The Averaged Hydrostatic Boussinesq Equations in Generalized Vertical
Coordinates
Abstract
Due to their limited resolution, numerical ocean models need to be
interpreted as representing filtered or averaged equations. How to
interpret models in terms of formally averaged equations, however, is
not always clear, particularly in the case of hybrid or generalized
vertical coordinate models. We derive the averaged hydrostatic
Boussinesq equations in generalized vertical coordinates for an
arbitrary thickness weighted-average. We then consider various special
cases and discuss the extent to which the averaged equations are
consistent with existing model formulations. As previously discussed,
the momentum equations in existing depth-coordinate models are best
interpreted as representing Eulerian averages (i.e., averages taken at
fixed depth), while the tracer equations can be interpreted as either
Eulerian or thickness-weighted isopycnal averages. Instead we find that
no averaging is fully consistent with existing formulations of the
parameterizations in semi-Lagrangian discretizations of generalized
vertical coordinate ocean models. Perhaps the most natural
interpretation of generalized vertical coordinate models is to assume
that the average follows the model’s coordinate surfaces. However, the
existing model formulations are generally not consistent with
coordinate-following averages, which would require “coordinate-aware”
parameterizations that can account for the changing nature of the eddy
terms as the coordinate changes. Alternatively, the model variables can
be interpreted as representing either Eulerian or (thickness-weighted)
isopycnal averages, independent of the model coordinate that is being
used for the numerical discretization. Existing parameterizations in
generalized vertical coordinate models, however, are usually not fully
consistent with either of these interpretations. We discuss what changes
are needed to achieve consistency.