SPECTRA, VARIANCES AND LENGTH SCALES IN A MARINE STABLE BOUNDARY-LAYER DOMINATED BY A LOW-LEVEL JET

A flow situation over coastal waters of the Baltic Sea is studied. The boundary layer was characterized by stable stratification and the pre sence of a pronounced low level jet at very low height, 30-150 m, abov e the surface of the sea. The atmospheric surface layer was apparently extremely shallow; thus the non-dimensional wind gradients and temper ature gradients derived from measurements at 8 m do not show adherence to Monin-Obukhov similarity, in sharp contrast to findings from the s ame site at similar stability conditions but with no low level jet. In stead these quantities are shown to be governed by scales characterist ic of stable shear flow away from the surface. The height to the jet c entre appears to be an important quantity. Thus, for the cases with th e lowest jet height values (30-50 m), some turbulent characteristics o f the how (non-dimensional velocity standard deviations and the correl ation between the longitudinal and vertical velocity) have values simi lar to those found for the zero pressure-gradient laboratory boundary layer over a flat plate (the so called 'canonical' boundary layer) rat her than the typical values found in atmospheric boundary-layer flow. It was inferred that the large scale fluctuations known as 'inactive' turbulence, as well as gravity waves, were suppressed in this case.