The main tool used by the stratospheric modelling group is the Belgian Assimilation System for Chemical ObsErvations (BASCOE). Developed at BIRA-IASB, it is used to study and to monitor the chemical composition of the Earth stratosphere. Assimilation methods aim at optimizing a model state in order to reproduce a set of observations available for a given time window, as illustrated below. These methods have been developed in the eighties by meteorologists in order to improve the weather forecast. Combining real observations of the day and a numerical model, the assimilation methods allow numerical weather prediction systems to produce the best possible weather forecast. Since 2000, thanks to the increase of the number of satellite instruments dedicated to measurements of the atmospheric composition, assimilation methods have been applied to chemical observations.

The above animation illustrates the data assimilation principle in the case of ozone around 20 km during the development of the ozone hole above the South Pole. The animation period is between September 1 (image 0/120) and October 1 (image 120/120), 2008. The left animation shows ozone observed by the MLS satellite instruments every 6 hours during September 2008. The right animation shows the BASCOE model state (no assimilation) every 6 hours. The centre animation shows the results of the assimilation of MLS ozone observations by BASCOE every 6 hours.Note how data assimilation combine the better accuracy of the observations and the better coverage of the model: assimilated fields are not biased against observations and are filling unobserved regions by MLS.

The BASCOE system is based on a three-dimensional Chemical Transport Model (CTM) and can use two data assimilation methods: 4D-Var and EnKF. The CTM calculates the evolution of around 60 stratospheric chemical constituents taking into account the advection, the chemistry and the microphysics of the Polar Stratospheric Clouds (PSC). The CTM is driven by the wind and temperature analyses produced by meteorological centres like the European Centre for Medium range Weather Forecast (ECMWF). The chemical scheme includes around 200 reactions (gas phase, photodissociation, and heterogeneous reactions). Further information on BASCOE is available in Errera and Fonteyn (2001), Errera et al. (2008), Viscardy et al. (2010), Errera and Ménard (2012), Skachko et al. (2014, 2016).

Errera, Q. and Fonteyn, D., Four-dimensional variational chemical assimilation of CRISTA stratospheric measurements, J. Geophys. Res., 106, 12,253-12, 265 (2001).

Errera, Q., Daerden, F., Chabrillat, S., Lambert, J. C., Lahoz, W. A., Viscardy, S., Bonjean, S., and Fonteyn, D., 4D-Var Assimilation of MIPAS chemical observations: ozone and nitrogen dioxide analyses, Atmos. Chem. Phys., 8, 6169-6187 (2008).

Errera, Q. and Ménard, R.: Technical Note: Spectral representation of spatial correlations in variational assimilation with grid point models and application to the Belgian Assimilation System for Chemical Observations (BASCOE), Atmos. Chem. Phys., 12, 10015-10031, doi:10.5194/acp-12-10015-2012, 2012.

Skachko, S., Errera, Q., Ménard, R., Christophe, Y., and Chabrillat, S.: Comparison of the ensemble Kalman filter and 4D-Var assimilation methods using a stratospheric tracer transport model, Geosci. Model Dev., 7, 1451-1465, doi:10.5194/gmd-7-1451-2014, 2014.

Skachko, S., Ménard, R., Errera, Q., Christophe, Y., and Chabrillat, S.: EnKF and 4D-Var data assimilation with chemical transport model BASCOE (version 05.06), Geosci. Model Dev., 9, 2893-2908, doi:10.5194/gmd-9-2893-2016, 2016.

Viscardy, S., Errera, Q., Christophe, Y., Chabrillat, S., and Lambert, J.-C., Evaluation of ozone analyses from UARS MLS assimilation by BASCOE between 1992 and 1997, JSTARS3, 190-202 (2010).