Anna Sjöblom

Vertical structure in the marine atmospheric boundary layer and its implication for the inertial dissipation method

The structure of the marine atmospheric boundarylayer and the validity ofMonin–Obukhov similarity theory over the seahave been investigated using longterm measurements. Three levels of turbulencemeasurements (at 10 m, 18 mand 26 m) at Östergarnsholm in themiddle of the Baltic Sea have beenanalysed. The results show that turbulentparameters have a strong dependenceon the actual height due to wave influence.The wind profile and thus thenormalised wind gradient are very sensitiveto wave state. The lower part of theboundary layer can be divided into three heightlayers, a wave influenced layerclose to the surface, a transition layer andan undisturbed ‘ordinary’ surfacelayer; the depth of the layers is determinedby the wave state. This heightstructure can, however, not be found for thenormalised dissipation, which is onlya function of the stability, except duringpronounced swell where the actualheight also has to be accounted for. Theresults have implications for the heightvariation of the turbulent kinetic energy(TKE) budget. Thus, the imbalancebetween production and dissipation willalso vary with height according to thevariation of wave state. This, in turn,will of course have strong implicationsfor the inertial dissipation method, inwhich a parameterisation of the TKEbudget is used.

A Note on Velocity Spectra in the Marine Boundary Layer

Spectra of longitudinal and vertical velocity have been studied at a marine site, östergarnsholm, in the Baltic Sea during a period of six days with near-neutral or slightly unstable conditions, when the wave state gradually changed from pure wind sea to strong swell having approximately the same direction as the wind. During the pure wind sea phase, spectra are shown to adhere closely to general forms for the neutral atmospheric surface layer obtained from a new theory. As soon as the wave age goes slightly beyond that representative of pure wind sea conditions, the spectra deviate in shape from the ‘ideal’ forms. The spectral modification appears to start at a frequency typical of the swell component. As the wave age increases, it progresses in frequency as a downscale cascade, which is particularly prominent in the spectrum of the vertical component but which is also observed in the longitudinal component. In addition, there is a strong effect in the low-frequency part of the spectra. It is interpreted as an indirect effect of large-scale ‘inactive’ turbulence, which becomes progressively more important as wave-age increases. It is found that the ratio of the spectrum of the vertical component and the spectrum of the corresponding longitudinal component attains the theoretically predicted value of 4/3 for cases of developing sea (gale force wind) for frequencies above approximately 4 Hz but never much exceeds unity for cases with swell. It is argued that this is an indication of local anisotropy and that the inertial-dissipation method for determination of the momentum flux is inappropriate in the case of mixed seas or swell.

New findings concerning the structure of the marine atmospheric boundary layer over the Baltic Sea – possible implications for wind energy installations

Seven years worth of meteorological tower data from the site Östergarnsholm in the Baltic Sea have provided new information on the structure of the marine atmospheric boundary layer, of high relevance for off-shore wind energy siting. The present paper is a review based on a series of original papers, which have appeared recently or are about to appear in the meteorological literature. Waves on the water surface modify the flow considerably as soon as they become of sufficient length. Thus, during neutral conditions, a logarithmic wind profile is observed only when the waves are growing actively. During unstable stratification, which occurs at this site during more than half the time, several factors act together to reduce the magnitude of the wind shear to a considerable extent. Velocity spectra during neutral conditions and growing sea agree with corresponding spectra over land, which have recently been shown to have appreciably more low-frequency energy than previously assumed. The marine spectra are shown to be strongly dependent on wave-state. The effect of limited water depth on the structure of the marine surface layer is discussed.