Scale-Dependent Infrared Radiative Damping Rates on Mars and Their Role in the Deposition of Gravity-Wave Momentum Flux

摘要

Using a Curtis matrix model of 15 micron CO2 radiative cooling rates for the Martian atmosphere, we have computed vertical scale-dependent IR radiative damping rates from 0-200 km altitude over a broad band of vertical wavenumbers m = 2pi (1-500 km)-1 for representative meteorological conditions at 40 deg 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 approximately 50-80 km altitude, before gradually transitioning to scale-independent rates above approximately 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 approximately 0-50 km altitude, and is the dominant process at all altitudes for gravity waves with vertical wavelengths approx less than 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.