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The spectroscopic database adopted for the reference FM includes all the lines listed in HITRAN 2004. In order to limit the total number of spectral features to be considered in the optimised version of the KLIMA-IASI code, a sensitivity test was carried out in which we evaluated the impact of skipping, for each species, all the lines smaller than 10^-5 the largest line in the selected band. The computation of the most intense feature is performed, species by species, for all 47 bands in which we sub-divided the IASI spectral range.

Figure 10 shows the difference between the spectrum simulated with and without the line skipping.

As for the previous tests, the simulations have been performed over the full IASI spectral range and the nominal instrument noise in reported in the figure for comparison.

The largest differences are observed around 700 cm^-1 (CO₂ band) and are due to the approximation introduced by CO₂ lines skipping. Considering that in the optimised version of the KLIMA-IASI code the spectroscopic data for CO₂ are taken from the line-mixing database (Niro et al, 2005a,b) and are not affected by the filtering criteria applied to HITRAN 2004, we can adopt the proposed line skipping for all other species without introducing any major source of systematic error in the forward model.

Figure 10

Difference between the spectrum simulated with and without HITRAN 2004 line skipping.

Red line is the nominal instrumental noise;

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