The Depth of the Convective Boundary Layer as Observed by Radio Occultation with Mars Express

Mars Express has been conducting radio occultation experiments for more than 7 years, yielding profiles of the neutral atmosphere with a unique combination of fine vertical resolution, broad coverage in latitude and longitude, and sampling of local times near dawn and dusk. Most of the atmospheric profiles that sound the tropics in late afternoon, 15-17 h, contain a well-mixed convective boundary layer (CBL), which is easily resolved by radio occultation measurements. We have analyzed these profiles to determine properties such as the depth of the CBL, the potential temperature within the mixed layer, and their response to variations in surface conditions. Observations accumulated from several seasons consistently show that the depth of the CBL is strongly correlated with variations in surface elevation, or, equivalently, strongly anti-correlated with variations in surface pressure. In general, the CBL is deep (8-10 km) where the surface elevation is high, as in the Tharsis region, and shallow (4-6 km) where the surface elevation is low, as in Isidis and Amazonis. Owing to this "pressure effect," the elevation at the top of the CBL varies with longitude by more than 10 km (at fixed local time), resulting in strong zonal modulation in the vertical mixing of dust, water vapor, heat, and momentum, as well as modulation in the spatial structure of the aphelion cloud belt. The depth of the CBL also depends on surface temperature, which varies (at fixed local time) with latitude relative to the sub-solar point and with surface properties (albedo and thermal inertia). We are comparing these observations with simulations of the CBL by the UK spectral version of the LMD Mars General Circulation Model (GCM). Once validated, the GCM can provide new insight into large-scale circulation patterns that cannot be measured directly, such as Walker cell-like structures in the tropics that arise from variations in CBL depth.