Inverse modelling-based reconstruction of the Chernobyl source term available for long-range transport X. Davoine1 and M. Bocquet1,2 1CEREA, Joint Laboratory Ecole Nationale des Ponts et Chaussées/EDF R&D, France 2CLIME, Joint Project INRIA/Ecole Nationale des Ponts et Chaussées, France
Abstract. The reconstruction of the Chernobyl accident source term has been
previously carried out using core inventories,
but also back and forth confrontations between model simulations
and activity concentration or deposited activity measurements.
The approach presented in this paper is based on inverse modelling
It relies both on the activity concentration measurements and on the adjoint
of a chemistry-transport model.
The location of the release is assumed to be known, and one is looking
for a source term available for long-range transport that depends both
on time and altitude.
The method relies on the maximum entropy on the mean principle and
exploits source positivity.
The inversion results are mainly sensitive to two tuning parameters,
a mass scale and the scale of the prior errors in the inversion.
To overcome this hardship, we resort to the statistical L-curve method
to estimate balanced values for these two parameters.
Once this is done, many of the retrieved features of the source are robust
within a reasonable range of parameter values.
Our results favour the acknowledged three-step scenario,
with a strong initial release (26 to 27 April), followed by a weak
emission period of four days (28 April–1 May) and again a
release, longer but less intense than the initial one (2 May–6 May).
The retrieved quantities of iodine-131, caesium-134 and caesium-137
that have been released are in good agreement with the latest
reported estimations. Yet, a stronger apportionment of the total released
activity is ascribed to the first period and less to the third one.
Finer chronological details are obtained, such as a sequence of
eruptive episodes in the first two days, likely related to the
modulation of the boundary layer diurnal cycle.
In addition, the first two-day release surges are found to have
effectively reached an altitude up to the top of the domain (5000 m).
Citation: Davoine, X. and Bocquet, M.: Inverse modelling-based reconstruction of the Chernobyl source term available for long-range transport, Atmos. Chem. Phys., 7, 1549-1564, doi:10.5194/acp-7-1549-2007, 2007.