StephenGriffies
commented
7 years ago I agree with all these points. Let's hope the new interpolation fixes the issue.
BTW, atmospheric winds (and other fields) have traditionally been a pain to interpolate onto ocean grids. In the old days, spectral "Gibbs Ripples" emanated from the Andes. We can still see that from the CORE forcing in some of their fields. It is an age-old problem of transferring information across grids without losing information and without adding computational garbage.
ofa001
nichannah
aekiss
aidanheerdegen
russfiedler
PaulSpence
Symptoms
Sea ice in 0.25 and 0.1 runs shows stripes, eg these plots of aice_m (ice area aggregate) in 0.25 deg run /g/data3/hh5/tmp/cosima/access-om2-025/025deg_jra55_ryf_spinup7/output144/ice/OUTPUT/iceh.0145-12.nc in the Ross and Amundsen Seas:
The line plots are meridional transects vs grid index at various longitudes in the Amundsen Sea. This shows that the stripes have a scale of about 5 grid points. With the Mercator grid the spacing in MOM is about 0.1deg at this latitude so the scale is about 0.5deg. This is roughly the JRA55 resolution (resolution of JRA55 data in /g/data1/ua8/JRA55-do/RYF/v1-1/ depends on field; for zonal velocity it is slightly uneven 0.55~0.562deg in latitude: ncdump -v latitude /g/data1/ua8/JRA55-do/RYF/v1-1/RYF.u_10.1990_1991.nc but uniform 0.5625deg in longitude).
Stripes of the same physical scale are also seen in 1/10 runs.
Diagnosis
The ice banding is probably due to wind stress curl having steps (or spikes?) at the JRA55 grid scale due to the wind velocity interpolation method used (conservative piecewise constant?) in the regridding. This would give stripy Ekman transport that could advect the ice into bands or draw cool water up from below in stripes.
@nicjhan - what interpolation is currently being used for wind?
Cure?
Try using a smoother interpolation method for regridding uas_10m and vas_10m. I think at minimum the interpolated velocities should be continuous (eg piecewise (bi-)linear) so the curl is piecewise constant. But smoother may be better - eg 2nd order velocity interpolation to give continuous curl. Perhaps see what @PaulSpence did to fix this in MOM-SIS 1/4deg.
Conservative 2nd order is in development but not yet available. But it is not important to be conservative when interpolating wind velocity, as the ocean is driven by stress which is quadratically related and so will not be conservatively interpolated even if the velocity is.
It is important to retain conservatve interpolation for the other fields, so these should be left as-is (at least until conservative 2nd order becomes available).