In order to reduce the overall size of the KLIMA-IASI code and the computing time requested for the simulation of a single spectrum, the sampling interval of the fine frequency grid was reduced from the reference value of 0.001953125 cm^-1 (equal to 1/128 the spectral sampling of IASI instrument) to the coarser value of 0.00390625 cm^-1 (equal to 1/64 the frequency sampling of IASI instrument).

Figure 8 shows the difference between the spectrum simulated using the downgraded fine wave number grid and the reference fine wave number grid. The impact of this approximation can be evaluated by comparison with the nominal instrumental noise also reported in the figure.

The simulation has been performed on the full IASI spectral range. The larger differences are observed in the CO₂ band between 650-800 cm^-1 and in the O₃ band close to 1100 cm^-1, where the contribution of carbon dioxide and ozone concentration in the uppermost levels is important.

 Figure 8

Difference between the spectral radiances simulated using  the downgraded fine wave number grid

and the reference fine wave number grid. The red line represents  the nominal instrumental noise;

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

.