Mellor, G. L., and A. F. Blumberg, 1985: Modeling vertical and horizontal diffusivities with the Sigma Coordinate System. Monthly Weather Review, 113 (8), 1379-1383.

Abstract: The use of diffusive terms in numerical ocean models is examined relative to different coordinate systems. The conventional model for horizontal diffusion is found to be incorrect when bottom topographical slopes are large. A new formulation is suggested which is simpler than the conventional formulation when transformed to a sigma coordinate system and makes it possible to model realistically both surface Ekman and bottom boundary layers.