[MITgcm-support] Strange output fields/discontinuities

[Martin Losch]

Hi Enrico,

this looks to me as if there are probably more than one problem:

- Eta will always be non-zero ***underneath*** ice shelves. That is expected. You also need to have this to adjust for potentially (actually always) inaccurate loading of the ice shelf … which maybe already explains your large positive and negative values: If your 
  SHELFICEloadAnomalyFile = ‘pload_iceShelf_5km_hfac0.2Dr20_extended'
is not consistent with your initial conditions (T/S) within the ice cavities, then you can get very large adjustment processes. In your case it looks to me that your loadAnomaly is rather arbitrary and that the model needs extra Eta within the cavity and that leads to a reduction of sea ice outside of the cavity (the default model is not set up to adjust this type of volume change via the open boundaries, so it needs do it via sea level change, see PR#251 on github).
Check out the isomip example in verification/isomip to understand how eta works with the shelfice pkg.

non-zero eta over land is also possible with a linear free surface, but should be constant in space and time (the Poission equation for eta can be solved up to one constant, in some configurations this constant can be different from zero, probably because of obcs or similar). I think that’s the case for you results.
Your mean Eta (dynstat_eta_mean) is also ~-1.7m after one day (similar to the value over land). I assume it has to do with the obcs.

- obcs: there are funny values along the boundaries in the surface forcing, probably not problematic because they are not used, but for debugging, it would turn off the obcs pkg.

- your surface forcing definitely looks funny (i.e. what exf makes of it). You surface temperatures reach 3degC and more. Is that expected? I usually use the sea ice model with the “default set” (A2, B3). Did you try that?
The net heat flux (forcing_qnet_mean) is order 200W/m^2 downwards. Is that expected? It would explain the warming! Did you get the heat flux conventions right (positive = upward for latent and sensible heat)?

- I think that your sea ice is simply melting because of your forcing in the middle of the domain, that would also explain the excess of fresh water in the middle of your domain and the strong downward freshwater flux underneath the remaining. Where does the warm water come from?

Martin

> On 9. Jan 2020, at 14:45, Pochini, Enrico <epochini at inogs.it> wrote:
> 
> Dear all,
> 
> I'm running a regional model of the Ross Sea with atmospheric forcing, sea ice and ice shelf
> I'm observing strange things in some output field, and in particular in some of the fields calculated by the EXF package. 
> 
> In the EXF module I'm giving in input pressure, winds, precipitation, evaporation, heat fluxes and downward lw and sw radiative fluxes, and I've set the #define to have the configuration  A2+B5 as indicated in EXF_OPTIONS.h. I'm also using the SEAICE package with mostly default parameters, OBCS with/without sponge layer.
> 
> I'm observing big negative values in Eta (and Eta not being zero on land or ice shelf), and big discontinuities at some boundary in the FU,FV, EmPmR, Qnet fields. I didn't use the interpolation option, as I remapped the fields offline and they don't have discontinuities in the original files. Some seaice-related fields have also discontinuities at domain edges, and seaice is missing in some region (maybe because of low thickness that was cut off).
> 
> I attach the configuration and the output files; in results there is a STDOUT, and files with suffix "extra" contain surface climate fields as calculated by the model. The ERA5 file is the first timestep of the climate input forcing, the glorys_2000 file (reanalysis data) is the first timestep of a monthly average that I used to make initial conditions and set conditions at OB. 
> 
> In netcdf results, 2016 is a week output, 8064 28 day output, starting date 2000-01-01
>  code.tar.gz
>  ERA5_ross_3hrs_2000_regrid_sample.nc.tar.gz
>  glorys_2000_monthly_final_jan.nc.tar.gz
>  input_nmlst.tar.gz
>  results.tar.gz
> What do you think is causing the discontinuities in my fields?
> 
> Enrico
> 
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