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Sensitivity tests of the KLIMA-IASI Forward Model S. Del Bianco

Sensitivity tests of the KLIMA-IASI Forward Model S. Del Bianco. KLIMA-IASI algorithm: First Requirement. To obtain ultimate performances for CO 2 retrieval from single IASI spectra, in order to limit the extent of spatial and temporal

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Sensitivity tests of the KLIMA-IASI Forward Model S. Del Bianco

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  1. Sensitivity tests of the KLIMA-IASI Forward Model S. Del Bianco

  2. KLIMA-IASI algorithm: First Requirement To obtain ultimate performances for CO2 retrieval from single IASI spectra, in order to limit the extent of spatial and temporal averaging necessary to achieve the level of accuracy required for OCO and GOSAT validation. A target accuracy of 0.3% (1 ppm out of 370 ppm) on regional scales (1000 x 1000 km) at monthly intervals was established, which is consistent with observational requirements of both the NASA and JAXA missions.

  3. KLIMA-IASI algorithm: Second Requirement To limit the overall size of the program, as well as the computing time required for processing single IASI spectra, in sight of future integration with the ESA G-POD system We have assumed 1.0 Gbyte as the maximum size allowed for the integration into the G-POD system. The requirement on maximum computing time has been relaxed to 1 day for the iterative retrieval process from single spectrum. Assuming an average number of 4 iterations and a factor of two between the computing times of a forward model run with and without Jacobian calculation, this can be translated into a requirement of maximum computing time allowed for the FM calculation of a single spectrum without Jacobian equal to 180 min.

  4. Accelerated Retrieval Model (ARM) is the trade-off born from the previous requirements. The impact of the approximations introduced in ARM on the accuracy of the forward model has been evaluated with respect to the nominal IASI NESR. Subsequent sensitivity tests conducted on the ARM version of the KLIMA-IASI code aimed at quantifying the bias on CO2 retrieval products due to the forward model approximations.

  5. In order to test the sensitivity of the KLIMA-IASI algorithms to individual approximations applied to the FM module, a FM reference configuration has been identified. The 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

  6. Fine frequency grid used to simulate the high-resolution spectrum 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.

  7. Modelling of the 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). Spectroscopic Database All the spectral features listed in the HITRAN 2004 spectroscopic database are taken into account. Spectroscopic data for carbon dioxide are taken from the dedicated database used by the CO2 line-mixing model.

  8. Modelling of 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.

  9. Instrument Field of View The effects due to the field of view of the instrument are not taken into account.

  10. Approximations adopted for the KLIMA-IASI AFM with respect to the reference configuration

  11. Sensitivity tests for the optimisation of the fine frequency grid

  12. Sensitivity tests for the optimisation of the CO2 line-mixing model

  13. Sensitivity tests for the optimisation of the spectroscopic database

  14. Sensitivity tests for the optimisation of the atmospheric line shape

  15. Sensitivity tests for the optimisation of the atmospheric vertical grid

  16. Configuration of the AFM

  17. Performances of the KLIMA-IASI Accelerated Forward Model Conclusions G-POD requirements -> OK for AFM with band selection FM accuracy wrt nominal NESR -> OK FM errors will be included in the VCM of the observations In the next presentation Retrieval performance and Bias due to the FM approximations

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