Turbulence spectra above the surface friction layer in a convective boundary layer

Turbulence measurements were made over a smooth and extensive playa surface at the SLTEST site in Western Utah in May and June 2005. Sonic anemometers were located at 9 levels from 1.42 to 25.7 meters above the surface. Here we report velocity spectra from highly convective periods when the modulus of the Obukhov length was less than 2 meters and winds were from the North, in which direction the surface was uniform for about 100 kilometers. In these conditions most of our instruments were above the surface friction layer. Our `outer' spectra are generally consistent with results for the `free convection layer' obtained at Kansas and Minnesota, but they display some highly significant details that have not been noted previously. We found that the outer spectra of horizontal velocity components are independent of height and may be reduced to a universal form by scaling them using (z_iε_o)^2/3, where z_i is inversion height (estimated from the spectral peak of the top anemometer) and ε_o is the outer dissipation rate. We also found that vertical velocity spectra displayed a universal form in the region of the spectrum peak when plotted against normalized frequency fz/U and scaled using (z ε_o)^2/3. At frequencies lower than the peak frequency we found scaled spectral energy to decrease more rapidly with increasing height. We interpret our results as showing that the dominant turbulence process at these levels is not `free convection', but is the mechanical action of eddies from the outer inertial sub-range interacting with the ground. We also deduce that the outer convective velocity scale should be (z_iε_o)^2/3, not the Deardorff velocity scale. These results are consistent with the local dynamics being constrained by their embedding in the whole flow, which is to say the whole turbulent flow in the convective boundary layer behaves as a single, self-organizing system.