Netcdf format from MOM6

[anton-seaice]

When running the current 0.25 degree configs, MOM6 produces netcdf output of this form:

work/MOM_IC_1.nc work/MOM_IC_2.nc work/MOM_IC.nc

The files are of format "64-bit-offset" (aka cdf-2) and the first two files are size 3.6GB. Which is the maximum file size supported by this format, and therefore leads to the multiple netcdf files when it doesn't fit in one file.

Having multiple output files itself is not a problem, but this file format doesn't support compression, which leads to file sizes ~x2 as large as compressed files.

It doesn't look like there are configuration options in MOM to change this (see https://github.com/NCAR/MOM6/blob/2b1201a87259c912c93e4c11039b8e59971e3c26/src/framework/MOM_netcdf.F90#L127). To change it we would need to set the filetype (NF90_NETCDF4) in the nf90_create() calls and set deflate level in nf90_def_var() calls. We could do this as a patch, or add it to the MOM namelist?

@angus-g Has there been any discussion of updating formats amongst the MOM developers? Do you know how other groups handle this?

[anton-seaice]

It looks like we don't use the MOM_netcdf module I linked and its controlled by FMS. For some reason we use the "deprecated_io" option from FMS.

With "deprecated_io", I think the format is controlled by cpp macros, and so we need to turn off the "+large_file" option in spack, and then the default will be netcdf4.

Alternatively, we could try using the "fm2_io" option, (i.e. turn off "+deprecated_io") but I assume there was a good reason this option is set (@micaeljtoliveira) ?

[anton-seaice]

@micaeljtoliveira confirmed removing +deprecated_io is ok

Once we have done that, we can add

&fms2_io_nml
    netcdf_default_format = "netcdf4"
    deflate_level = 1
/

+deprecated_io is not in the access-nri om3 build , so we can wait for that and then make the config update needed.

[dougiesquire]

I'm moving this to priority:high because we should do this before any long runs for parameter tests. I'll do a new development release of ACCESS-OM3 using the COSIMA build infra so that we have something to test ASAP

[anton-seaice]

I think you need to remove "+large_file +deprecated_io" from https://github.com/COSIMA/spack-config/blob/7a477783782e62b0f743937c7c512b5ca42835f4/environments/access-om3-0_x_0/spack.yaml#L20 and then build. Then in the config update input.nml

[anton-seaice]

Closed through :

• Updates to 1deg_jra55do_ryf ACCESS-NRI/access-om3-configs#91
• 1deg jra55do iaf.cherrypicks ACCESS-NRI/access-om3-configs#102
• 025deg jra55do ryf.cherrypicks ACCESS-NRI/access-om3-configs#103
• 1deg jra55do ryf.cherrypicks ACCESS-NRI/access-om3-wav-configs#44
• 1deg jra55do iaf.cherrypicks ACCESS-NRI/access-om3-wav-configs#45