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Surface processes

Surface temperature evolution is governed by the balance between incoming fluxs (direct solar insolation, thermal radiation from the atmosphere and the surface itself and turbulent heat fluxs) and thermal conduction in the soil. The parametrization of this last process is often rather crude in terrestrial GCMs where a great part of the surface temperature are either imposed or computed in oceanic models. For a dry planet like Mars, an accurate parametrization of heat conduction is crucial to determine correctly surface temperatures and their response to diurnal, synoptic and seasonal forcing. A new parametrization was therefore developed for the Martian version of the LMD GCM.

The time evolution of the temperature under the surface is given by a classical conduction equation

equation735

where the conductive flux tex2html_wrap_inline5541 is given by

equation743

and where tex2html_wrap_inline5413 and C are the soil conductivity and specific heat per unit volume, respectively. In the simple case of a vertically homogeneous soil (which is assumed here), it can easily be shown that the model, as far as the time evolution of the surface temperature is concerned, is only dependent on the soil thermal inertia tex2html_wrap_inline5547 .

Although atmospheric GCMs often use force-restore schemes with one or two layers to simulate the time evolution of the surface temperature, it is much more accurate and straightforward to perform a direct temporal integration of these equations using a multi-layer difference scheme in the ground [4, 16]. This was found to be numerically cheap enough even for a large number of layers in the soil: with 11 levels, this parametrization only represents 0.1 tex2html_wrap_inline5549 of the CPU-time of the Martian GCM. The soil model is similar to that presented by Warrilow  et al. (1981).

The accuracy of the model was checked by computing the phase and intensity of the surface temperature oscillation forced by a sin varying surface flux. For periods in the range from 0.3 to 2000 sols, the model produces errors of less than 1 tex2html_wrap_inline5549 on the intensity and phase shifts lower than tex2html_wrap_inline5553 .


next up previous contents
Next: CO2 condensation-sublimation Up: Les paramétrisation physique du Previous: Convective adjustment

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