3.6.4 Parameters: Tracer equations

This section covers the tracer equations i.e. the potential temperature equation and the salinity (for the ocean) or specific humidity (for the atmosphere) equation. As for the momentum equations, we only describe for now the parameters that you are likely to change. The logical variables tempDiffusion tempAdvection tempForcing, and tempStepping allow you to turn on/off terms in the temperature equation (same thing for salinity or specific humidity with variables saltDiffusion, saltAdvection etc.). These variables are all assumed here to be set to '.TRUE.'. Look at file model/inc/PARAMS.h for a precise definition.

initialization

The initial tracer data can be contained in the binary files hydrogThetaFile and hydrogSaltFile. These files should contain 3D data ordered in an (x,y,r) fashion with k=1 as the first vertical level. If no file names are provided, the tracers are then initialized with the values of tRef and sRef mentioned above (in the equation of state section). In this case, the initial tracer data are uniform in x and y for each depth level.

forcing

This part is more relevant for the ocean, the procedure for the atmosphere not being completely stabilized at the moment.

A combination of fluxes data and relaxation terms can be used for driving the tracer equations. For potential temperature, heat flux data (in W/m ) can be stored in the 2D binary file surfQfile. Alternatively or in addition, the forcing can be specified through a relaxation term. The SST data to which the model surface temperatures are restored to are supposed to be stored in the 2D binary file thetaClimFile. The corresponding relaxation time scale coefficient is set through the variable tauThetaClimRelax (in s). The same procedure applies for salinity with the variable names EmPmRfile, saltClimFile, and tauSaltClimRelax for freshwater flux (in m/s) and surface salinity (in ppt) data files and relaxation time scale coefficient (in s), respectively. Also for salinity, if the CPP key USE_NATURAL_BCS is turned on, natural boundary conditions are applied i.e. when computing the surface salinity tendency, the freshwater flux is multiplied by the model surface salinity instead of a constant salinity value.

As for the other input files, the precision with which to read the data is controlled by the variable readBinaryPrec. Time-dependent, periodic forcing can be applied as well following the same procedure used for the wind forcing data (see above).

dissipation

Lateral eddy diffusivities for temperature and salinity/specific humidity are specified through the variables diffKhT and diffKhS (in m /s). Vertical eddy diffusivities are specified through the variables diffKzT and diffKzS (in m /s) for the ocean and diffKpT and diffKpS (in Pa /s) for the atmosphere. The vertical diffusive fluxes can be computed implicitly by setting the logical variable implicitDiffusion to '.TRUE.'. In addition, biharmonic diffusivities can be specified as well through the coefficients diffK4T and diffK4S (in m /s). Note that the cosine power scaling (specified through cosPower--see the momentum equations section) is applied to the tracer diffusivities (Laplacian and biharmonic) as well. The Gent and McWilliams parameterization for oceanic tracers is described in the package section. Finally, note that tracers can be also subject to Fourier and Shapiro filtering (see the corresponding section on these filters).

ocean convection

Two options are available to parameterize ocean convection: one is to use the convective adjustment scheme. In this case, you need to set the variable cadjFreq, which represents the frequency (in s) with which the adjustment algorithm is called, to a non-zero value (if set to a negative value by the user, the model will set it to the tracer time step). The other option is to parameterize convection with implicit vertical diffusion. To do this, set the logical variable implicitDiffusion to '.TRUE.' and the real variable ivdc_kappa to a value (in m /s) you wish the tracer vertical diffusivities to have when mixing tracers vertically due to static instabilities. Note that cadjFreq and ivdc_kappacan not both have non-zero value.

Copyright © 2006 Massachusetts Institute of Technology

Last update 2011-01-09