Turbulence Spectra and Dissipation Rates in a Wind Tunnel Model of the Atmospheric Convective Boundary Layer

Abstract

A model of the atmospheric convective boundary layer (CBL) is realized in the thermally stratified wind tunnel of the Institute of Hydrology and Water Resources, University of Karlsruhe. Further experimental results from this model are presented. The wind tunnel with a test section 10 m long, 1.5 m wide, and 1.5 m high allows one to generate a quasi-stationary, horizontally evolving CBL, characterized by convective Richardson numbers Ri?T up to 10 and RiN up to 20, with the bottom shear/buoyancy dynamic ratio u*/w* in the range of 0.2 to 0.5. The convective regime in the tunnel is dominated by bottom-up forcings. Effects of entrainment in the simulated CBL play a secondary role. The spectra of turbulence in the wind tunnel flow are calculated from high-resolution velocity component and temperature time series, simultaneously measured using laser Doppler and resistance-wire technique, respectively. The spectra from the mixed core of the CBL and in the entrainment zone display pronounced inertial subranges. The ratio of vertical to horizontal velocity spectra in these subranges is within the interval from 1.3 to 2, which is slightly larger than could be expected for purely isotropic turbulence. Different maxima in the production ranges of the spectra are related to dominant turbulence scales in the wind tunnel flow. The energy-containing ranges of the wind tunnel spectra exhibit plateaulike shapes resulting from modification of the turbulence production by the flow shear. The comparison with atmospheric spectra and spectral data from water tank and large eddy simulation studies of the CBL suggests that the turbulence spectral regime in the tunnel flow has much in common with its atmospheric prototype. The turbulence kinetic energy dissipation rate and the destruction rate of temperature fluctuations are evaluated based on the relationships resulting from the Kolmogorov theory for the inertial-subrange spectra. The dissipation rates obtained are within the scatter ranges of data from atmospheric measurements and model studies of convection. High dissipation values in the lower portion of the simulated CBL are indicative of the shear enhancement of turbulence production in the wind tunnel convective flow.