Variational assimilation of IASI SO<sub>2</sub> plume height and total column retrievals in the 2010 eruption of Eyjafjallajökull using the SILAM v5.3 chemistry transport model
Creators
- 1. Finnish Meteorological Institute, Erik Palménin aukio 1, 00560 Helsinki, Finland
- 2. COMET, Atmospheric, Oceanic and Planetary Physics, University of Oxford, Parks Road, Oxford, OX1 3PU, UK
Description
This study focuses on two new aspects of inverse modelling of volcanic
emissions. First, we derive an observation operator for satellite retrievals
of plume height, and second, we solve the inverse problem using an algorithm
based on the 4D-Var data assimilation method. The approach is first tested
in a twin experiment with simulated observations and further evaluated by
assimilating IASI SO2 plume height and total column retrievals in a
source term inversion for the 2010 eruption of Eyjafjallajökull. The
inversion resulted in temporal and vertical reconstruction of the SO2
emissions during 1–20 May 2010 with formal vertical and temporal
resolutions of 500 m and 12 h.
The plume height observation operator is based on simultaneous assimilation
of the plume height and total column retrievals. The plume height is taken
to represent the vertical centre of mass, which is transformed into the
first moment of mass (centre of mass times total mass). This makes the
observation operator linear and simple to implement. The necessary
modifications to the observation error covariance matrix are derived.
Regularization by truncated iteration is investigated as a simple and
efficient regularization method for the 4D-Var-based inversion. In the twin
experiments, the truncated iteration was found to perform similarly to the
commonly used Tikhonov regularization, which in turn is equivalent to a
Gaussian a priori source term. However, the truncated iteration allows the
level of regularization to be determined a posteriori without repeating the
inversion.
In the twin experiments, assimilating the plume height retrievals resulted
in a 5–20 % improvement in root mean squared error but simultaneously
introduced a 10–20 % low bias on the total emission depending on assumed
emission profile. The results are consistent with those obtained with real
data. For Eyjafjallajökull, comparisons with observations showed that
assimilating the plume height retrievals reduced the overestimation of
injection height during individual periods of 1–3 days, but for most of the
simulated 20 days, the injection height was constrained by meteorological
conditions, and assimilation of the plume height retrievals had only small
impact. The a posteriori source term for Eyjafjallajökull consisted of
0.29 Tg (with total column and plume height retrievals) or 0.33 Tg (with
total column retrievals only) erupted SO2 of which 95 % was injected
below 11 or 12 km, respectively.
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