The Marine Atmospheric Boundary Layer During the SOFIA ASTEX Experiment:
Turbulence Structure and Island Effects on the Flow

The SOFIA experiment was performed in June 1992 in the Azores region, as a part of the ASTEX field experiment. The turbulent structure of the Marine Atmospheric Boundary layer (MABL) was investigated with the French instrumented research Fokker 27 aircraft. The air-sea exchanges (fluxes of sensible heat, latent heat and momentum, computed from low-altitude runs) were analysed in terms of parameterization (drag coefficients). The turbulence behavior of the low MABL was described by means of the Monin-Obukhov scaling theory. Variances of the turbulent velocities and of temperature and moisture, as well as the corresponding dissipation rates were studied in order to establish the energetic budget of the MABL at various levels. Furthermore, the turbulence structure was described in terms of the relative contribution to the transfers of the various "events": up and down, moist and dry, warm and cold,....

The mesoscale features of the MABL in the downstream area of the Santa Maria island were analyzed with aircraft data and with constant volume balloons launched within the MABL from the island. These latter were tracked by differential Omega system, and measured temperature and moisture along their trajectory. 3-D high-resolutions trajectories allowed a description of the internal structure of the MABL in the wake of the island. The island effects on the flow were evidenced using eulerian analysis of aircraft together with the "quasilagrangian" view of the balloons.