next up previous contents
Next: Vertical turbulent mixing Up: Les paramétrisation physique du Previous: Les paramétrisation physique du

Radiation

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).

For thermal radiation,

effects of both atmospheric carbon dioxide and dust are included. Particular attention was given to the parametrization of absorption by the tex2html_wrap_inline5473 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 tex2html_wrap_inline5473 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 tex2html_wrap_inline5473 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 tex2html_wrap_inline5473 m band. Scattering is not taken into account because of the strong isotropy of the thermal radiation.

Solar 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.

The atmospheric dust content

is specified as a mixing ratio constant in both time and space, except for the vertical distribution which is taken according to Pollack et al.  (1990). Beyond simplicity, the reason for this choice is that dust transport has not been included in the GCM until now. This strong approximation may affect the global tex2html_wrap_inline5481 condensation-sublimation cycle which primarily depends on the local dust-content at low latitudes [13]. The optical parameters of dust (such as single scattering albedo and asymmetry factor) are taken from Pollack et al.  (1979) .


next up previous contents
Next: Vertical turbulent mixing Up: Les paramétrisation physique du Previous: Les paramétrisation physique du

HOURDIN Christophe
Wed Jan 13 11:16:49 MET 1999