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.
