Diffusivity is also sometimes known as the coefficient of diffusion. Diffusion occurs as a result of a gradient in the spatial distribution of mass concentration, temperature or momentum. The diffusivity may be very different in the vertical and horizontal directions. The construction vertical_X_diffusivity means the vertical component of thediffusivity of X due to motion which is not resolved on the grid scale of the model. "Due to tides" means due to all astronomical gravity changes which manifest as tides.No distinction is made between different tidal components. The specification of a physicalprocess by the phrase due_to_process means that the quantity named is a single term in a sum of terms which together compose the general quantity named by omitting the phrase.