Ongoing projects

The vertical grid adopted for the reference FM is the same that is used to represent IASI L2 operational products for temperature and water vapour, consisting of 90 levels in the pressure range from 1050.0 hPa to 0.005 hPa. In the accelerated configuration of the forward model, the vertical resolution of the pressure grid was degraded: the 21 lowermost levels from 1050.0 hPa to 478.4 hPa (2 km) were the same as in the reference vertical grid. For the upper levels, only one every third pressure level was maintained in the new grid.

The total number of pressure levels in this approximation is 44.

Figure 12 shows the difference between the spectrum simulated with the downgraded and the reference vertical grid.

The largest differences are observed around 670 cm^-1 in the middle of the CO₂ band, where the CO₂ content in the upper tropospheric and lower stratospheric levels provide a significant contribution to the observed radiance and where a coarser vertical resolution of the atmospheric model at higher altitude may affect the accuracy of the simulation.

Figure 12

Difference between the spectrum simulated with and without the vertical grid reduction.

The red line is the nominal instrumental noise;

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