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Sensitivity Tests and Performances

of the KLIMA-IASI Retrieval Model

In this section, the results of sensitivity tests carried out to evaluate the performances of the KLIMA-IASI Accelerated Retrieval Model are presented. All tests have been performed using simulated IASI measurements and selecting as retrieval target the temperature profile, the water vapour profile, the total and partial columns of carbon dioxide and ozone and the Earth skin temperature.

A quantitative assessment of the quality of the retrieval products has been derived by computing:

• the retrieval error on the target parameters (including the random and the systematic components);

•  the bias between the retrieved and the true value of the target parameters due the approximations of the accelerated retrieval code.

A series of five retrieval tests was conducted, by combining different options for the set up of the FMs used to generate the simulated measurements and to perform retrieval calculations.

In Table 5,, we report a summary of the configurations adopted in the individual cases, indicating:

• (a) which version of the forward model (reference FM or accelerated FM) was used;
• (b) whether the uncertainties on forward model parameters were taken into account in the retrieval process (with or without the VCM of the FM);
• (c) whether retrieval calculations are performed on the full IASI range or on a selection of spectral bands.

Table 5

Retrieval set-up for the sensitivity tests

For each test several runs of the retrieval simulation were repeated with different realizations of the measurement noise, in order to account for the statistical variability of the retrieval products. The number of retrieval simulations performed for the individual test is 4 for the reference case (test 1) e 25 for the other cases (test 2 to 5).

Basic features of the Accelerated Retrieval Model

The basic features of the inverse model implemented in all retrieval tests can be summarised as follows:

• Multi Target Retrieval;

• Pressure retrieval grid for temperature: equal to the retrieval grid of IASI L2 operational products for the first 21 levels from 1050.0 hPa to 478.54   hPa (0-2 km); for the levels above only one every third pressure level is maintained. The total number of levels of the retrieval grid for temperature is 44;

• Pressure retrieval grid for H₂O: equal to the retrieval grid of IASI L2 operational products for the first 21 levels from 1050.0 hPa to 478.54 hPa (0-2 km); for the levels above only one every third pressure level is maintained. The total number of levels of the retrieval grid for temperature is 44;

• Pressure retrieval grid for CO₂: equal to the retrieval grid of IASI L2 operational products for the first 4 levels from 1050.0 hPa to 978.98 hPa (~0.3 m); for the levels above only one every second pressure level is maintained up to 499.54 hPa (~ 5.6 km). The total number of levels of the retrieval grid for temperature is 12;

• Pressure retrieval grid for O₃: one every third pressure level of the retrieval grid of IASI L2 operational products is taken from 1050.0 hPa to 759.16 hPa (~2.4 km). The total number of levels of the retrieval grid for temperature is 4;

• Marquardt parameters: 0.01 for all targets;

• Marquardt and Gauss dumping factor: 3;

• Use of the optimal estimation.

The target parameter of the retrieval are:

• The profile of temperature;

• The profiles of water vapour, carbon dioxide, and ozone;

• The band dependent earth surface temperature.

The atmospheric scenario (true atmospheric state and a priori values) selected for all the retrieval tests were taken from the IG2 climatology developed by University of Leicester for the operational analysis of MIPAS-ENVISAT data (Remedios et al, 2007.). In particular:

• True atmospheric state - IG2 profiles, October 2008 at Northern mid-latitude [20°N - 65°N]

• A priori atmospheric state - IG2 profiles, October 2007 at Northern mid-latitude [20°N - 65°N]

These IG2 profiles were used for the retrieved gases and temperature and pressure profiles.

The thermal contrast between the Earth surface and lowest atmospheric layer (defined as the difference between Earth skin temperature and the air temperature at ground level) was taken equal to zero for the true atmospheric state and equal to +10 K for the a priori state.

The a priori uncertainties that we have assumed on the retrieval targets are reported in Table 6.

Table 6

A priori errors and correlation length used for the retrieval tests