A spectrally-resolved, seasonally-varying energy budget for the Martian atmosphere, based on observational reanalyses.

Abstract:

The pathways and intensity of the flow of kinetic and potential energy through exchanges between different scales of motion are key characteristics of turbulent atmospheres and oceans. These exchanges of energy between scales reflect the dynamical response of an atmosphere to the thermodynamic processes that drive its climate and circulation, and are an important diagnostic of its dynamical circulation regime. The well-known Lorenz energy cycle measures the exchanges of KE and PE between eddies and the zonal mean circulation, but more recent approaches compute exchanges across the whole spatial spectrum, effectively connecting the Lorenz energy cycle with Kolmogorov turbulence. The resulting formalism is essentially an energy budget at each scale, with different scales being connected by nonlinear energy cascades. Using the MACDA assimilated analysis of observed temperature and dust in the Martian atmosphere, we have computed for the first time an observationally based, seasonal and multiannual Martian spectral energy budget, by applying an algorithm that was first used for Earth GCMs. KE and PE are well characterised by a Kolmogorov-like k^-5/3 spectrum with no evidence of the k^-3 inertial range found at large scales in Earth's kinetic energy spectrum. The Martian atmosphere seems therefore to be in a different dynamical regime from Earth. Within the k^-5/3 inertial range, KE and PE spectral energy fluxes show a more or less uniformly downscale energy cascade within this range, though with evidence of a very weak inverse cascade of KE at large scales. The larger eccentricity in Mars' orbit compared to Earth also induces a complex energy interaction between the seasons, with PE and KE fluxes and reservoirs varying significantly during the year. This also contrasts with the Earth's atmosphere which is much less seasonally dynamic. Major dust storm events present in an extended MACDA dataset exhibit a distinct increase in the intensity of the eddy energy cascades even though the yearly energy input from the Sun remains the same. Indeed the year to year annual mean spectral energy fluxes vary only weakly, regardless of whether Mars has a global dust storm or not. This is important as it suggests that other seasons can be potentially less energetic than usual in a year that has a global dust storm event.