Saving Iris Cubes containing GRIB-2 data is changing 'Fixed Surface' settings to inconsistent values.

[mgange659]

Octets 23-34 in Section 4 of GRIB-2 messages store settings relating to the vertical coordinates describing
the GRIB-2 data.

For GRIB-2 files loaded into a cube, iris.save makes these alterations before saving in a new file

Type of first fixed surface=6 (Maximum Wind Level) changed to Type of first fixed surface=1 (Ground)
Type of first fixed surface=7 (Tropopause) changed to Type of first fixed surface=1 (Ground)

and for all screen-level parameters at 1.5 metres:-
Scale Factor of first fixed surface=1 (101) changed to Scale Factor of first fixed surface=0 (100)
Scaled Value of first fixed surface=15 (1.5m) changed to Scaled Value of first fixed surface=2 (2m)

[trexfeathers]

@SciTools/peloton: this is a known limitation due to our minimal implementation; good to have it recorded officially.

It is unclear when we will have resource to address this. Any information on priority / other context would be helpful for decision making (remembering that this a public repo!). Thanks 🙂

[mgange659]

This issue is currently LOW priority. I am exploring possible options for creating re-gridded GRIB products from full sized MDDA GRIB-2 files so that older code can be decommissioned.

[pp-mo]

I think we can't expect iris-grib to preserve the scaling, but ideally it would always selectt a sensible one.
I think that links this to #205