In order to test the sensitivity of the KLIMA-IASI algorithms to individual approximations applied to the FM module, a reference configuration of the forward model was identified.

The approximate values of spectral radiance simulated using different optimisation options were compared with the synthetic spectrum generated by the reference FM configuration and the resulting differences were evaluated with respect to the nominal NESR of the IASI instrument.

The list of forward model approximations tested in the sensitivity study included the following aspects:

• Fine frequency grid used to simulate the high-resolution spectrum
• Modelling of the Line-Mixing Effects
• Spectroscopic Database
• Modelling of Atmospheric Line Shape
• Vertical grid
• Instrument Field of View

With respect to these aspects, the reference configuration set-up for the KLIMA-IASI FM can be described as follows:

• Fine frequency grid - a regular grid with steps of 0.001953125 cm^-1 - equal to 1/128 the spectral sampling of the IASI instrument (0.25 cm^-1) is adopted. A multiple of the frequency sampling of the ILS is used in order to avoid interpolation, when the convolution between the ILS and the simulated high-resolution spectrum is computed. Tests have shown that the use of a finer frequency grid results in systematic effects smaller than the nominal instrumental noise and affecting the uppermost altitude levels, where the information provided by the IASI measurements is negligible. Figure 6 shows the difference between the simulated spectrum in the spectral interval [650 - 800] cm^-1 calculated using a fine frequency grid with steps of 0.0009765635 cm^-1 (equal to 1/256 of IASI spectral sampling) and the reference spectrum. In the figure is also reported the nominal instrumental noise as reference for the approximation. The simulation has been performed in the CO₂ from 650 cm^-1 to 800 cm^-1 where the higher levels contribute and more narrow lines are observed

• Line-mixing Effects - CO₂ line-mixing effects are included and their contribution is estimated according to the model by Niro et al. (2005a and 2005b). are taken into account. Spectroscopic data for carbon dioxide  are taken from the dedicated database used by the CO₂ line-mixing model.

• Atmospheric Line Shape - All atmospheric lines are modelled up to  ±25 cm^-1 from the line centre.

• Vertical Grid - The vertical grid is the same of the IASI L2 products, i.e. consists of 90 pressure levels from 1050.0 hPa to 0.005 hPa.

• Instrument Field of View - The effects due to the field of view of the instrument are not taken into account (see the section on KLIMA-IASI upgrades for more details about the impact of this approximation on the accuracy of the FM).

Figure 7 shows the spectrum simulated using the reference FM.

Figure 6

Difference between the spectrum simulated with 0.0009765635 cm^-1 (1/256 of IASI spectral sampling)

and the reference fine frequency grid. The red line is the nominal instrumental noise;

the green line is the nominal instrumental noise reduced by a factor of 10.

Figure 7

Synthetic spectral radiance in the wave number range [650-2670] cm^-1,

simulated using the KLIMA-IASI Reference Forward Model