LOW-WAVE-NUMBER SPECTRAL CHARACTERISTICS OF VELOCITY AND TEMPERATURE IN THE ATMOSPHERIC SURFACE-LAYER

The structure df atmospheric surface layer turbulence low wavenumbers was analyzed using 56 Hz triaxial velocity and temperature measurement s above a uniform dry lake bed, A key feature of this experiment was t he, small roughness length of the surface that resulted in a small rou ghness Reynolds number. Under near-neutral atmospheric stability condi tions, a -1 power law was observed in both measured velocity and tempe rature spectra which is consistent with previously proposed dimensiona l analysis for rough and smooth turbulent boundary layer flows. The wa venumber at which the -1 power law terminates and the -5/3 power law c ommences was derived as a function of the Kolmgorov and von Karman con stants, Good agreement between the predicted and the measured transiti on. wavenumber from -1 to -5/3, was holed for fully rough-flow conditi ons. However, this was ndt the case fdr other roughness conditions. Th e similarity theory constants for the neutral case were determined and they compared favorably with other laboratory and field studies. For unstable atmospheric conditions, directional dimensional analysis was used to predict the slopes of the power spectra of temperature and vel ocity. It was demonstrated that for moderately unstable conditions, th e temperature and vertical velocity power spectra exhibited a -1 power law, but the longitudinal velocity exhibited a -2 power law. The agre ement between predicted and measured power laws was within experimenta l errors. Some differences between the constants determined from this experiment and other experiments are also discussed.