At the ocean bottom, z=-H, there can be no flux of heat or salt:
There can also be no mass flux through the bottom leading to
where subscript b denotes the lower boundary.
The remaining boundary conditions are
where and are the shear stress components at the bottom and is the friction velocity at the bottom. The bottom stresses are determined by matching the velocities , at the bottommost grid point to a logarithmic law of the wall:
where the drag coefficient is determined from log-law:
is the roughness scale, of the order of a centimeter or so for moderately rough bottom. Prescription (63) and (64) assumes that the bottommost grid point is in the log layer. If the model vertical resolution is not adequate to satisfy this condition, the drag coefficient is set to a value of 0.0025, so that a conventional quadratic drag law results. The model algorithm chooses the larger of this value and the value given by equation (64), to satisfy this prescription.