Dear Alison,
thank you very much for your quick reply, I'm very glad that we seem to
converge to a good solution on how to add the requested sea-ice
variables to the CF convention. In particular, I certainly appreciate
that the concept of sea-ice extent is quite confusing, and that a clear
definition is certainly required. Notwithstanding such definition, we
discourage the use of sea-ice extent for model evaluation in CMIP6
SIMIP, for the reasons outlined here:
http://www.the-cryosphere.net/8/229/2014/
In detail:
> So we will use the existing name
> sea_ice_extent (m2).
> The definition should be amended to make it a requirement to supply the threshold and to say something about the geographical area over which extent is calculated:
> 'The term sea_ice_extent means the total area of all grid cells in which the sea ice area fraction equals or exceeds a threshold, often chosen to be 15 per cent. The threshold must be specified by supplying a coordinate variable or scalar coordinate variable with the standard name of sea_ice_area_fraction. The horizontal domain over which sea ice extent is calculated is described by the associated coordinate variables and coordinate bounds or by a coordinate variable or scalar coordinate variable with the standard name of "region" supplied according to section 6.1.1 of the CF conventions.'
>
> OK?
This sounds very good to me.
>
> I have checked the CF conventions document again regarding this point. Section 6.1.1, "Geographic regions", states: "When data is representative of geographic regions which can be identified by names but which have complex boundaries that cannot practically be specified using longitude and latitude boundary coordinates, a labeled axis should be used to identify the regions. We recommend that the names be chosen from the list of standardized region names whenever possible. To indicate that the label values are standardized the variable that contains the labels must be given the standard_name attribute with the value region." In the case of the northern and southern hemispheres, the boundaries are not "complex" and can conveniently be described using the usual coordinate variables, so I think in fact we should stick to doing that for CMIP6. In the definition of the name I have allowed for the use of a 'region' coordinate or scalar coordinate variable because it is conceivable that someone may at some stage w
ish to calculate sea_ice_extent over an irregularly shaped area. However, I now think that we don't need to add northern_hemisphere and southern_hemisphere to the standardized region list. OK?
>
I fully agree, thanks for looking this up!
Sea-ice area:
>
> We should stick with using sea_ice_area, as currently defined, for this quantity.
>
Great, thanks!
Sea-ice volume:
> Agreed. So we will use the existing name
> sea_ice_volume (m3)
> and add the definition:
> ' "X_volume" means the volume occupied by X within the grid cell.'
> OK?
Yes, this is good, thanks!
> I appreciate that we may not be able to finalise all the sea ice names during August when many people are on leave. The next update of the standard name table won't take place until September 13th (again due to the holiday season) so we might be able to agree some, if not all, the names by then. We need to expand on the definition text for all the proposed names to bring them into line with existing names so I will prepare a list summarizing the current state of play, with full definitions, ready for you to review once you are back in the office.
Great, we'd be very happy to have further comments and/or suggestions
regarding our proposed variables. I find the exchange on this email list
to be truly helpful and extremely constructive. Thanks to everyone!
All the best for now,
Dirk
Received on Fri Aug 05 2016 - 05:24:45 BST
