4A/OP – operational release for 4A radiative transfer model
4A for Automatized Atmospheric Absorption Atlas is a fast and accurate line-by-line radiative transfer model particularly efficient in the infrared region of the spectrum.
4A allows the fast computation of the transmittance of discrete atmospheric layers, and of the radiance at a user defined observation level, thanks to the use of a comprehensive database (the atlases) of monochromatic optical thicknesses for up to 43 atmospheric molecular species (Scott and Chédin, 1981). The atlases are created once and for all by using the line-by-line and layer-by-layer model, STRANSAC (Scott, 1974), in its latest 2000 version with up to date spectroscopy from the GEISA spectral line catalogue (Jacquinet-Husson et al., 1999, 2003, 2008). This concept has been developed and maintained at the Laboratoire de Météorologie Dynamique LMD. NOVELTIS is now in charge of the consolidation and the distribution of 4A, in accordance with the convention signed between CNES, LMD/CNRS and NOVELTIS.
4A computes the optical thickness due to gaseous absorption for each atmospheric model layer in a spherical atmosphere and the 4A output is the radiance spectrum in a user-defined spectral domain in the infrared region; the usual spectral domain is between 600 and 3000 cm-1.
4A can be used for a wide variety of surface and earth atmospheric conditions; its use could be extended to extra-terrestrial atmospheric conditions.
- Spectra are computed at high spectral resolution (the nominal spectral resolution is 5.10-4 cm-1 but it can be changed by users).
- Spectra can also be convolved with various types of instrument function.
- Jacobians on user-defined layers can also be computed :
- Partial derivatives of the radiance with respect to the temperature and gas mixing ratio. They allow the model coupling with an inversion algorithm for the atmospheric constituent retrieval from infrared radiance measurements.