[PI] Quality Flag Handling

[abooton]

Provision of support for "Quality Flags"

Required work
The relevant PRs to implement support for quality flags are below:

• Include support for standard_name modifiers Support Standard Name Modifers (not units!) #3393
• Decide on how to handle units and standard name modifiers Decide what to do about standard name modifiers units #3408
• Include support for Ancillary Variables Include AncillaryVariables and restructure Cube metadata. #3422
• Included loading/saving of Ancillary Variables (outstanding issue Add Support for Loading/Saving Ancillary Variable to/from NetCDF #3473)
• Support for status flags (outstanding issue [PI] Ensure flags are correctly loaded/saved to/from NetCDF #3474)

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Description:

Quality flags can be encoded in CF compliant data by using the "ancillary variable" and with "flag" values. This convention allows for a more explicit descriptions of the "quality flags" that one would like to associate with the data. (See snipped below for example)

According to the CF conventions: http://cfconventions.org/Data/cf-conventions/cf-conventions-1.7/cf-conventions.html#ancillary-data

However, at present, there are a couple of small issues when creating a "flag" auxiliary coordinate, as the current implementation does allow the issues below to be correctly handled:

1. The status_flag standard name should be recognised as so if the cube name is also a standard_name. (At present it come in as a long name.) See Appendix C, Standard Name Modifiers.
2. The ancillary_variable should be a local attribute, as it is directly associated the specific data. (i.e. like a latitude coord would be local to the data).
3. status_flags should not have units (they are described instead). See Appendix C, Standard Name Modifiers.

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The following code was used to generate the flag coordinate, the meta data for which is shown above:

[lbdreyer]

Delaying this til the next Iris release (Iris 3).

Unfortunately we have run out of time to properly implement this (in the way we want to which would include an improvement to how Cube dimensional metadata is handled), so this will instead be included in the Iris 3 release. Hopefully that shouldn't be an issue as Iris 3 is only a month away!

[lbdreyer]

The relevant PRs to implement support for quality flags are below:

• Include support for standard_name modifiers Support Standard Name Modifers (not units!) #3393
• Decide on how to handle units and standard name modifiers Decide what to do about standard name modifiers units #3408
• Include support for Ancillary Variables Include AncillaryVariables and restructure Cube metadata. #3422
• Included loading/saving of Ancillary Variables (outstanding issue Add Support for Loading/Saving Ancillary Variable to/from NetCDF #3473)
• Support for status flags (outstanding issue [PI] Ensure flags are correctly loaded/saved to/from NetCDF #3474)

Note - list copied to top of issue for easier use (26/11/2019).

[stephenworsley]

In the following line in the netCDF file:

air_temperature:coordinates = "flag forecast_period forecast_reference_time time" ;

it seems as though the appearance of flag is probably a mistake. The var_name flag should only be referenced by ancillary_variables. i.e. in the line:

air_temperature:ancillary_variables = "flag" ;

[abooton]

Support for status flags (was added in issue #3474). However, the "what's new" seems to have been overlooked?

[stephenworsley]

A "what's new" entry has now been added by #3801 (and fixed by #3816). This closes the issue.