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Parameter-field estimation for atmospheric dispersion: application to the Chernobyl accident using 4D-Var

Abstract : Atmospheric chemistry and air-quality numerical models are driven by uncertain forcing fields: emissions, boundary conditions, wind fields, vertical turbulent diffusivity, kinetic chemical rates, etc. Data assimilation can help to assess these parameters or fields of parameters. Because such parameters are often much more uncertain than the fields diagnosed in meteorology and oceanography, data assimilation is much more of an inverse modelling challenge in this context. In this article these ideas are experimented with by revisiting the Chernobyl accident dispersion event over Europe. A fast four-dimensional variational scheme (4D-Var) is developed, which seems appropriate for the retrieval of large parameter fields from large observation sets and the retrieval of parameters that are nonlinearly related to concentrations. The 4D-Var, and especially an approximate adjoint of the transport model, is tested and validated using several advection schemes that are influential on the forward simulation as well as on the data-assimilation results. Firstly, the inverse modelling system is applied to the assessment of the dry and wet deposition parameters. It is then applied to the retrieval of the emission field alone, the joint optimization of removal-process parameters and source parameters and the optimization of larger parameter fields such as horizontal and vertical diffusivities or the dry-deposition velocity field. The physical parameters used so far in the literature for the Chernobyl dispersion simulation are partly supported by this study. The crucial question of deciding whether such an inversion is merely a tuning of parameters or a retrieval of physically meaningful quantities is discussed. Even though inversion of parameter fields may fail to determine physical values for the parameters, it achieves statistical adaptation that partially corrects for model errors and, using the inverted parameter fields, leads to considerable improvement in the simulation scores. Copyright c 2011 Royal Meteorological Society
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Contributor : Nathalie Gaudechoux Connect in order to contact the contributor
Submitted on : Wednesday, June 6, 2012 - 4:59:09 PM
Last modification on : Friday, January 21, 2022 - 3:18:31 AM

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Marc Bocquet. Parameter-field estimation for atmospheric dispersion: application to the Chernobyl accident using 4D-Var. Quarterly Journal of the Royal Meteorological Society, 2012, 138 (664), pp.664-681. ⟨10.1002/qj.961⟩. ⟨hal-00705045⟩



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