Mach and Froude Numbers on Mars

Mars atmospheric global circulation models exhibit transonic jet streaks during northern winter, which motivates this study of the Mach number, Ma (the ratio of flow speed to the speed of sound), and Froude number, Fr (the ratio of flow speed to the speed of buoyancy waves), as a function of season and location. Two global reanalyses spanning Mars Years (MY) 24 to 32 are used as input, EMARS and OpenMARS. The study's vertical coordinate is potential temperature, theta, ranging from theta = 400 to 1100 K (from z approximate to 32 to 66 km); the floor is set to the lowest level that avoids intersecting mountains. Area-weighted global means and standard deviations and 5 yr temporal means using the complete years MY 25, 26, 29, 30, and 31 are compared. EMARS and OpenMARS show general agreement below theta = 700 K (z approximate to 53 km), where the observational constraints are strongest, but can vary significantly at higher levels. Both reanalyses contain transonic jet streaks in every northern winter sampled. The Fr signal is roughly twice the Ma number signal, as expected where the temperature lapse rate is small compared to the dry adiabatic lapse rate. Mach numbers are similar in both reanalyses but show larger year-to-year variability in OpenMARS. Maps of standard deviations indicate a depression between the main peaks in Tharsis and higher variability in Mare Boreum than Mare Australe. The main conclusion is that the atmosphere of Mars routinely operates in the compressible regime (0.3 < Ma < 0.8), unlike Earth. Aspects of all flow-speed meteorology are discussed.

Automated summary

This study investigates the variations of Mach number and Froude number in the Martian atmosphere in different seasons and locations. The results show that the Martian atmosphere operates in the compressible regime and exhibits transonic jet streaks. The results from different reanalysis datasets are generally consistent at lower altitudes, but can vary significantly at higher altitudes.