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Article Dans Une Revue Journal of the Atmospheric Sciences Année : 2005

Net exchange reformulation of radiative transfer in the CO2 15um band on Mars

Résumé

The Net Exchange Formulation (NEF) is an alternative to the usual radiative transfer formulation. It was proposed by two authors in 1967, but until now, this formulation has been used only in a very few cases for atmospheric studies. The aim of this paper is to present the NEF and its main advantages, and to illustrate them in the case of planet Mars. In the NEF, the radiative fluxes are no more considered. The basic variables are the net exchange rates between each pair of atmospheric layers i,j. NEF offers a meaningful matrix representation of radiative exchanges, allows to quantify the dominant contributions to the local heating rates and provides a general framework to develop approximations satisfying reciprocity of radiative transfer as well as first and second principle of thermodynamic. This may be very useful to develop fast radiative codes for GCMs. We present a radiative code developed along those lines for a GCM of Mars. We show that computing the most important optical exchange factors at each time step and the others exchange factors only a few times a day strongly reduces the CPU time without any significant precision lost. With this solution, the CPU time increases proportionally to the number N of the vertical layers and no more proportionally to its square N^2. We also investigate some specific points such as numerical instabilities that may appear in the high atmosphere and errors that may be introduced if inappropriate treatments are performed when reflection at the surface occurs.
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Dates et versions

hal-00113218 , version 1 (13-11-2006)

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Jean-Louis Dufresne, Richard A Fournier, Christophe Hourdin, Frérédric Hourdin. Net exchange reformulation of radiative transfer in the CO2 15um band on Mars. Journal of the Atmospheric Sciences, 2005, 62 (9), pp.3303-3319. ⟨10.1175/JAS3537.1⟩. ⟨hal-00113218⟩
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