Dear Dirk, Martin, Jonathan,
I agree that it is useful to have 'threshold' in the name itself where a quantity could in theory be calculated for any threshold. This makes the name more general and therefore potentially more useful in the future. I also agree that we should add a definition to the existing sea_ice_extent name.
Initially, Dirk sent me a list of variables to preview before proposing the standard names to the mailing list. In it there were several variables relating to sea ice extent, area and volume, some of which refer to a threshold and others which don't. I've reproduced my original comments below with some additional explanation.
1. Current CF proposal: sea_ice_extent_in_region
CMIP6 short name: siextentn
Long name: Sea ice extent North
Units: 10^6 km?
Description: Total area of all Northern-Hemisphere grid cells that are covered by at least 15 % areal fraction of sea ice
My suggestion:
Standard name: area_extent_of_grid_cells_with_sea_ice_above_threshold (canonical units: m2)
+ new standardized region name 'northern hemisphere' which could be supplied in a scalar coordinate variable.
The standard name definition can be written so as to allow a threshold of sea_ice coverage to be specified, in this case 15%, using a scalar coordinate variable. If no threshold is specified it is assumed to be zero. This would follow the practice adopted for existing names such as time_when_flood_water_falls_below_threshold.
I appreciate that the existing name sea_ice_extent doesn't have a definition, and maybe we could write something that would allow us to use it for the aforementioned variable, but I don't like that solution for the following reason. The existing name sea_ice_area has the definition ' "X_area" means the horizontal area occupied by X within the grid cell', i.e. it is the area of the sea_ice itself. By analogy, I would expect sea_ice_extent to be defined as something like ' "X_extent" means the horizontal area occupied by X summed across the horizontal domain of the data variable as described by the associated coordinate variables and coordinate bounds or a scalar coordinate variable with a standard name of "region" '. However, if I have understood Dirk's variable correctly, it is the sum of the area of the grid cells that contain at least 15 per cent sea ice, not the area of the sea ice itself, and I would favour introducing a new standard name for that concept. Plus it gives us the opportunity to mention the
threshold.
Dirk's original list contained an analogous variable for the southern hemisphere for which we could use the same standard name. Instead of using a "region" scalar coordinate variable, we could specify the latitude and longitude extents of the northern and southern hemispheres using coordinate variables as others have already suggested. I'd be happy with either approach, so on that point I'll go along with the majority decision.
2. Current CF proposal: sea_ice_area_in_region
CMIP6 short name: siarean
Long name: Sea ice area North
Units: 10^6 km?
Description: Total area of sea ice in the Northern hemisphere
My suggestion:
Standard name: sea_ice_area_extent (canonical units: m2)
+ new standardized region name 'northern hemisphere' or specify horizontal domain via coordinate variables.
I think the existing name of sea_ice_extent (with added definition as above: "X_extent" means the horizontal area occupied by X summed across the horizontal domain of the data variable as described by the associated coordinate variables and coordinate bounds or a scalar coordinate variable with a standard name of "region") can definitely be used for this variable, and for added clarity I'm suggesting we also modify the name itself. Thus sea_ice_extent would become an alias of sea_ice_area_extent. This name would also work for the analogous southern hemisphere quantity in Dirk's original list.
3. Current CF proposal: sea_ice_volume_in_region
CMIP6 short name: sivoln
Long name: Sea ice volume North
Units: 10^3 km?
Description: Total volume of sea ice in the Northern hemisphere
My suggestion:
Standard name: sea_ice_volume_extent (canonical units: m3)
+ new standardized region name 'northern hemisphere' or specify horizontal domain via coordinate variables.
We have an existing name sea_ice_volume which does not have a definition. I think we should add a definition and by analogy with sea_ice_area I suggest it should be ' "X_volume" means the volume occupied by X within the grid cell'. To indicate the sum of the volumes across a horizontal domain we would then need a new standard name. For consistency with sea_ice_area_extent I suggest 'sea_ice_volume_extent' for this name. This would be defined as ' "X_volume_extent" means the volume occupied by X summed across the horizontal domain of the data variable as described by the associated coordinate variables and coordinate bounds or a scalar coordinate variable with a standard name of "region" '. Again, this name would work for both northern and southern hemispheres.
What do others think of these ideas?
Please note that all Dirk's proposed names can be viewed in the CEDA vocabulary editor:
http://cfeditor.ceda.ac.uk/proposals/1?status=active&namefilter=&proposerfilter=Dirk&descfilter=&unitfilter=&yearfilter=&commentfilter=&filter+and+display=Filter. I will update the entries as our discussions progress.
Best wishes,
Alison
------
Alison Pamment Tel: +44 1235 778065
Centre for Environmental Data Analysis Email: alison.pamment at stfc.ac.uk
STFC Rutherford Appleton Laboratory
R25, 2.22
Harwell Campus, Didcot, OX11 0QX, U.K.
> -----Original Message-----
> From: CF-metadata [mailto:cf-metadata-bounces at cgd.ucar.edu] On Behalf
> Of martin.juckes at stfc.ac.uk
> Sent: 25 July 2016 10:16
> To: cf-metadata at cgd.ucar.edu
> Subject: [CF-metadata] CMIP6 Sea Ice MIP: Integrated quantities
>
> Dear Jonathon, Dirk,
>
> I agree with Jonathon that it is a good idea to follow the approach used in
> other names (e.g.
> number_of_days_with_air_temperature_above_threshold) and use a
> coordinate for the threshold value .. though I would prefer to make it
> obligatory to state the threshold in order to avoid ambiguity.
>
> Another issue is the domain .. the variable is being defined as the total area
> of cells above the threshold within a specific domain (the N. hemisphere of
> S. hemisphere in the CMIP6 data request). The threshold examples that I
> could find in the existing standard names are all temporal statistics (e.g.
> number of days with quantity X above a threshold within a year) and so use
> a climatological time variable. Here we want to specify the area over which
> cells are being summed. Would it make sense to use a 2nd coordinate
> variable, with standard name "region" specifying the region (this would also
> require adding "northern_hemisphere" and "southern_hemisphere" to the
> region list)? Alternatively, we could use latitude and longitude coordinate
> variables with bounds attributes to declare the region.
>
> The region will be in the long name, of course, but it would be good to have
> an agreed approach to encoding it in CF,
>
> regards,
> Martin
>
>
> Dear Dirk
>
> > For sea_ice_extent, there is currently no definition in the CF
> > convention. This variable is somewhat odd in a CF context, as it only
> > makes sense when applied over several grid cells.
> >
> > Hence, we suggest to adopt a definition which implies usage over several
> > grid cells:
> >
> > sea_ice_extent: Total area of all grid cells that are covered by a
> > minimum of a given area fraction [usually 15 %) of sea ice in a
> > specified region".
>
> I agree that it needs a definition. It is like sea ice area (m2) in being an
> extensive quantity which is integrated over the area considered, but I agree
> that extent is not meaningful unless there is some discretisation of area.
> I would suggest that we could allow for the possibility of the threshold
> being other than 15%, following practice for other standard names (as in
> recent emails of Martin's), like this
>
> sea_ice_extent: Total area of all grid cells in which the sea ice area
> fraction equals or exceeds a threshold. By default the threshold is 15%.
> The threshold can be specified by supplying a coordinate variable or scalar
> coordinate variable with standard_name of sea_ice_area_fraction.
>
> Would something like that be OK?
>
> Best wishes
>
> Jonathan
>
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Received on Thu Aug 04 2016 - 18:32:23 BST
