Top-down/bottom-up diffusion

From Glossary of Meteorology

top-down/bottom-up diffusion

An atmospheric boundary layer theory that splits turbulent transport into two components, one that is entrained into the boundary layer top and then diffused downward, and one that is injected from the earth's surface and brought upward into the boundary layer.

These two linearly decomposed components of a passive scalar assume that the top-down component has zero flux at the surface while the bottom-up component has zero flux at the top. This theory allows some aspects of vertical transport across a mixed layer to be modeled, even though the actual vertical gradient of the passive scalar is zero, which is often the case for vigorous buoyant convection.