TOP-DOWN, BOTTOM-UP DIFFUSION EXPERIMENTS IN A WATER CONVECTION TANK

Large-eddy simulation (LES) results indicate that turbulent scalar dif fusion in the convective atmospheric boundary layer (CBL) has interest ing 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 replic a of the original Deardorff-Willis laboratory convection tank. The aut hors measured mean gradients and inferred flux profiles for two scalar s, temperature, and dye concentration. Using these data, vertical prof iles of the top-down, bottom-up gradient functions and eddy diffusivit ies are calculated. Experimental results for these profiles are in goo d agreement with the LES predictions.