Wind tunnel model study of turbulence regime in the atmospheric convective boundary layer

Results from a wind tunnel model of the sheared atmospheric convective boundary layer (CBL) are presented and discussed. The model is realized in the thermally stratified wind tunnel of the Institute of Hydrology and Water Resources Planning (IHW), University of Karlsruhe. Design of the wind tunnel, and the technique employed for velocity and temperature measurements are described. The performed experiments reproduce convective conditions in the atmosphere when both capping inversion and hydrostatic stability of the free-atmosphere air are not very strong. Under such conditions, the turbulence regime in the CBL is mainly determined by the bottom-up convective and shear forcings, whereas the heat flux of entrainment and temperature fluctuation magnitude within the inversion layer are comparatively small. Temporal averaging has been employed for deriving turbulence statistics from the velocity and temperature time series measured. These statistics are compared with their counterparts from atmospheric measurements, and from numerical and laboratory model studies. Surface shear and elevated shear were found to be essential factors modifying the CBL turbulence regime. The elevated shear turned out to be an inhibitor of entrainment at the CBL top. Turbulence enhancement has been observed in the transition zone between the unmixed convective layer in the neutral environment and the well-mixed CBL capped by a temperature inversion. The turbulence spectra obtained display short but pronounced inertial intervals in large-wavenumber ranges. The characteristic value of the turbulent Reynolds number Re-l in the simulated CBL is 3.10(3). The vertical to horizontal velocity spectral ratio is slightly higher than the local-isotropy value of 4/3. Velocity spectra measured in the lower portion of the CBL show the buoyancy to be the dominant factor of turbulence production at smaller wavenumbers, while the shear contribution increases with the wavenumber growth. The combined effect of buoyant and shear forcings apparently leads to elongation and flatness of the production ranges in the velocity spectra. The inertial-subrange theory relationships have been used for evaluation of the turbulence dissipation rates from the velocity spectra. The turbulence kinetic energy dissipation rates in the CBL with shear were found to be comparatively large over the whole turbulized region.