Given that Aronnax does not (yet?) support vanishingly thin layers, we may not need a particularly complex algorithm to implement this feature.
Potential issues:
in n+1/2 layer mode there is no constraint on total thickness, so this will likely lead to simulations in which the model continues to accrue fluid
in n layer mode we will need to ensure that the fluxes balance so that there is no net change in volume
a naive implementation is likely to result in equilibrium solutions in which all layers have equal thickness (linear stratification), which is qualitatively different to the equilibrium in a z-level model, where each level would end up with the same value of density (vertically homogeneous profiles with no stratification)
In some situations it will be useful to represent the transfer of fluid from one layer to another by diapycnal (across density surfaces) diffusion.
Some examples of how other, more complex, models have implemented this idea:
Given that Aronnax does not (yet?) support vanishingly thin layers, we may not need a particularly complex algorithm to implement this feature.
Potential issues: