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Keywords:

  • power spectrum;
  • cospectrum;
  • quadrature spectrum;
  • coherence;
  • Taylor hypothesis

Abstract

Standard meteorological measurements from a number of masts around two Danish offshore wind farms have been used to study the spectral structure of the mesoscale winds, including the power spectrum, the co- and quadrature spectrum and the coherence. When average conditions are considered, the power spectra show universal characteristics, in agreement with the findings in literature, including the energy amplitude and the −5/3 spectral slope in the mesoscale range transitioning to a slope of −3 for synoptic and planetary scales. The integral time-scale of the local weather is found to be useful to describe the spectral slope transition as well as the limit for application of the Taylor hypothesis. The stability parameter calculated from point measurements, the bulk Richardson number, is found insufficient to represent the various atmospheric structures that have their own spectral behaviours under different stability conditions, such as open cells and gravity waves. For stationary conditions, the mesoscale turbulence is found to bear some characteristics of two-dimensional isotropy, including (1) very minor vertical variation of spectra; (2) similar spectral behaviour for the along- and across-wind components; and (3) the along- and across-wind components at one point are not correlated. Copyright © 2012 Royal Meteorological Society