Scale-dependent infrared radiative damping rates on Mars and their role in the deposition of gravity-wave momentum flux

Abstract

Using a Curtis-matrix model of 15 μm CO 2 radiative cooling rates for the martian atmosphere, we have computed vertical scale-dependent IR radiative damping rates from 0 to 200 km altitude over a broad band of vertical wavenumbers ∣ m∣ = 2 π(1–500 km) −1 for representative meteorological conditions at 40°N and average levels of solar activity and dust loading. In the middle atmosphere, infrared (IR) radiative damping rates increase with decreasing vertical scale and peak in excess of 30 days −1 at ∼50–80 km altitude, before gradually transitioning to scale-independent rates above ∼100 km due to breakdown of local thermodynamic equilibrium. We incorporate these computed IR radiative damping rates into a linear anelastic gravity-wave model to assess the impact of IR radiative damping, relative to wave breaking and molecular viscosity, in the dissipation of gravity-wave momentum flux. The model results indicate that IR radiative damping is the dominant process in dissipating gravity-wave momentum fluxes at ∼0–50 km altitude, and is the dominant process at all altitudes for gravity waves with vertical wavelengths ≲10–15 km. Wave breaking becomes dominant at higher altitudes only for “fast” waves of short horizontal and long vertical wavelengths. Molecular viscosity plays a negligible role in overall momentum flux deposition. Our results provide compelling evidence that IR radiative damping is a major, and often dominant physical process controlling the dissipation of gravity-wave momentum fluxes on Mars, and therefore should be incorporated into future parameterizations of gravity-wave drag within Mars GCMs. Lookup tables for doing so, based on the current computations, are provided.