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In the reference forward model line-mixing effects are taken into account, by adding to the simulated radiances the contribution of CO₂ line mixing modelled over the full spectral range covered by IASI. A significant reduction of the computing time is obtained in the ARM by evaluating the contribution of  line-mixing on a sub-set of spectral intervals.

In order to identify the minimum sub-set compatible with the accuracy requirements of the ARM, we calculated the difference between synthetic spectra simulated over the full IASI spectral range,  with and without the contribution of CO₂ line-mixing. The residuals are plotted in Figure 9, along with the nominal instrumental noise.

The modelling of CO₂ line-mixing can be limited to the spectral intervals, where these differences are larger than the nominal instrument noise, i.e. to the bands [650-800] cm^-1 and [2000-2500] cm^-1.

  Figure 9

Difference between the spectrum simulated with and without the CO₂ line mixing model.

 The red line is the nominal instrumental noise;

the green line in the nominal instrumental noise reduced by a factor 10