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 so meone may at some stage wish 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 _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
