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Next: 3.16.8 Tracer equations
Up: 3.16 Customizing MITgcm
Previous: 3.16.6 Equation of state
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In this section, we only focus for now on the parameters that you are
likely to change, i.e. the ones relative to forcing and dissipation
for example. The details relevant to the vector-invariant form of the
equations and the various advection schemes are not covered for the
moment. We assume that you use the standard form of the momentum
equations (i.e. the flux-form) with the default advection scheme.
Also, there are a few logical variables that allow you to turn on/off
various terms in the momentum equation. These variables are called
momViscosity, momAdvection, momForcing, useCoriolis,
momPressureForcing, momStepping and metricTerms and are
assumed to be set to '.TRUE.' here. Look at the file
model/inc/PARAMS.h for a precise definition of these
variables.
- initialization
-
The velocity components are initialized to 0 unless the simulation
is starting from a pickup file (see section on simulation control
parameters).
- forcing
-
This section only applies to the ocean. You need to generate
wind-stress data into two files zonalWindFile and
meridWindFile corresponding to the zonal and meridional
components of the wind stress, respectively (if you want the stress
to be along the direction of only one of the model horizontal axes,
you only need to generate one file). The format of the files is
similar to the bathymetry file. The zonal (meridional) stress data
are assumed to be in Pa and located at U-points (V-points). As for
the bathymetry, the precision with which to read the binary data is
controlled by the variable readBinaryPrec. See the matlab
program gendata.m in the input directories under
verification to see how simple analytical wind forcing data
are generated for the case study experiments.
There is also the possibility of prescribing time-dependent periodic
forcing. To do this, concatenate the successive time records into a
single file (for each stress component) ordered in a (x,y,t) fashion
and set the following variables: periodicExternalForcing to
'.TRUE.', externForcingPeriod to the period (in s)
of which the forcing varies (typically 1 month), and
externForcingCycle to the repeat time (in s) of the forcing
(typically 1 year - note: externForcingCycle must be a
multiple of externForcingPeriod). With these variables set
up, the model will interpolate the forcing linearly at each
iteration.
- dissipation
-
The lateral eddy viscosity coefficient is specified through the
variable viscAh (in ms). The vertical eddy
viscosity coefficient is specified through the variable
viscAz (in ms) for the ocean and
viscAp (in Pas) for the atmosphere. The
vertical diffusive fluxes can be computed implicitly by setting the
logical variable implicitViscosity to '.TRUE.'.
In addition, biharmonic mixing can be added as well through the
variable viscA4 (in ms). On a spherical polar
grid, you might also need to set the variable cosPower
which is set to 0 by default and which represents the power of
cosine of latitude to multiply viscosity. Slip or no-slip conditions
at lateral and bottom boundaries are specified through the logical
variables no_slip_sides and no_slip_bottom. If
set to '.FALSE.', free-slip boundary conditions are
applied. If no-slip boundary conditions are applied at the bottom, a
bottom drag can be applied as well. Two forms are available: linear
(set the variable bottomDragLinear in s) and
quadratic (set the variable bottomDragQuadratic in
m).
The Fourier and Shapiro filters are described elsewhere.
- C-D scheme
-
If you run at a sufficiently coarse resolution, you will need the
C-D scheme for the computation of the Coriolis terms. The
variable tauCD, which represents the C-D scheme coupling
timescale (in s) needs to be set.
- calculation of pressure/geopotential
-
First, to run a non-hydrostatic ocean simulation, set the logical
variable nonHydrostatic to '.TRUE.'. The pressure
field is then inverted through a 3D elliptic equation. (Note: this
capability is not available for the atmosphere yet.) By default, a
hydrostatic simulation is assumed and a 2D elliptic equation is used
to invert the pressure field. The parameters controlling the
behaviour of the elliptic solvers are the variables
cg2dMaxIters and cg2dTargetResidual for
the 2D case and cg3dMaxIters and
cg3dTargetResidual for the 3D case. You probably won't need to
alter the default values (are we sure of this?).
For the calculation of the surface pressure (for the ocean) or
surface geopotential (for the atmosphere) you need to set the
logical variables rigidLid and implicitFreeSurface
(set one to '.TRUE.' and the other to '.FALSE.'
depending on how you want to deal with the ocean upper or atmosphere
lower boundary).
Next: 3.16.8 Tracer equations
Up: 3.16 Customizing MITgcm
Previous: 3.16.6 Equation of state
Contents
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