Séminaire

The exploration of the Martian environment yields many examples of planetary boundary layer [PBL] phenomena commonly encountered on Earth: convective cloud streets, dust devils, afternoon growth of mixing layer associated to turbulent fluctuations of near-surface temperature, nighttime stable conditions with low-level jets. Yet, in many respects, the PBL dynamics on Mars is much more extreme than its terrestrial counterpart, although it can be seen as a large dusty desert. Due to the thin CO2 atmosphere and the low thermal inertia of the surface, covered by dust, the Martian PBL is radiatively controlled and undergoes a strong diurnal cycle with near-surface gradients of temperature obeying super-adiabatic regimes in the daytime and very-stable regimes in the nighttime with several K m−1 values. In the afternoon, the mixed layer is sometimes almost as deep as one atmospheric scale height (H0 ∼ 10 km) and in many cases more than 4 km deep, much deeper than afternoon PBL in most regions on Earth. 3D explicit high-resolution (50/100m) non hydrostatic simulations (so called Large-Eddy Simulations or LES) have demonstrated that the PBL dynamics and fluxes associated to the superadiabatic near-surface gradients comprises powerful narrow updrafts with vertical velocities 10 − 20 m s−1 and broad downdrafts with vertical velocities 5 − 10 m s−1 organized in a polygonal cellular structure. As is the case with terrestrial climate, those convective processes are sub-grid scale in global circulation models [GCM] and regional climate models [RCM] and must be parameterized. Parameterizing PBL vertical transport in GCMs is a key element to simulate correctly large-scale variability of wind and temperature, volatile transport and mixing and surface-atmosphere interactions [e.g. dust lifting]. Subgrid scale processes are now commonly represented in terrestrial models by mass-flux schemes of shallow convection. During this seminar, we will discuss the applicability of such schemes to Martian boundary layer convection, and present an adaptation of the LMDZ thermals model to Martian large-scale and regional-scale meteorological models.

MP Lefebvre (0144272799)