Turbulence velocity spectra dependence on the mean wind at the bottom of a valley

A spectral analysis of the surface boundary layer turbulence at the bottom of a valley is presented. The spectra were classified according to wind speed, wind direction with respect to the valley axis and stability condition. The results reveal that the vertical velocity spectra have a well defined peak in all cases. The average spectra are better defined for mean winds parallel to the valley axis. For transversal winds, a large scatter occurs. The peak of the vertical velocity spectra happens at smaller frequencies for winds parallel to the valley axis than when the winds are transversal to the valley. The stability dependence of the spectral peak frequency of the vertical component is larger for winds larger than 1 m s(-1). The spectra of the horizontal components show low-frequency peaks, probably associated to mesoescala exchange. The frequency peak of the horizontal spectra does not depend on stability for convective conditions. For stable conditions, the peak frequency increases as it gets more stable. Local isotropy occurs at large frequencies for both wind directions with respect to the valley axis. (C) 2009 Elsevier B.V. All rights reserved.