The marine atmospheric boundary layer during SEMAPHORE. II: Turbulence profiles in the mixed layer

The marine atmospheric boundary layer (MABL) analysed over the midlatitude ocean during SEMAPHORE (Structure des Echanges Mer-Atmosphere, Proprietes d es Heterogeneites Oceaniques: Recherche Experimentale) was topped by a brok en stratocumulus layer. Around midday, this layer was decoupled from the su rface. The mixed layer which develops under the influence of surface buoyan cy flux therefore stays well below the cloud layer. in this paper, we prese nt a parametrization of turbulence moments and spectral characteristics in this mixed layer. The data were gathered in autumn 1993 by two instrumented aircraft in the Azores region. We began by analysing a large number of run s, each over 100 km in length, in order to determine what scales must be ta ken into account for estimating turbulence moments. The turbulence characte ristics were thus calculated on 241 straight and level runs performed at va rious altitudes in the mixed layer during 17 flights over a period of 10 da ys, with various winds and surface fluxes. The turbulence profiles were nor malized using the convective scales, computed from the mixed-layer thicknes s (not the whole MABL thickness), and the surface fluxes. The contribution to buoyancy of temperature and moisture fluctuations are of the same order of magnitude, but they are of the same sign in the lower part of the mixed layer and of the opposite sign in the upper part. The result is that buoyan cy flux vanishes at the top of the mixed layer, and entrainment is weak. Th ese features are significantly different from what is observed in continent al convective boundary layers. We also examine the usefulness of skewness p rofiles (mainly of moisture skewness) in understanding mixed-layer dynamics (mainly of the respective contributions of the surface and top processes).