Dense water flowing down a slope is at best only poorly represented in
z-level coordinate models. However, this situation is remedied within
MOM by enabling option bbl which parameterizes the bottom
boundary layer (hereafter referred to as BBL) by assuming frictional
dynamics within a thin layer that effectively ``hangs'' underneath
bottom-most ocean cells. Although the surface under which the BBL
cells are ``hung'' is disconnected due to changes in the number of
vertical levels given by 
,
the BBL cells are mapped onto a
continuous two dimensional surface so that each BBL cell is in
communication with its four nearest neighbors. For practical reasons,
this two dimensional surface is defined to be at constant depth within
level k=km. For the interior model, level k=km is treated as if it
were always land. Within the BBL model (level k=km), there are no
land cells ...only a two dimensional continuously connected lattice
of BBL cells. The BBL cells communicate vertically through advection
and diffusion with the bottom-most interior ocean cells above them.
Note that land cells may exist between the bottom-most ocean cell and
the BBL cell beneath it.
This configuration works well for steeply sloping topography but is inadequate in regions where the topographic slope is less than the grid aspect ratio. Refer to Chapter 26. Enabling option partial_cell resolves these mildly sloping regions and the BBL cells are then ``hung'' underneath partial-cells which allows the BBL to feel shallow as well as steep topographic slopes. As without option partial_cell, the BBL cells populate level k=km.
Just as with option partial_cell, the bbl option must be used right from the beginning of an experiment. It is not intended as an option which can be enabled in the middle of an experiment because it will wipe out all information in level k=km. For further information, refer to http://www.gfdl.gov/ a1g/bbl.html.