PPM advection#419
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There was some code duplication in the implementation of PPM tracer advection, which was eliminated in favour of adding an additional conditional check.
This scheme uses slope data from neighbouring cells to calculate the edge values that are used for constructing the interpolating parabola within a cell. This differs from the Huynh (PPM:H3) scheme in that the stencil is larger.
The non-Huyhn advection reconstruction requires a larger stencil to compute interface values. To ensure the group pass mechanism updates when the stencil is invalid, we change the stencil value to 3 for this advection method.
This is an incomplete introduction to the tracer advection algorithm in MOM6. It tries to cover aspects of the flux advection algorithm, including numerical considerations, as well as the subcell reconstruction methods that are used for calculating the actual fluxes.
iangrooms
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May 12, 2026
Fix how fluxes sent to MARBL are treated when DT_THERMO is bigger than coupling interval
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Here's all that I think is required to implement the PPM reconstruction for tracer advection. We basically use the limited slope from adjacent cells (which was already being calculated for PLM) to determine estimates for interface values, which slightly increases the stencil requirement.
I don't know that adding another boolean flag is the most elegant method of switching between schemes. If there are further reconstructions planned, maybe an ALE-like enum would be more appropriate.
I've also started documentation for tracer advection in a .dox file, as is becoming the norm for the rest of the repository.