TURBULENCE STRUCTURE OF THE BOUNDARY-LAYER BELOW MARINE CLOUDS IN THESOFIA EXPERIMENT

The SOFIA (Surface of the Ocean: Flux and Interaction with the Atmosph ere) experiment, included in the ASTEX (Atlantic Stratocumulus Transit ion EXperiment) field program, was conducted in June 1992 in the Azore s region in order to investigate air-sea exchanges, as well as the str ucture of the atmospheric boundary layer and its capping low-level clo ud cover. We present an analysis of the vertical structure of the mari ne atmospheric boundary layer (MABL), and especially of its turbulence characteristics, deduced from the aircraft missions performed during SOFIA. The meteorological situations were characteristic of a temperat e latitude under anticyclonic conditions, i.e., with weak to moderate winds, weak surface sensible heat flux, and broken capping low-altitud e cloud cover topped by a strong trade inversion. We show that the mix ed layer, driven by the surface fluxes, is decoupled from the above cl oud layer. Although weak, the surface buoyancy flux, and the convectiv e velocity scale deduced from it, are relevant for scaling the turbule nce moments. The mixed layer then follows the behavior of a continenta l convective boundary layer, with the exception of the entrainment pro cess, which is weak in the SOFIA data. These results are confirmed by conditional sampling analysis, which shows that the major turbulence s ource lies in the buoyant moist updrafts at the surface.