Non-local thermodynamic equilibrium in general circulation models of the Martian atmosphere 1. Effects of the local thermodynamic equilibrium approximation on thermal cooling and solar heating

Calculations of CO2 thermal cooling and near-IR solar heating rates under non-local thermodynamic equilibrium (non-LTE) situations have been performed to understand and evaluate the effects of non-LTE on the energy balance of the upper atmosphere of Mars. We find that the 15-mu m cooling rates can be in error if LTE is assumed above 80 km. In general, the correct non-LTE values are significantly smaller than the LTE values above about 85 km, but the magnitude and sign of the error depend on the temperature structure and the top altitude of the model and, to a lesser extent, on the collisions with atomic oxygen. A detailed analysis of the relevance of the upper boundary layer and a suggested buffer region are presented for both LTE and non-LTE. Based on general considerations of the thermal profile in the mesosphere and lower thermosphere, recommendations for general circulation models (GCM) are presented as a first guide for minimizing the LTE cooling rates inaccuracies. The error of assuming LTE on the CO2 near-IR solar heating rates is found to be about 20% at 85 km and increases strongly above this altitude. The dependences of this LTE-non-LTE difference on rate coefficients, thermal structure, surface pressure, and solar zenith angle (SZA) are studied. In contrast to the large effect of the SZA on the solar heating rate, we find it is not important for the LTE-non-LTE relative difference, which permits a simple tabulation of the non-LTE effect as a function of pressure only. A table of LTE correction factors for the solar heating rate is included for its potential use as a fast yet accurate operative scheme within GCMs.