Structure and dynamics of the Martian boundary layer: remote sensing, Large-Eddy Simulations and comparative meteorology

We describe the structure of the Martian convective Boundary Layer (BL) by a novel approach combining modelling and data analysis. Radio-occultation (RO) temperature profiles acquired recently on board the Mars Express orbiter are compared to Large-Eddy Simulations (LES) performed through a Martian adaptation of the non-hydrostatic Weather, Research and Forecast (WRF) model which makes use of the Martian physical parameterizations (radiative transfer, soil scheme, ...) developped at Laboratoire de Météorologie Dynamique (LMD).

Through the Mars Express RO technique, boundary layer depths in various Martian regions have been measured for the first time. We show that the dramatic regional variations of the BL depth revealed by RO are quantitatively reproduced by Martian windless free-convection LES. Intense BL dynamics is found to underlie the measured depths (up to 9 km): vertical speed up to 20 m/s, heat flux up to 2.7 Km/s and turbulent kinetic energy up to 26 m2/s2.

Under specific conditions, both model and measurements show a distinctive positive correlation between surface topography and BL depth. Our interpretation is that, in the tenuous CO2 Martian near-surface environment, the daytime BL is to first order controlled by the infrared radiative heating, fairly independent of elevation, which implies a simple correlation between the BL potential temperature and the inverse pressure ("pressure effect"). No prominent "pressure effect" is in action on Earth where sensible heat flux dominates the BL energy budget. Both RO observations and numerical simulations confirm the terrain-following behaviour of near-surface temperature on Mars induced by the dominant radiative influence, but also point out that the contribution of the Martian sensible heat flux is not negligible.

On Earth the daytime BL warms ‘from below' by the sensible heat flux incoming from the heated surface. On Mars the daytime BL warms ‘from inside and from below' respectively by the infrared radiative heating (plus the visible absorption by the dust) and the sensible heat flux. The strong radiative control of the Martian convective BL implies a generalized formulation for the BL dimensionless quantities, making use of the maximum vertical eddy heat flux (reached hundreds of meters above the ground and seven times larger than near-surface value on Mars). Based on this formulation and the variety of simulated BL depths by the LES, new similarity relationships for the Martian convective BL in quasi-steady midday conditions are derived. Rigorous comparisons between the Martian and terrestrial BL are now made possible by such similarity laws.