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H. Savijarvi (UH/PHYS/ATM), D. Crisp (JPL), A.-M. Harri (FMI/GEO)
We suggest a framework for radiation scheme intercomparisons for Mars atmospheric models. An atmospheric mean profile T(z) (based on Mariner 9 IRIS observations) is defined in 39 fixed-altitude points. CO2 mass mixing ratio is fixed to 95.3~%, surface temperature to 210~K and surface pressure to 6.0~mb. Optional trace gases include water vapour at 27~% relative humidity (giving 10.0~micron precipitable water content), and O3, CO and O2 with their typical constant mass mixing ratios. Well-mixed dust is assumed to have visible optical depths of 0, 0.3, 0.6, 1.0 and 5.0.
A Spectral Resolving Model (SRM) with line-by-line multiple scattering multi-stream code (Crisp) is used as a least-compromise reference model. The surface albedo and surface longwave (LW) emissivity vary spectrally in the SRM but yield broadband values of 0.24 and 0.96, respectively.
We report some initial LW calculations with the SRM, and with two University of Helsinki Mars model versions, 'old' and improved. In the no-dust case, a CO2-only atmosphere produces downwelling LW flux of 15.7~W/m2 in the SRM. Including water vapour increases this to 18.0~W/m2 but the other trace gases have practically no effect neither separately nor together. Dust at the visible optical depth of 0.3 increases the downwelling flux further to 20.5~W/m2, and at the weak dust storm value of 1.0, to 26.8~W/m2, so dust and water vapour both have a significant effect in the longwave.
In comparison with the SRM LW fluxes and cooling rates, the UH Mars model's 'old' emissivity scheme proved reasonable in the no-dust case. Some simple refinements made it quite good, with flux differences being within 0.5~W/m2 at all altitudes. Dust was included in the improved version via the grey approximation. This seemed to work well, provided that the dust mass LW absorption coefficients were optimized separately for the upwelling and downwelling fluxes.