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2.2.1 The Bottom Friction

Even in multi-level ocean models where there is an attempt to model in great detail the vertical structure, the effort to resolve boundary layers is usually confined to the surface mixed layers (see section 4 ``Mixed-Layer Models''), paying little attention to the so-called ``benthic'' or bottom boundary layer (i.e. the turbulent frictional layer along the ocean bottom). Generally, most ocean models resort to the application of some sort of drag law in the lowest layer to represent the turbulent frictional processes there. The use of lateral (i.e. horizontal) friction, on the other hand, is an almost universal constraint imposed on all hydrodynamic numerical models by their inability to follow the nonlinear cascade of energy below scales of the mesh size. This friction effectively prevents the pileup of energy in a numerical simulation in the wavelength range corresponding to the mesh size (the nonlinear terms in 3-D turbulence cause an energy cascade down toward high wave numbers or short length scales). The form of the bottom friction term can take either a linear or nonlinear form; both formulations have been used in the past in ocean models of various types.