Stability modifications of the Frøya wind spectrum

Abstract

A general-stability wind spectrum for the marine boundary layer being an extension of the Frøya wind spectrum ( Andersen and Løvseth, 2006) is proposed in order to quantify spectral stability modifications for marine operations and wind turbines. The analysis of the Frøya data was based on results from the literature: Stability modifications are given by the ratio of a geometrical length scale Z SC increasing linearly with height to a crude representation of the Monin–Obukhov length L. It was a problem that the Frøya data were not available in digital form. Based on a slightly conservative approach restricted to periods below 6 min, the major spectral stability modification was found to be a linear scaling, whereas frequency shifting was less evident. A smooth representation of the linear scaling factor versus the above ratio was found correspond to monotonic decrease for stable conditions and unlimited monotonic increase for unstable conditions. The modification vanishes as the wind speed increases. In view of the large scatter in the results for near-neutral stability and the need for better resolution for wind speed and static stability, it is necessary to re-analyse the Frøya database (in digital form) or to analyse some other relevant data set.