Abstract

The vertical diffusion of a passive conservative scalar through the convective PBL is actually the superposition of what we call `top-down' and `bottom up' processes. These component processes are driven by scalar fluxes at the mixed-layer top and bottom. We argue that the vertical asymmetry in the buyoant production of the turbulent kinetic energy should cause the top-down and bottom-up diffusivities to differ. If so, we show that the diffusivity for the total process is poorly behaved; it depends on the scalar flux profile and can have singularities.We present results from large-eddy simulations which confirm that the top-down and bottom-up diffusivities are different, the latter being substantially larger. We present simple parameterizations for these diffusivities and the corresponding mean gradients. The results are useful in `unmixed layer' models and in Fielder' integral closure for scalar flux.