A condensation temperature is introduced, following Pollack et al. (1981), as
with in K and pressure p in hPa.
Both atmosphere and surface temperatures are prevented from falling
below
by precipitating atmospheric
onto the surface.
if the temperature of a given layer falls below (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.
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
(where R is the gas constant
and is the condensation temperature corresponding to the
pressure chosen as a reference, here
mbar).
J kg
and
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.