4A/OP

The software 4A/OP subroutine, called 4A/OP-sub hereafter, is the subroutine version of the code 4A-SWIR-TIR.

The objective of the software is to allow the fast simulation of the atmospheric radiative transfer in particular over the thermal and near infrared range either with a “pseudo-infinite” (high) resolution or with a spectral resolution of the simulated instruments.

4A output is the radiance spectrum in a user-defined spectral domain in the infrared and near-infrared region; the usual spectral domain is between 495 and 13290 cm^-1 (20-0.76 µm). 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). They can also be convolved with various types of instrument function. Jacobians, i.e. partial derivatives of the radiance with respect to the temperature and gas mixing ratio, can be also computed. They allow the model coupling with an inversion algorithm for the atmospheric constituent retrieval from infra-red radiance measurements.

The 4A/OP-sub is an heritage of the 4A program for the needs of the inversion issues and was developed under funding by CNRS and from CNES programs. Further development is supported by CNES.

The main scientific functions included in 4A/OP-sub are listed below:

• User-defined spectral emissivity/reflectance/brdf functions;
• Spherical atmosphere;
• Solar spectral contribution;
• Limb viewing including refraction;
• Scattering for molecular, aerosol and cirrus contribution;
• Simulation in polarized light;
• Jacobian computation of temperature profile, atmospheric molecule profiles, surface pressure, emissivity and temperature and aerosol optical thickness;
• Convolution with instrument functions;
• The new possibility to define the input spectral limits and the instrumental functions in wavelength.