The radiative transfer code is adapted from the one developed in the context of terrestrial modeling by Fouquart and Bonnel (1980) for solar radiation and Morcrette et al. (1986) for thermal radiation. This code, originally developed for the climate GCM of Laboratoire de Météorologie Dynamique, has since been included by Morcrette in the operational model of the European Centre for Medium-Range Weather Forecasts (ECMWF).
effects of both atmospheric carbon dioxide and
dust are included.
Particular attention was given
to the parametrization of absorption by the
m band with inclusion of Doppler effect [3].
Cooling rates computations were carefully validated by
comparison to line-by-line integrations. The model, in which Doppler
effects are introduced, is very accurate up to 70 km.
The thermal spectrum is divided into three parts, one for the core of
the
m band, one for the wings and the third one for
the remaining part of the spectrum.
For the three parts, the
transmission by dust is computed using grey absorption approximation.
For the two intervals of the
m band,
the total transmissivity is evaluated as the product between
transmissivity of dust and that of carbon dioxide.
Strictly speaking, it can be shown that
this evaluation of the combined transmissivity is valid
when there is no correlation between the spectral variations of the
two absorbers. This is generally assumed for dust and
carbon dioxide in the
m band.
Scattering is not taken into account because of the strong isotropy of
the thermal radiation.
in the original code developed by Fouquart and Bonel (1980), the upward and downward fluxs are obtained from the reflectances and transmittances of the layers. The interaction between gaseous absorption and scattering (by dust, molecules or clouds) is introduced using the photon path distribution method. At this stage, only absorption and scattering by dust (already present in the version of the code used at the ECMWF) is included in the Martian version although absorption by the near infra-red bands of carbon dioxide may become non negligible for very non-dusty conditions. The transmittances and reflectances of the layers are computed using the Delta-approximation to account for the strong asymmetry of the aerosol phase function.