Vertical diffusion

The vertical diffusion terms are expressed for an arbitrary variable, $\varphi$, using a second order approximation for various vertical coordinates as

$$D_{z\,\varphi} = {\frac{1}{\rho}}{\frac{\partial}{\partial z}} \l... ...\partial z}}}\right) = {\frac{1}{\rho}}{\frac{\partial F_\varphi }{\partial z}}$$ (105)

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.