Vertical cross-spectral phases in neutral atmospheric flow

Abstract

The cross-spectral phases between velocity components at two heights are analyzed from observations at the Høvsøre test site and from the field experiments under the Cooperative Atmosphere-Surface Exchange Study in 1999. These phases represent the degree to which turbulence sensed at one height leads (or lags) in time the turbulence sensed at the other height. The phase angle of the cross-wind component is observed to be significantly greater than the phase for the along-wind component, which in turn is greater than the phase for the vertical component. The cross-wind and along-wind phases increase with stream-wise wavenumber and vertical separation distance, but there is no significant change in the phase angle of vertical velocity, which remains close to zero. The phases are also calculated using a rapid distortion theory model and large-eddy simulation. The results from the models show similar order in phasing, but the slopes of the phase curves are slightly different from the observations, especially for low wavenumbers.

Keywords:

• cross-spectra
• phases
• atmospheric turbulence
• wavenumber
• vertical distance

Acknowledgments

This study is a part of the Ph.D. project funded by Siemens Wind Power A/S and WindScanner.dk, which is funded by the Danish Agency for Science. We are also obliged to the COMWIND project funded by Danish Council of Strategic Research (DSF-contract: 09-067216). The National Center for Atmospheric Research is sponsored by the National Science Foundation.