The Belgian Assimilation System for Chemical ObsErvations (BASCOE) is dedicated to the assimilation of stratospheric chemical observations. Assimilation methods aim at optimizing a model to reproduce a set of observations. These methods have been developed in the eighties by the meteorologists in order to improve the weather forecast. Combining real observations of the day and a numerical model, the assimilation methods allow to produce the best possible prediction for the near future. More recently, 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 BASCOE system is based on the four dimensional variational (4D-Var) assimilation method and a three dimensional chemical transport model (3D-CTM). The aim of this method is to adjust the initial conditions of the model in order to reproduce a set of available observations over a specific time period (i.e., the assimilation window). The CTM calculates the evolution of 57 stratospheric chemical constituents taking account of advection and chemistry. The CTM is driven by the wind and temperature
analyses produced by the European Centre for Medium range Weather Forecast (ECMWF). The chemical scheme includes 199 gas-phase, photodissociation, and heterogeneous reactions which are ruled by a parameterization of stratospheric sulfate aerosols and Polar Stratospheric Clouds (PSCs). Further information on BASCOE are available in Errera et al. (2008).
Observations assimilated by BASCOE are the vertical distribution of ozone and other chemical tracers measured by satellite instruments. While BASCOE was initially developed to assimilate observations from the MIPAS instrument, its use is now generalized to other satellite observations such as UARS MLS or EOS Aura MLS. The ozone analyses (i.e., the optimized model fields) from MIPAS and UARS MLS are freely available through our ftp server. Finally, near real-time analyses of EOS Aura MLS observations are now operated with the MACC projects.