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CO2 condensation-sublimation

A condensation temperature is introduced, following Pollack et al.  (1981), as

equation760

with tex2html_wrap_inline5555 in K and pressure p in hPa. Both atmosphere and surface temperatures are prevented from falling below tex2html_wrap_inline5555 by precipitating atmospheric tex2html_wrap_inline5481 onto the surface.

In the atmosphere,

if the temperature of a given layer falls below tex2html_wrap_inline5555 (as an effect of dynamics or other physical processes), condensation occurs, in an amount appropriate to restore, by latent heat release, the condensation temperature corresponding to the local pressure. All condensed carbon dioxide instantly precipitates to the ground without sublimation. Surface pressure is modified in agreement with the total amount of precipitation.

At the surface,

the temperature of the frost is kept at the condensation value either by condensing atmospheric tex2html_wrap_inline5481 or by sublimating tex2html_wrap_inline5481 ice.

The sublimation-condensation scheme exactly conserves both energy and mass. The energy balance on the caps is mainly controlled by albedo and emissivity of ice, which are unfortunately poorly known. The impact of those parameters on the annual pressure cycle has been carefully analysed in a recent study by Pollack et al. (1993). Direct measurements [5, 11], and model studies [15] yield large ranges of values: about 0.7-1. and 0.4-0.8 for emissivity and albedo, respectively. For this study, these quantities were somewhat arbitrarily fixed to 0.8 and 0.6 respectively (they were not tuned to fit Viking data). The approximation for the pressure-vapor curve used in paper 1 is replaced by a more accurate relationship based on the Clausius-Clapeyron relation for perfect gas. Assuming that the latent heat of sublimation L is independent of temperature, the vapor pressure curve reduces to

  equation767

(where R is the gas constant and tex2html_wrap_inline5575 is the condensation temperature corresponding to the pressure chosen as a reference, here tex2html_wrap_inline5577  mbar). tex2html_wrap_inline5579  J kg tex2html_wrap_inline5491 and tex2html_wrap_inline5583  K are fixed in the range of experimental values [6, e. g. ,]. The change of the pressure-vapor curve was found to have a minor effect on the atmospheric mass budget.


next up previous contents
Next: Aspects informatiques Up: Les paramétrisation physique du Previous: Surface processes

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