The marine atmospheric boundary layer during SEMAPHORE. I: Mean vertical structure and non-axisymmetry of turbulence

Abstract

This paper describes the vertical structure of the marine atmospheric boundary layer, analysed using aircraft measurements in the Azores region during SEMAPHORE (Structure des Echanges Mer-Atmosphère, Propriétés des Hétérogénéités Océaniques: Recherche Expérimentale). the flights were performed in the middle of the solar day, during near-anticyclonic conditions, with weak-to-moderate winds and weak surface sensible-heat flux, over a homogeneous oceanic area. the boundary layer was characterized by a mixed layer, driven by surface fluxes, which was not coupled to the overlying, broken stratocumulus, layer. This decoupling is demonstrated by the shape of the continuous profile of the dissipation rate of turbulent kinetic energy, computed from the continuous slant profiles performed by the aircraft. This method makes it possible to determine the thickness of the mixed layer, which is a relevant scale for the parametrization of the profiles. the turbulence field is analysed through the wavelength of the spectrum peak of the vertical velocity, computed on 392 runs more or less parallel or perpendicular to the mean wind. It demonstrates a stretching of the most energetic eddies along the mean wind. the consequences of this non-axisymmetric behaviour of the turbulent field on the flux estimates from aircraft data are discussed.