The Vertical Structure of Second-Order Turbulence Moments in the Stable Boundary Layer from SABLES98 Observations

Observations of the stable boundary layer (SBL) obtained during the SABLES98 experiment are analyzed in order to investigate the vertical variations of the momentum flux τ > 0 , the heat flux Q ( < 0 in stable conditions) and the turbulent kinetic energy E . The traditional SBL is identified on the basis of the momentum and heat fluxes respectively decreasing and increasing with height; the vertical scales obtained from the profiles of τ , Q and E give indications about the depth of the boundary layer, and are shown to be different for the different statistical moments. The upside-down SBL cases are defined by the momentum flux and the turbulent kinetic energy increasing with height, while the heat flux can increase or decrease with height. Also in this case the vertical scales differ for the different statistical moments. The observations show that the stability (evaluated from the gradient Richardson number or the Obukhov length) is not a univocal index that discriminates between traditional and upside-down cases. The scales allow a compact description of the vertical structure of the traditional and upside-down SBL in terms of the considered statistical moments and of the gradient Richardson number, and are used to interpret the variation of the Obukhov length with height.