The atmospheric boundary layer under stable stratification (Stable Boundary Layer, here called as SBL) is difficult to describe and model. Sometimes the turbulence in SBLs with strong stability is intermittent and patchy, allowing the upper portions of the boundary layer to decouple from surface forcings. The turbulence structure and transport process of SBL with strong stability have not yet been fully clarified, because of the difficulties of measurement and the complexity associated with unsteadiness, non-uniformity and sensitivity to terrain slope of SBL. In the present study, we have developed a simulation method for SBL by using a specially designed thermally stratified wind tunnel. To produce thermally stratified flows, the tunnel is equipped with two independent temperature systems, which consist of an airflow heating unit and a floor temperature-controlling unit. We have investigated the turbulence phenomena of SBLs for a wide range of stability, particularly focusing on the effects of strong stratification on turbulent boundary layers.

　　In parallel with wind tunnel experiments, to understand the turbulence features and fluid dynamics in detail, we have also performed numerical simulations of SBL under the boundary conditions similar to those in the wind tunnel experiments. The numerical studies based on a finite-difference method (FDM) are direct Navier-Stokes simulations without any turbulence model (DNS). The number of grid points in the x, y and z-directions are 601x101x91 in a Cartesian grid. Under the Boussinesq approximation, the governing equations consist of the Navier-Stokes, continuity and energy equations for 3D incompressible stratified flow.

Results

A type of stably stratified flows, in which the mean temperature increases upward almost linearly, is successfully created by heating the wind tunnel airflow and by cooling the test-section floor. The Reynolds number, Re, based on the boundary layer thickness, h, ranges from (2.9 - 5.3)x10000 and the bulk Richardson number, Ri(=(g/To)・(Ta-Ts)h/UaxUa), ranges from 0 to 1.38. Here, Ua and Ta are the ambient air speed and temperature, Ts and To are the surface and reference temperatures. Stable stratification rapidly suppresses the fluctuations of streamwise velocity and temperature as well as the vertical velocity fluctuation. Momentum and heat fluxes are also significantly decreased with increasing stability and become nearly zero over the whole boundary depth of the boundary layer with very strong stability. From the flow visualization in both wind tunnel experiment and DNS, wave-like motions driven by buoyancy and waves due to the Kelvin-Helmholtz instability can be observed locally and intermittently in a SBL flow with very strong stability. DNS results show that the occurrence of the shear instability depends on the local Richardson number in the SBL flow. Moreover, the cross-spectral analysis of velocity and temperature fluctuations suggests that internal waves also occur in the upper part of SBLs. Thus, for SBL flows with very strong stratification, the flow structure is entirely different from SBLs with weak stability, showing the coexistence of turbulence and waves.