4A/OP

The operational version of 4A, a fast and accurate radiative transfer model for the Thermal, Short Wave and Far Infrared

The 4A/OP software package is the operational version of the fast and accurate radiative transfer model 4A (Automatized Atmospheric Absorption Atlas), developed, maintained and validated at LMD.
The 4A high spectral resolution line-by-line radiative transfer model (Scott and Chédin, 1981) pioneered the idea of bypassing the time-consuming line-by-line process by calculating once and for all Look-Up-Tables (LUT) of compressed monochromatic optical depths («the Atlases»).
Originally designed for the Earth atmosphere, the 4A concept has easily and successfully been extended to the Jovian atmosphere in the frame of the Voyager experiment.
Related to the current and future space activities or planned experiments, the spectral domain extends now from 100 to 14000 cm-1.

4A allows accurate computations: The Atlases are created by the line-by-line and layer-by-layer model, STRANSAC (Scott, 1974), at a spectral resolution of 5*10-4 cm-1, with state-of-the-art physics (line mixing and pressure shift for several species) based on inputs from the latest version of the GEISA spectroscopic database (or from any equivalent spectroscopic database);
4A allows fast computation of monochromatic or ISRF-convolved transmittances, radiances, and Jacobians for any geometrical/optical path or observation level, according to the user’s choice;
4A allows fast analytical computation of the Jacobians (Chéruy et al, 1995): these partial derivatives of the radiances with respect to temperature, gas mixing ratio, emissivity or albedo, aerosol optical depth and surface pressure are the backbone of online inversion algorithms for the retrieval of surface or atmospheric properties.

The operational version of 4A was developed in the late 90’s by Noveltis and LMD, under the aegis and with the support of CNES. Since then, 4A/OP is regularly improved, updated and extended by LMD, CNES and NOVELTIS.

4A/OP Jacobians — Example of the variation of the 4A Jacobians versus pressure with respect to perturbations of respectively 1K and 10% on the temperature profile (left) and CO2 concentration (right) for the mean tropical atmosphere of the TIGR dataset

Limb transmittances — Example of 4A-simulated transmittances (green) versus ACE-FTS observed transmittances (red) spectra (top) and residuals spectrum (bottom) for a 11.612 km tangent height observation, in the 2450-2550cm-1 range.

Illustration of the 4A/DISORT model is given below. It shows the spectral dependence – between 650 and 900 cm-1 or 15.4 µm - 11 µm -, of the sensitivity of IASI channels to an aerosols layer of IR optical depth 1.5 (at 10 µm), at an altitude of 2400 m (solid black line and right ordinate). Superimposed are spectra using an atmosphere increased by 20% of water vapour (magenta) or ozone (cyan), and, for comparison, 20%, 10%, and 5% of the aerosol contribution (all results on the left ordinate). Black dots (•) indicate channels selected for aerosol retrieval in this spectral region. Example below is for a mean tropical atmosphere).
spectral dependence – between 650 and 900 cm-1 or 15.4 µm - 11 µm -, of the sensitivity of IASI channels to an aerosols layer of IR optical depth 1.5 (at 10 µm), at an altitude of 2400 m (solid black line and right ordinate). Superimposed are spectra using an atmosphere increased by 20% of water vapour (magenta) or ozone (cyan), and, for comparison, 20%, 10%, and 5% of the aerosol contribution (all results on the left ordinate). Black dots (•) indicate channels selected for aerosol retrieval in this spectral region. Example below is for a mean tropical atmosphere).

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