A fast method to compute Three-Dimensional Infrared Radiative Transfer in non scattering medium

The field of numerical simulation of Atmospheric Radiation has seen the development of more accurate and faster methods to take into account absorption by atmospheric constituent. Thanks to High Performance Computing, multi-band model is often used to perform the integration of the transmission over a spectral band. Nevertheless, the coupling of radiative transfer with fluid dynamics in local-scale forecast model is time consuming.

A parametrization of broadband emissivity based on the Yamamoto's analytical functions is proposed here. For incoming and outgoing directions, a gray absorption coefficient which only depends on upward and downward emissivity is computed. The analytical expression of the upward and downward gray absorption coefficient derives from the Cooling-To-Space approximation. Under this hypothesis, a comparison with emissivity functions and a Correlated-K Distribution shows good results on heating rate and incident flux on the ground.

For non homogeneous path, this parametrization was tested on atmospheric profiles with measurements from ParisFOG field experiments at different hours.

All this comparisons were made with three different mesh heights to test the sensibility to the boundary condition at the top of the domain. Actually, in radiative fog episode, only the first hundreds meters need to be simulated . The results have shown the good behavior of the upper boundary condition on computed radiance.