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Sensitivity Tests and Performances

of the KLIMA-IASI Forward and Retrieval Models

 

 

The core activity of Phase 1 consisted in the sensitivity study carried out to determine the best trade-offs between accuracy and efficiency of the KLIMA-IASI algorithm, to implement them in a final version of the code, hereafter indicated as KLIMA-IASI Accelerated Retrieval Model (ARM), and to evaluate the ARM performances for the  retrieval of carbon dioxide from IASI radiances.
Two major drivers and the associated requirements oriented the sensitivity study:

 

 

 
  • 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.

     

 

  • 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.

 

The sensitivity study was carried out in two steps:

 

Sensitivity Tests on KLIMA-IASI Forward Model

A series of approximations was introduced to improve the efficiency of the code and to meet the requirements on program size and computing time. The impact of these approximations on the accuracy of the forward model was evaluated and the results were used for fine-tuning individual choices and implementing an overall procedure optimally suited for the selected target.

 

Sensitivity Tests on KLIMA-IASI Retrieval Model

 

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 implemented by the trade-off process.