Global and Regional Climate Statistics of Mach and Froude Numbers in the EMARS and OpenMARS Reanalyses from MY 24 to 33

We are using the EMARS and OpenMARS global reanalyses, which span Mars Years (MY) 24 to 32 (into 33 for EMARS), to characterize the atmospheric dynamics on Mars from the point of view of the Mach number field, Ma, the ratio of flow speed to the speed of sound waves, and the Froude number field, Fr, the analogous ratio of flow speed to the speed of buoyancy waves (gravity waves). Key values of Ma are the incompressible range, 0.0 to 0.3, the compressible range, 0.3 to 0.8, and the transonic range, 0.8 and above. Mars proves to be more challenging to model than Earth because Ma spans all three of these ranges, each of which typically requires a different numerical algorithm for advection, whereas Earth’s fastest jets do not exceed 0.3. In both EMARS and OpenMARS, a significant portion of the atmosphere is compressible (Ma > 0.3), and in both data sets the northern winter polar jet is transonic, with occasional supersonic jet streaks. Similarly, Fr tends to be supercritical in northern winter. We calculate the area-weighted global fraction that is incompressible, f_incomp, as a function of season, which shows that typically less than half of the atmosphere is incompressible during northern and southern winters, increasing to more than half during the fall and spring seasons. We have broken down the Ma and Fr climate statistics for the 30 traditional Mars quadrangles. As illustrations, we show comparisons between Tharsis (MC-9), next-door Arcadia (MC-3), and Hellas (MC-28), and between the two polar regions, Mare Boreum (MC-1) and Mare Australe (MC-30). Except for Tharsis (MC-9), in these other four quadrangles, the maximum Ma is always transonic in the domain of study (potential temperatures above 400 K, corresponding to average altitudes above 35 km). The standard deviation of Ma is noticeably less for Tharsis (MC-9) than Arcadia (MC-3), and noticeably less for Mare Australe (MC-30) than Mare Boreum (MC-1). In addition to highlighting modeling issues, the un-Earth-like behavior of Ma and Fr on Mars motivates future physical studies, for example, how supersonic jet streaks (Ma > 1) interact with stationary tides, and how supercritical hydraulics (Fr > 1) affects polar circulations.