Source term, dispersion modelling and data assimilation
Several atmospheric dispersion models (FLEXPART, SILAM and WRF-Chem) are used in VAST to simulate the dispersion of volcanic ash and SO2 emitted during an eruption. The below movie loop shows an example of the transport of ash and SO2 emitted during the 2011-Grimsvötn eruption.
Figure 1: Modelled dispersion of ash and SO2
from the Grimsvötn eruption in 2011.
The accuracy of the model forecast is highly dependent on the input
parameters to the model simulations, such as the accuracy of the meteorological
data (wind, temperature, precipitation) and the determination of the emissions
from the volcano (the source term).
Source term determination
Using well defined source terms provide more accurate forecasts of the
spread and concentration levels of the volcanic emission clouds. The main VAST
technique for source term determination is an inversion scheme based on the
work of (Seibert, 2000). The inversion method couples
atmospheric transport models such as FLEXPART
with satellite observations.
The models are firstly used to evaluate how releases of ash or SO2
at different vertical levels above the volcano are transported in different
directions (see figure below). The inversion method compares the different
model simulations with the satellite data to estimate the most probable source
Figure 2: Illustration
of the basic principle of the inversion method used for source term
The inversion method can retrieve the source term parameters
It is assumed that the location of the volcano and the initial ash
particle size distribution are known (or approximated). The method estimates an
effective source term that not necessarily depicts the true source emissions
but is the source term needed for the atmospheric transport model to simulate
the downwind emissions clouds in best agreement with observation data.
The VAST inversion method has already been
applied and tested for several different volcanic eruptions. The Grimsvötn-2011 eruption is particularly
interesting as ash and SO2 were released to different altitudes and
transported to different directions;
- eruption onset and end time
- the mass eruption rate
- eruption column height
- initial vertical distribution
- Ash was released mostly to low altitudes andwas transported mostly south-eastwards.
- SO2 was released to high altitudes and was
Figure 3: The source
term for ash and SO2 emitted during the 2011-Grimsvötn eruption as
re-constructed using the VAST inverse modelling system. The height of the
emissions, the time of the emissions as well as the strength of the emissions
The inversion method was able to distinguish between ash and SO2 and
retrieve source terms that gave more accurate transport simulations when
validating with independent observation data (Moxnes et al, 2013: in prep).
The VAST inversion method was also used to successfully estimate source terms for
the eruptions Jebel at Tair-2007 (Eckhardt, 2008), Kasatochi-2008 (Kristiansen, 2010) and Eyjafjallajökull-2010 (Stohl et al, 2010).
1. Eckhardt, S., et al (2008),
Estimation of the vertical profile of
sulfur dioxide injection into the atmosphere by a volcanic eruption using
satellite column measurements and inverse transport modeling, Atmos. Chem.
Phys., 8, 3881–3897, doi:10.5194/acp-8-3881-2008.
2. Kristiansen, N. I., et al (2010), Remote sensing and inverse transport modeling of the Kasatochi eruption
sulfur dioxide cloud, J. Geophys. Res., 115, D00L16,
Seibert, P. (2000), Inverse
modeling of sulfur emissions in Europe based on trajectories, in Inverse
Methods in Global Biogeochemical Cycles, Geophys. Monogr. Ser., vol. 114, , pp.
147–154, AGU, Washington, D. C.
Stohl, A., et al (2011)
Determination of time- and height-resolved volcanic ash emissions for
quantitative ash dispersion modeling: The 2010 Eyjafjallajokull eruption, Atmos.
Chem. Phys., 11, 4333-4351, doi:10.5194/acp-11-4333-2011.
Moxnes, E., et al (2013) Separation of ash and sulfur dioxide
during the 2011 Grímsvötn eruption – source term estimates and transport
modeling constrained by satellite observations, In preparation