[MITgcm-support] Setting up external wind stress in ocean model

[Chichi Zhou]

Hi Jean-Michel,

Thank you for your previous reply. I conducted a test by changing the `ustressperiod` in `pkg/exf` to zero, but the comparison results did not improve.

Here's what I did for the test using `pkg/exf`:
1. `ustressperiod = 0`, `umax = 33.0`, `readStressOnCgrid = .TRUE.`
2. `ustressperiod = 0.2` (the interval of my wind stress field), `umax = 33.0`, `readStressOnCgrid = .TRUE.`
3. `ustressperiod = 0.2`, `umax = 15.0`, `readStressOnCgrid = .TRUE.`

In all three cases, a `ustressfile` was specified with dimensions `(nx, 1, nt)`, and `ustress(1:nx, 1, 1:nt) = 0.3`.

The results of the three tests (`uVel`, `windstress`, and `uwind`) are exactly the same. According to the output files, `windstress = 0.3 N/m^2`, which is consistent with my intended setup. However, compared to the results without using `pkg/exf`, where I set up constant wind stress directly in `input/data`, the results using `exf` show a smaller wind-driven current in the surface region (the error is around 0.05 m/s when running for 4 hours).



Best,
Chichi

From: MITgcm-support <mitgcm-support-bounces at mitgcm.org> on behalf of Jean-Michel Campin <jmc at mit.edu>
Date: Monday, July 15, 2024 at 12:07
To: mitgcm-support at mitgcm.org <mitgcm-support at mitgcm.org>
Subject: Re: [MITgcm-support] Setting up external wind stress in ocean model
Hi Chichi,

For temporally constant wind stress:
a) without pkg/exf:
   wind-stress is supposed to be provided on C-grid ;
   and with periodicExternalForcing=F (i.e., the default), it's constant in time.
b) with pkg/exf:
   need to specify grid location (either: readStressOnAgrid=T or readStressOnCgrid=T,
   in data.exf, EXF_NML_01);
   and setting "ustressperiod = 0." will result in constant in time wind-stress forcing.
Note that for this case, if using spacial interpolation, the code for constant wind-stress
is a bit more complicated than for other exf forcing fields (see comments in exf_init_varia.F,
lines 65-69).

So, in your case, I would try to change ustressperiod setting to zero and check if the
comparison improves.

Cheers,
Jean-Michel

On Fri, Jul 12, 2024 at 09:24:09PM +0000, Chichi Zhou wrote:
> Hello everyone,
>
> I'm conducting a 2-D simulation in the x-z plane and would like to add spatially and temporally constant wind stress 0.3 N/m^2 at ocean surface. I have tried two methods:
>
> (1) Specify the wind file in `input/data` and do not use the EXF package.
>
> &PARM05
> hydrogThetaFile='T.init',
> bathyFile='flat_bottom',
> uVelInitFile='U.init',
> zonalWindFile='windx.bin',
>
> Here, 'windx.bin' is a 1-D array with dimensions (nx, 1), where windx (1:nx, 1) = 0.3. ?nx? is the total number of grid points in the x direction.
>
> (2) Set up the wind stress in `input/data.exf`.
>
> &EXF_NML_02
> ustressfile         = 'windx.bin',
> ustressperiod       = 0.2,
> &
>
> Here  'windx.bin' is converted from a matrix  with dimensions (nx, 1, nt).  windx (1:nx, 1, 1:nt) = 0.3. `nx` is the grid points in the x direction, and ?nt? is the total time steps. ?ustressperiod = 0.2? because `dt = 0.2s`.
>
> However, the output velocity and temperature fields from the two numerical simulations are different. I am confused about the reason of this and would appreciate any suggestions you might have. Thank you in advance!
>
> Best,
> Chichi
>

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