Scaling properties of velocity and temperature spectra above the surface friction layer in a convective atmospheric boundary layer

Abstract

Abstract.We report velocity and temperature spectra measured at nine levels from 1.42 meters up to 25.7 m over a smooth playa in Western Utah. Data are from highly convective conditions when the magnitude of the Obukhov length (our proxy for the depth of the surface friction layer) was less than 2 m. Our results are somewhat similar to the results reported from the Minnesota experiment of Kaimal et al. (1976), but show significant differences in detail. Our velocity spectra show no evidence of buoyant production of kinetic energy at at the scale of the thermal structures. We interpret our velocity spectra to be the result of outer eddies interacting with the ground, not "local free convection".We observe that velocity spectra represent the spectral distribution of the kinetic energy of the turbulence, so we use energy scales based on total turbulence energy in the convective boundary layer (CBL) to collapse our spectra. For the horizontal velocity spectra this scale is (z i o ) 2/3 , where z i is inversion height and o is the dissipation rate in the bulk CBL. This scale functionally replaces the Deardorff convective velocity scale. Vertical motions are blocked by the ground, so the outer eddies most effective in creating vertical motions come from the inertial subrange of the outer turbulence. We deduce that the appropriate scale for the peak region of the vertical velocity spectra is (z o ) 2/3 where z is height above ground. Deviations from perfect spectral collapse under these scalings at large and small wavenumbers are explained in terms of the energy transport and the eddy structures of the flow.We find that the peaks of the temperature spectra collapse when wavenumbers are scaled using (z 1/2 z

Mean scalar concentration profile in a sheared and thermally stratified atmospheric surface layer

“…The generation and the impinging mechanisms of large eddies onto the atmopsheric surface layer are diverse and vary with atmospheric stability. For experiments in which the terrain was flat and the surface cover was uniform, these mechanisms may include detached eddies generated by shearing motions in the neutral boundary layer, convective motion in the outer layer of the convective boundary layer, and attached eddies initiated by instabilities within the atmopsheric surface layer [8,[43][44][45]. Even for neutral conditions, laboratory studies have also documented the impingement of large (and very large) structures onto the 'logarithmic' region at high Reynolds number [46][47][48].…”

The BLLAST field experiment: Boundary-Layer Late Afternoon and Sunset Turbulence

“…In this context, van Driel and Jonker (2011), based on an idealized LES and 0-D model study of a non-stationary PBL, suggest considering the time it takes for the energy to travel from the surface up to the top of the boundary layer. McNaughton et al (2007), Sorbjan (2010Sorbjan ( , 2012 and Kumar et al (2006) also proposed new scalings that could be tested in the context of transitory phases, like the local Richardson number and Nieuwstadt scalings. A question that is still poorly understood is the following: how long does the CBL remain quasi-stationary during the AT, or, equivalently, for how long does the convective scaling apply as the surface flux decreases?…”

Surface layer similarity in the nocturnal boundary layer: the application of Hilbert-Huang transform

“…The surface layer is defined as the layer where shear-generated turbulence is predominant over buoyancy-generated turbulence. McNaughton et al (2007) emphasized such property by naming it as the surface friction layer. In our case, z/L was generally of the order of 1.…”