Dear All,
Based on this paper:
https://link.springer.com/content/pdf/10.1023/A:1011520432200.pdf
I am wondering whether the BBL parameterisations implemented in Nemo are actually totally physical.
It does make sense to open gates between non-adjacent grid cells located at bathymetry jumps if there is downward density gradient, but to me 2 things are actually missing:
1/ The downslope pressure gradient which includes the bathymetry gradient (i.e.: dH/dx) is nowhere to be seen in the model (or is it ?). And for good reasons since it is a pressure gradient that is not between adjacent grid cells, and can be even between grid cells with more than one k indice difference. But I think it is possible to compute it based on the tools already existing in trabbl, and include it in dynhpg. In a geostrophic or quasi-geostrophic flow, this pressure gradient creates a strong alongslope source of momentum that does not exist right now, and (perhaps) prevents the alongslope advection of dense plumes.
2/ The real downslope plume advection is actually a balance between friction terms (including the Ekman layer), and geostrophy. I don’t think it can be computed by the model, but it can be parameterised based on the paper mentioned above, and replace the advection velocity in the BBL parameterisation, or its equivalent diffusion coefficient.
The point here, dense water plumes do not actually go down very much, unless the flow is critical or super-critical, their rate of descent (ratio between downslope and alongslope velocities for the plume) is actually less than 5%.
These are my thoughts, I would be interested in any feedback on this topic.
Kind regards,
Robinson