The vertical diffusion terms are expressed for an arbitrary variable, , using a second order approximation for various vertical coordinates as
The upper-boundary fluxes (top-of-atmosphere) are zero, while the lower boundary fluxes are determined by the water, frozen-water or land surface models (or an average of the fluxes from these different surfaces or sub-grid tiles). The diffusivity and land surface flux are described in more detail in the Physics section.