Check Geostrophic_Velocities_from_Sea_Level.ipynb

[julia-neme]

This one was comparing geostrophic surface velocities from sea level to those of the first cell instead of usurf vsurf, but otherwise OK. I've also updated intake loading.

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anton-seaice commented on 2026-07-16T02:00:18Z
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Have you tested this with MOM6 ?

I guess you also need to get the right combination of

      dxCu: # dxCu is delta x at u points [L ~> m]

      dyCu: # dyCu is delta y at u points [L ~> m]

      dxCv: # dxCv is delta x at v points [L ~> m]

      dyCv: # dyCv is delta y at v points [L ~> m]

instead of dxu and dyu

it looks like uo and vo are 3d fields as well ?

julia-neme commented on 2026-07-20T02:49:38Z
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True! I can replace with SSU and SSV,but I don't think they're available for any run. I'll add the caveat.

And regarding the right combination, since discovering that I can use .derivative() you don't need to think about combining interp and diff, so I might change the script to use that.

(see comment below)

anton-seaice commented on 2026-07-20T03:27:41Z
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I think using .derivative would calculate quanitites at the cell centre, so the model velocities need to be interpolated to cell centres then?

SSU and SSV are just the top layer in uo and vo right? So it's ok to use them, just note it in the table ?

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anton-seaice commented on 2026-07-16T02:00:20Z
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Why id boundary = 'extend' ?

I think boundary = None might work

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anton-seaice commented on 2026-07-19T23:31:51Z
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can these two lines be combined ?

variable=['dxu','dyu']

these are just merged below anyway ?

julia-neme commented on 2026-07-20T02:47:29Z
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I wanted that yes! But intake only opened one of them. So I had to open 1by1 and merge.

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anton-seaice commented on 2026-07-19T23:31:52Z
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Line #2.    detadx = grid.interp(grid.diff(ds['sea_level'], 'X', boundary='extend'), 'Y', boundary='extend') / ds['dxu']

It's a bit hard to follow this, but it looks like this:

1. takes the diff of the 'sea_level' field. This should interpolate to the middle of the top face.
2. interpolates those to the middle of the cell
3. then divides by the dxu - but surely dxu is the width of the top face?

shouldn't step 3 be either:

a. divide by dxt - the width across the middle of the cell, or

b. moved to between steps 1 and 2

(Assuming B-grid)

This calculation presumably would need changing for C-grid, to get rid of the .interp ?

anton-seaice commented on 2026-07-19T23:59:12Z
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Hangon - i dont have that correct, ill revisit

anton-seaice commented on 2026-07-20T01:00:17Z
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sea_level is a tracer/center point quantity, so this:

1. takes a diff across two center points to get the quantity in the middle of a face,
2. interpolates across two middle of face quantities to get the cell corner
3. divides by the width/length of the corner cell

so that is correct for B-grid,

julia-neme commented on 2026-07-20T02:48:10Z
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Maybe I should change this to use the .derivative() function instead!

[anton-seaice]

Hangon - i dont have that correct, ill revisit

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anton-seaice commented on 2026-07-20T00:51:13Z
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@charles-turner-1 - this only works on older intake-catalogs, the new ones gives this error:

ESMDataSourceError: Failed to load dataset with key='ocean.fx.xu_ocean:3600.yu_ocean:2700.point'
                 You can use cat['ocean.fx.xu_ocean:3600.yu_ocean:2700.point'].df to inspect the assets/files for this key.

anton-seaice commented on 2026-07-20T01:05:57Z
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I think there is a workaround for this already ?

charles-turner-1 commented on 2026-07-20T02:49:58Z
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Yeah, there should be - in fact, I think it should already be in the newest environments.

I’ll double check when I’m off the plane. The code needs a more sophisticated fix anyway which is already on the todo list

[anton-seaice]

sea_level is a tracer/center point quantity, so this:

1. takes a diff across two center points to get the quantity in the middle of a face,
2. interpolates across two middle of face quantities to get the cell corner
3. divides by the width/length of the corner cell

so that is correct for B-grid,

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[anton-seaice]

I am not sure what to do for C-grid / MOM6

This recipe calculates the geostrophic velocities at the cell corner (b-grid) point and compares to model speed at the cell corner.

So for C-grid, to calculate speed, should we also interpolate to the model velocities to the corner point to calculate speed?

[anton-seaice]

I think there is a workaround for this already ?

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[julia-neme]

I wanted that yes! But intake only opened one of them. So I had to open 1by1 and merge.

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[julia-neme]

So no change needed?

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[julia-neme]

True! I can replace with SSU and SSV,but I don't think they're available for any run. I'll add the caveat.

And regarding the right combination, since discovering that I can use .derivative() you don't need to think about combining interp and diff, so I might change the script to use that.

(see comment below)

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[charles-turner-1]

Yeah, there should be - in fact, I think it should already be in the newest environments.

I’ll double check when I’m off the plane. The code needs a more sophisticated fix anyway which is already on the todo list

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[anton-seaice]

I think using .derivative would calculate quanitites at the cell centre, so the model velocities need to be interpolated to cell centres then?

SSU and SSV are just the top layer in uo and vo right? So it's ok to use them, just note it in the table ?

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