Top-Down and Bottom-Up Diffusion of a Scalar in the Convective Boundary Layer

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.