Awesome, I browsed through Lellouche et al. 2018 again and included the following points in the email to the helpdesk:
*) In Lellouche et. al (2018) [3] it says:
“The physical configuration is based on the tripolar ORCA12 grid type (Madec and Imbard, 1996) with a horizontal resolution of 9 km at the Equator, 7 km at Cape Hatteras (midlatitudes) and 2 km toward the Ross and Weddell seas.”
[5] is not available for free without subscription, but the abstract describes the mesh. Is there any non-conservative interpolation involved to generate the data that is downloadable via this portal? If yes, is there access to the variables on the original mesh so we can do more accurate mass/volume balances? Or was there some kind of conservative interpolation that would allow a mass/volume balance to be accurate on the provided 1/12 degree grids?
*) In Lellouche et. al (2018) [3] it says:
“A “partial cell” parameterization (Adcroft et al., 1997) is chosen for a better representation of the topographic floor (Barnier et al., 2006) and the momentum advection term is computed with the energy- and enstrophy-conserving scheme proposed by Arakawa and Lamb (1981).”
This looks like it could impact volume balances. Again, would this require access to the original mesh like the above point?
*) In Lellouche et. al (2018) [3] it says:
“Z coordinates are used on the vertical; the 50-level vertical discretization retained for this system has a decreasing resolution from 1 m at the surface to 450 m at the bottom and 22 levels within the upper 100 m.”
Is there any vertical stretching as described in [2]?
In the following see the entire ticket including references.
Hi, I’m using the CMEMS Mercator forecast/analysis product and I would like to make a calculation of barotropic volume fluxes. If I understand the paper by Lellouche et al. ([3]), I think the product is based on NEMO 3.1.
Currently I’m trying to calculate the volume flux into the northern Bay of Bengal, northward of the zonal transect at 17 degrees north. I’m currently assuming that the mass/volume balance is:
dV/dt = \int_{transect} v dA + P - E + R
where
*) dV/dt is the change of volume due to free surface movements (variable ‘zos’ in the CMEMS Mercator notation)
*) v is the meridional velocity and the integral is over the transect at 17 north.
*) P and E are precipitation and evaporation, respectively (see Eq. 6.3 in the Nemo manual [1])
*) R is the river runoff (which may be significant in the region, the runoff of the GBM delta alone has a climatological peak on the order of 0.1 Sv in late summer)
For velocity, I’m using daily averaged 3d meridional velocity (‘vo’ in the CMEMS Mercator notation). For ‘zos’ (free surface) I’m using daily averages too. The grid spacings are given by the e1t, e2t and e3t variables and the masks are available too from the static files ‘GLO-MFC_001_024_coordinates.nc’ and ‘GLO-MFC_001_024_mask_bathy.nc’.
For E and P, I’m using ERA5 data. For R, I’m using Dai and Trenberth 2002 data.
The problem is that the the volume flux through the boundary (first term on the RHS above, \int_{transect} v dA seems much too large, much larger than all other terms combined. I’m getting volume fluxes on the order of 1 Sv throughout the first three months of 2020. If i have done the calculation correctly, this would be equivalent to an absurd 5 m sea level rise in the domain within 3 months. Am I being naive about how to calculate this? Is it actually possible to do this type of calculation with daily averaged data? I’m aware that wave induced mean flow can in theory be a problem, but I guess this would be orders of magnitude smaller?
I have tried to stretch the vertical coordinate according to Eq. 2.24 in [2], although I have no clue which vertical coordinate the Mercator analysis/forecast product actually uses (the information is not in the user manual, unless I’ve missed it, but see the excerpt from Lellouche et al. (2018) below).
Here are some considerations that have been brought up in a nemo-ocean.discourse thread [4] where I asked the same question:
*) In Lellouche et. al (2018) [3] it says:
"The physical configuration is based on the tripolar ORCA12 grid type (Madec and Imbard, 1996) with a horizontal resolution of 9 km at the Equator, 7 km at Cape Hatteras (midlatitudes) and 2 km toward the Ross and Weddell seas."
[5] is not available for free without subscription, but the abstract describes the mesh. Is there any non-conservative interpolation involved to generate the data that is downloadable via this portal? If yes, is there access to the variables on the original mesh so we can do more accurate mass/volume balances? Or was there some kind of conservative interpolation?
*) In Lellouche et. al (2018) [3] it says:
"A “partial cell” parameterization (Adcroft et al., 1997) is chosen for a better representation of the topographic floor (Barnier et al., 2006) and the momentum advection term is computed with the energy- and enstrophy-conserving scheme proposed by Arakawa and Lamb (1981)."
This looks like it could impact volume balances. Again, would this require access to the original mesh like the above point?
*) In Lellouche et. al (2018) [3] it says:
"Z coordinates are used on the vertical; the 50-level vertical discretization retained for this system has a decreasing resolution from 1 m at the surface to 450 m at the bottom and 22 levels within the upper 100 m."
Is there any vertical stretching as described in [2]?
Thanks so much for your kind help!
Regards, Stefan
[1] https://www.nemo-ocean.eu/doc/node38.html
[2] https://www.nemo-ocean.eu/doc/node9.html
[3] Lellouche, J. M., Greiner, E., Le Galloudec, O., Garric, G., Regnier, C., Drevillon, M., … & Le Traon, P. Y. (2018). Recent updates to the Copernicus Marine Service global ocean monitoring and forecasting real-time 1∕ 12 high-resolution system. Ocean Science, 14(5), 1093-1126.
[4] https://nemo-ocean.discourse.group/t/computing-barotropic-flux-through-a-transect-from-daily-averaged-output
[5] Madec, G. and Imbard, M.: A global ocean mesh to overcome the North Pole singularity, Clim. Dynam., 12, 381–388, 1996.
