Study of the wind speed u-component spectra by using low-response instrumentation: a model comparison

Abstract

A set of spectra for the wind speed u-component at 6 and 100 m heights taken by a cup-anemometer at CIBA site (Low Atmospheric Research Center, Spain) is presented. Turbulent conditions at the surface layer are evaluated by using a small meteorological mast and applying a typical profile method based on a least-squares approach over five heights. Under stable conditions, quite good agreement is obtained between observed spectra and the Olesen et al. (1984) and Kaimal et al. (1972) models for the slopes of the different regions into which stable spectra are divided. However, the spectral energy density is underestimated by the data. Also, a gap is clearly found in the low frequency range. The peak frequency is right-shifted. This shift could be explained by considering that the cup-anemometer does not sense some turbulence energy (specifically the convective part of the spectrum), but is also can be interpreted as mechanical turbulence losses due to buoyancy forces damping out the 3-dimensional eddies scaled by h, the height of the boundary layer. Under unstable conditions, the Olesen et al. (1984) model performs quite good when z i , the depth of the convective boundary layer, is evaluated by using the formulation of San José and Casanova (1988). Finally, a sensitivity test for z i is presented.