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New grids #172
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This issue has been mentioned on ACCESS Hive Community Forum. There might be relevant details there: https://forum.access-hive.org.au/t/cosima-working-group-announce/238/73 |
Hi Andrew, |
Good points to think about! Might be nice to bring this discussion up at a COSIMA weekly meeting? |
IInteresting questions. It is a rare opportunity to change the grid. Hard to know where and how much to invest in grid changes. Probably less modifications is best, considering the effort required to determine results. Item 4 above seems like a no-brainer modification though. |
Raising a new point here: @pwongpan and I realised that large tabular grounded iceberg D15 is not in the 0.25* grid (and presumably the .1 and 1.0 grids either). Even without cavities being implemented, I'd argue that D15 would be better treated as land pixels than open ocean pixels for the following reasons:
Can we consider prescribing this as land pixels for the OM3 grid? Obviously when cavities are implemented, it would make more sense to describe this as a cavity, but until then I think land is better than water. Just a discussion point - there may be ocean things I haven't thought of :) |
Out of interest, are there any grounded icebergs included in the current bathymetry? I think we just use bathymetry from GEBCO that doesn't have any icebergs in it? |
I suspect none. @dpath2o might know more |
Hi @adfraser @adele-morrison @dpath2o I would think it odd from a coupled model perspective to have the icebergs as land points in any new grid. For Dan's project it might be useful but he can put them in for that, but these grids will need to be used more widely. Upto now we havent been using icebergs in the grid, and we just had to redo the Antarctic because minor ice shelves weren't handled well in earlier bathymetry version. We need to improve things for the next iterations in access-om3/cm3 |
Thanks @ofa001 - yeah I realise they will need to be used widely but I think including D15 could be a special case for the "base" grid. Just a thought. |
The difference in area for some of these cells (as described in 126) seem fairly drastic, so this looks worth investigating further. From Bi et al 2013: In panel a), it looks like we would move the poles south-west to maximise the distance from the ocean. I suspect they will need to be 180° in longtitude apart for us not to break something.
In theory, moving this to the longitude to with the smallest length of ocean would reduce the amount of communication between MPI tasks but the effect would very marginal. It would be nice to have Australia in the middle of our plots :)
Its nice to have the equator on the edge of grid cells, just because its easier to understand I think (and maybe easier to analyse?)
Sounds like we have consensus on doing this, but leaving the ice shelves landmasked for now.
Coarsening in the Arctic could help with 1. ? But will refining the grid in the Antarctic force us into a shorter time-step also?
I guess we will keep these refinements for 1 degree (Bi et al 2013):
We should make sure the new grid files are cf-complaint. CMS have a compliance checker which would help: http://climate-cms.wikis.unsw.edu.au/CF_checker |
Hi @anton-seaice In realtion to the Antarctic, the choice of grid there in the BI et al "auscom" grid used in access-om as well was to be "square" so that n/s resolution was close to the e-w resolution as the we went further south, it might look a bit skewed on the plot. I would support additional resolution as its our region of interest, but we still need to keep some resolution in the Arctic (for both ice and ocean processes). But its less of a priority. |
Thanks :) Ill correct my post . |
On further thought on this point, by definition the location of the longitudinal seam is the longitude of one of the tripoles. From the Murray 1995 paper: The curves are constructed on a NH stereographic projection about foci, F1 and F2 , lying on either side of the N Pole (N), which is the origin of the projection (Fig. 9). The foci are situated at latitude phiF (...) and 90° east and west of the symmetry meridian, which is at longitude l0. The focal arc and the symmetry meridian define the x and y axes, respectively. The key point being the symmetry meridian (90° offset in longitude from the tripoles), which is continuous across the orthogonal and stereographic sections of the grid: The definition precludes us from defining the location of the longitudinal seam differently than the tripole, although there may not be a mathematical reason why the longitudinal seam and the tripoles need to be aligned in longitude. |
@adfraser Also just pointed out that moving the tripoles south will force the resolution in the Arctic to be lower, which is probably desirable for us. |
Apologies for the talking to myself: @pwongpan asks if we should consider IBSCO bathymetry in the Southern Ocean. It looks like the GEBCO_2023 has assimilated latest fairly up-to-date IBSCO already. From https://www.gebco.net/data_and_products/gridded_bathymetry_data/gebco_2023/: The SRTM15+ base grid has been augmented with the gridded bathymetric data sets developed by the four Seabed 2030 Regional Centers to produce the GEBCO_2023 Grid. The Regional Centers have compiled gridded bathymetric data sets, largely based on multibeam data, for their areas of responsibility. These regional grids were then provided to the Global Center. ... For the polar regions, complete grids were provided due to the complexities of incorporating data held in polar coordinates. The compilation of the GEBCO_2023 Grid from these regional data grids was carried out at the Global Centre, with the aim of producing a seamless global terrain model. |
As long as the new GEBCO has IBCSO2, not IBCSO1? IBCSO2 was a major update |
Yeah it does: see data_contributors and search Southern Ocean |
That's great then. I consider IBCSO2 the best available for the SO |
Thanks all. I don't know how complicated it is, but it would be great to have a platform/cookbook where we could regenerate the bathymetry based on the new bathymetric observations (by icebreakers, Expendable Bathythermograph etc.) from the grid of interest. |
@pwongpan these tools exist and have been used for past models (ACCESS-OM2, panan): https://github.com/COSIMA/domain-tools |
It is suggested in the ocean grid generator guide when doing this that the South Pole should be offset to maximise the distance from the Ice Shelves (which will lead to less tiny grid cells which could be ocean). The Ross Ice Shelf looks to extent past 85°S, so its probably worth considering. |
Hmm, yes, good point @anton-seaice. That would make analysis a pain doing zonal averaging though!!! |
It could/should be possible to start the 'displaced pole' section at say 78°S, so for most configurations it would all be within the landmask. |
The number of truncations in year 4 is lesser than year 1.
The default value is zero, but in years 3, 4 and 5, the maximum number of daily truncations is around 10, which consistently occurs in August. |
OK that makes sense. If the truncations mostly occur at the same location in Hudson Bay we should probably edit the topography there. If we need to do a lot of this sort of editing it could be worthwhile getting |
It would be worthwhile inspecting the GEBCO bathymetry in that location to decide how to edit the topography, e.g. like we did here. |
Maybe comparing to the GFDL OM5 bathymetry in Hudson Bay would be helpful? |
Could also compare to the original OM2 bathymetry |
I apologise for the delay in getting back to this issue. I was occupied with the truncation file issue and wanted to confirm that the truncations are indeed happening at Hudson Bay, and they are consistent with the hourly output. I’ve opened an issue to report the bug in MOM6 regarding the truncation files NOAA-GFDL/MOM6 Issue #736. The truncations are happening at a cell in Hudson Bay with a depth of 20m in the new topography. In the GEBCO dataset, this area is 100m deep, whereas in the OM2 topography, the strait is closed. Given our discussion during the meeting, we agreed to close it.
@gustavo-marques, Changing MAXTRUNC isn't affecting the solution |
@aekiss As mentioned in the previous posts, In the new bathymetry, the depth at the crashing location is around 20m, while it's 100m in GEBCO 2024. I also noticed that Hudson's Bay is removed in the OM2 bathymetry—why was this done, and how can I replicate it? Would deepening the 20m strait to match GEBCO 2024 help? Also, the test run with 75 vertical levels crashed at the same location due to truncation errors, just like with 50 levels. |
It was probably done because that area created instabilities, but not sure that was documented at the time. |
Looks like the narrowness of the channel meant the gridcell-average of GEBCO was reduced to 20m by including regions of land. You could try increasing to 100m, but this is probably an unimportant channel (unless it carries a lot of runoff) so could be filled in. |
The OM2 topo includes Hudson Bay - I guess you meant Foxe Basin, north of Hudson Bay? |
More importantly, it looks like the new topography is missing these seas which are present in OM2:
I guess deseas is removing them? These straits might also need opening:
It would be a good idea to plot the land mask difference from OM2 to spot any others, particularly in the Indonesian Archipelago. |
Would also be a good idea to plot the range of wet cell sizes and choose the land mask to eliminate excessively small cells, e.g. Gulf of Ob - see https://github.com/COSIMA/ACCESS-OM2-1-025-010deg-report/blob/master/figures/grid/grid.ipynb |
These edits would be most easily and reproducibly done with |
Hudson Bay or Foxe Basin area its in north might have been adjusted as it is close to the tri-pole, but yes we need Red Sea, and Gulf, Black Sea is a tricky one depending on resolution as the link to Med is through a very narrow strait. |
This is slightly worrying non? Like the topo generation should be robust to this and only look at ocean cells ? I had a quick look in gen_topo, but it would take some more time to figure out if it has awareness of land masks. |
@ezhilsabareesh8 I've fixed |
Thanks @aekiss, I will try to do the hand edits at crash locations.
Yes deseas is removing them. Here is the topography generated without deseas, will have a look at land mask difference. |
The current OM2 0.25 deg topo This uses I expect we'll want to retain many of the edits in |
@aekiss are you saying we should dive straight in and deepen Denmark Strait etc now? You don’t think it’s worth having a look first to see if there’s an issue with the transport to see if it’s needed? I guess I’m optimistic some things might be improved with different model physics, and the updated GEBCO product? Perhaps at least we could check if the same edits in topog_edits.txt have also been made in the OM5 topography before diving in? |
It's a bit complicated. The current OM2 0.25 deg topo
So the topography and land mask are modified (and modify each other) at multiple steps, with many of the largest topography changes actually being imposed via the land mask. We'll need to think about which of these changes we want to inherit into OM3. Many of the edits at straits etc are needed to correctly reproduce the depths of unresolved sills and I expect would be a better starting point than 0.25° averages of the new GEBCO. Filling in the ocean for small cells near the tripoles is also likely to be a good idea to get CFL stability with a decent timestep. |
This issue has been mentioned on ACCESS Hive Community Forum. There might be relevant details there: https://forum.access-hive.org.au/t/cosima-twg-meeting-minutes-2024/1734/19 |
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Looks like we'll need to modify |
@aekiss I have updated the However, I am unclear about why the minimum T-cell size is set to 6 km for regions north of 60°N and 11.7 km for regions south of 60°S. In addition, I have regridded the OM2 topography onto the new grid and plotted the mask differences. It appears that additional land has been added near the tripole region, particularly close to Kara Strait, when the T-cell size is limited. I also noticed that the Caspian Sea is not masked, but I believe this will be corrected once deaseas processing is applied. Topography after cutting off T cells of size less than 6 km: Land mask difference compared to OM2: (Blue: Ocean added, Yellow: Land added) |
The SH cutoff is due to Mercator scaling of dyt only being applied between 65S and 65N, so dyt is constant south of 65S. So the 11.7 km SH limit is due to the grid itself, not the topography. |
I think it would be good to have Kara Strait open, as in OM2 - maybe use a cutoff slightly smaller than 6km? |
I agree with @aekiss that keeping Kara Strait open may be a good idea it looked like a lot of additional land had been added to close it off, and we are now on a C-gris so it doesn't need to be as many points wide to have a realistic flow through. |
Before redoing the topography (#68), we should take the opportunity to check whether the grid can be improved.
e.g. consider
Does anything else spring to mind?
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