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The KLIMA algorithm, which has been adapted for processing IASI data, is the Millimetre-Wave Atmospheric Retrieval Code (MARC) (Carli et al., 2007) originally written to analyze the Millimetre-wave Airborne Receiver for Spectroscopic CHaracterization of Atmospheric Limb-Sounding (MARSCHALS) measurements (Oldfield et al., 2001). The MARC software was designed for the analysis of limb-sounding observations in the millimetre-waves and substantial modifications were required to extend its capabilities to the processing of data acquired by passive sensors that operate at higher frequencies and with a different viewing geometry. An upgraded version of the code was implemented and tested for application to the inversion of nadir-sounding observations performed in the middle/far infrared spectral region by the balloon-borne FT spectrometer REFIR-PAD (Palchetti et al., 2007 and Bianchini et al., 2007). The code was used to retrieve temperature and water vapour information by simultaneously fitting the water vapour profile, the temperature profile and the Earth skin brightness temperature using the spectrum from 100 to 1000 cm^-1 (Del Bianco et al., 2007). The new features introduced in the MARC code for the analysis of the REFIR-PAD measurements were mostly driven by the need to extend the forward model capabilities to the frequency range (100-1400 cm^-1) and to viewing mode (nadir sounding) of the instrument. Accordingly, the MARC code was modified to read the HITRAN database (Rothman et al., 2005 and Gordon et al., 2007), and the related atmospheric continuum was implemented (Clough et al., 2005), while to take into account the CO₂ line-mixing a dedicated database and line-by-line cross section routines was used (Niro et al., 2005a,b).

Moreover, the new observation geometry required the implementation of the retrieval of surface temperature. As for the MARC code, the surface was modelled as a black body multiplied by a frequency independent emissivity. Different instrumental features had to be modelled: the side band effects included in the model of the MARSCHALS heterodyne spectrometer were neglected and new pertinent ILS and FOV functions were taken into account. Spectral radiance and frequency units commonly adopted in the millimetre-wave range (Kelvin and GHz, respectively) and used therefore in the original version of the code were modified in the REFIR version, where frequency is expressed in cm^-1 and spectral radiance in nW/(cm² sr cm⁻¹) . The evolution of the MARC code into the version used for the analysis of REFIR spectra constituted the basis for the development of the retrieval code labelled as KLIMA and aimed at processing Level 1 IASI data. The KLIMA-IASI code is the result of further upgrading of both the forward model, to perform accurate and efficient RT calculation taking into account the IASI instrument characteristics, and of the inverse model to retrieve CO₂ concentration.

In Table 1 the main differences between the REFIR and IASI instrument are reported.