Statistics of Conservative Scalars in the Convective Boundary Layer

We represent the three-dimensional, time-dependent field of a passive, conservative scalar in the conservative boundary layer as the sum of “top-down” and “bottom-up” components created by the scalar fluxes through the top and bottom, respectively. A simple similarity hypothesis for these component fields allows us to extract their statistics from large-eddy simulations of the boundary layer. We find that the top-down and bottom-up fields are strongly correlated, but that their statistics are quite different. Analysis of the budgets of their fluxes and variances shows clearly some of the differences between top-down and bottom-up diffusion. We present simple formulas for scalar variance profiles which are in good agreement with observations.