| 17 Sep 2025
Revisiting the parameterization of dense water plume dynamics in geopotential coordinates in NEMO v4.2.2
Abstract. Based on analytical study of the physics of dense overflow plumes, we suggest a new version of the bottom boundary layer parameterization of dense water plumes for geopotential vertical coordinates models. This parameterization is implemented in the NEMO ocean engine, based on a modification of the already existing tools to represent dense water plumes, and for the case of dense plumes that can be considered as geostrophic or quasi-geostrophic flows. A test case is designed to test the performance of the parameterization on a critical area of the North Atlantic Ocean, the Iceland-Scotland ridge, that includes the Faeroe Bank Channel. A comparison with the existing parameterizations already implemented in the NEMO ocean engine show that our approach increases the bottom density along the plume pathway. A sensitivity experiment of the different possible stages of this new parameterization shows their gradual effects. The addition of a downslope bottom pressure gradient term in the primitive equations is its most important feature to boost the bottom dense plume advection, although the parameterization of the downslope advection of the dense plume also contributes.
This is an interesting and pragmatic approach to improving the parameterisation of dense water flows in NEMO. The evidence of a significant improvement is not definitive but there is sufficient to encourage further investigation.
The premise of the paper is to use an analytical model of plume dynamics to derive a parameterisation. The assumptions underlying the analytical model are discussed but the suitability of this approach in a wider range of applications is not made clear. For example, at what model resolution do the assumptions breakdown even for the same IFR/FBC area?
The parameterisation adds momentum to the model and the authors should consider implications for energy balance diagnostics.
The implementation is unlikely to have impacted the computational performance of the model greatly but the authors should specify the cost of adding their parameterisation. The results sections appear to have been rushed and could be presented better to improve readability (see comments below). The code changes appear well-contained and using a recent enough version of NEMO that replicating this work in the latest releases should be straight-forward.
line 50 'only this' instead of 'this only'
line 74 'the conclusions of which' instead of 'and which conclusions'
line 83 Not sure turbulence is the best example here. Turbulence closures often include empirical relationships derived from observations and laboratory experiments and are not simply analytical studies.
line 114 'from which' instead of 'which yields'
Paragraph starting line 172. I find the discussion of u and v as alongslope and downslope velocities confusing in this section relating to the model grid. Why should the slope always be aligned such? Surely, any real world application could have the roles reversed or any orientation in between?
Section 3.1
The sensitivity experiment is of insufficient length to evaluate the effect of the parameterisation in the target application of climate forecasting. Much longer tests in a global domain featuring all of the major overflows will be required before the universality of the approach and any impact can be assessed.
However, the tests do demonstrate some improvement and are a reasonable first step. The presentation needs to be improved however. Simple steps such as renaming DIFF as BD_DIFF (to make easier association with Beckman and Döscher) and ADV1 as CG_ADV1 (to make easier association with Campin and Goose) for example, would help. Figure 4 does not make for an easy comparison between the schemes and would benefit from addition panels showing DIFF-NOBBL, ADV1-NOBBL and ADV2-NOBBL.
Likewise, the comparisons of sections in figure 5 would benefit from a supplementary figure with columns NOBBL, DPLUME-FULL minus NOBBL, DIFF minus NOBBL, ADV1-NOBBL and ADV2 minus NOBBL
line 256 'a slight density increase' instead of 'a light density increase'
I see little value in appendix A as it stands. The only comparison is between NOBBL and BOM which achieves no new insights. It is already known that sigma coordinate models better preserve dense overflows (often too well). Unless equivalent comparisons with DPLUME-FULL are available, this appendix, and references to it, should be removed.