Top-Down, Bottom-Up Diffusion Experiments in a Water Convection Tank

Abstract

Large-eddy simulation (LES) results indicate that turbulent scalar diffusion in the convective atmospheric boundary layer (CBL) has interesting properties. A scalar introduced into the bottom of the CBL with no flux through the top (bottom-up diffusion) has a radically different eddy diffusivity profile than a scalar introduced at the CBL top with zero flux through the surface (top-down diffusion). To test these LES results, a set of experiments was designed and carried out in a replica of the original Deardorff-Willis laboratory convection tank. The authors measured mean gradients and inferred flux profiles for two scalars, temperature, and dye concentration. Using these data, vertical profiles of the top-down, bottom-up gradient functions and eddy diffusivities are calculated. Experimental results for these profiles are in good agreement with the LES predictions.