, this work is focused on
the modeling of volcanic ash dispersion and deposit applying the eulerian model FALL3D-6.2 to three
main eruptions that occurred in Patagonia in the past 21 years, they are from Hudson (August 1991),
Chaitén (May 2008) and Cordón del Caulle (June 2011) volcanoes.
Simulations were performed using the FALL3D-6.2 dispersion model coupled with the WRF-ARW
meteorological model. In the Hudson case, which was the oldest eruption, WRF-ARW was run in
diagnostic mode using ERA-INTERIM data set from ECMWF as initial field and boundary conditions,
while in case of Chaitén and Cordón del Caulle the meteorological model was run in forecast mode
using the GFS as initial field and boundary conditions. In the last two cases, the FALL3D-6.2 restart
option was used, where each run is initialized with the tridimensional volcanic ash concentrations and
bidimensional deposit fields resulting from the previous run.
Different methodologies were used to estimate the eruptive column height, using a combination of
rawinsonde and cloud top images, observations, scientific articles among others. The granulometry for
each event were estimated “in situ” for the Cordón del Caulle and from scientific articles for the older
cases.
The model results evaluation also depend on the information available at the time of the eruption. For
the Hudson case, the localization of the ash cloud was made using images already published from
sensor TOMS on board of the satellite NIMBUS 7, where from the detection of SO 2 present in the ash
cloud and the AI aerosol index, the cloud track can be determined. In case of Chaitén and Cordón del
Caulle, the comparison was made with sequences of MODIS and AVHRR sensors from the
AQUA/TERRA and NOAA 15-16-19 respectively, using ash detection algorithms based on the 11-12
µm channels Brightness Temperature Difference (BTD). In addition, deposit thickness measurements
from field campaigns were used to validate the modeled ash deposits.
The model results show a good representation of the localization of the volcanic ash. In few occasions,
the modelled ash cloud does not represent cloudy patterns probably associated with diffusive processes.
The forecasted ash deposits show a good agreement with the obtained from the field campaign
measurements in the case of Hudson and Chaitén. Meanwhile, for the Cordón del Caulle, it was not
posible to identify a pattern associated with the isopachs field calculated from the measurements. This
is probably due to the lack of temporal discrimination of the eruptive column height, as well as the
subestimation of the particles size in the total distribution calculations.