Dear Alison, Jonathan,
we do need to be careful about the distinction between snow and ice.
The generally reliable AMS glossary is not much help here: *firn<
http://glossary.ametsoc.org/wiki/Firn>" is part of the process of transformation from *snow<
http://glossary.ametsoc.org/wiki/Snow>* to *land ice* , *land ice<
http://glossary.ametsoc.org/wiki/Land_ice>* is a layer of *ice* formed on land and *ice<
http://glossary.ametsoc.org/wiki/Ice>* includes snow, which leaves the definition of "firn" a bit wanting.
For the glaciologists, the transformation snow --> firn --> ice appears to be well established, but there are other usages in which ice includes snow. In cloud physics it seems to be clear that snow flakes are ice crystals.
regards,
Martin
________________________________
From: CF-metadata <cf-metadata-bounces at cgd.ucar.edu> on behalf of Jonathan Gregory <j.m.gregory at reading.ac.uk>
Sent: 18 October 2018 14:52
To: Pamment, Alison (STFC,RAL,RALSP)
Cc: CF-metadata (cf-metadata at cgd.ucar.edu)
Subject: Re: [CF-metadata] ice_sheet/land_ice confusion
Dear Alison
Yes, I realise I'm being a bit provocative and perhaps rash in trying to get
rid of ice_on_land (because I think it's terribly confusing). Distinctions can
be made for practical purposes (e.g. in models) between ice and snow, although
in reality it's a continuum. I'm wondering who has a need for an area type
including *all* kinds of frozen water on land (ice sheets, glaciers, firn/neve,
rivers, lakes, ponds, frozen flood water, snowfall which has melted and
refrozen as ice, hailstones, frost) *except* snow - and if so, how do they
distinguish snow from the rest.
Best wishes
Jonathan
On Thu, Oct 18, 2018 at 12:29:01PM +0000, Alison Pamment - UKRI STFC wrote:
> Date: Thu, 18 Oct 2018 12:29:01 +0000
> From: Alison Pamment - UKRI STFC <alison.pamment at stfc.ac.uk>
> To: "CF-metadata (cf-metadata at cgd.ucar.edu)" <cf-metadata at cgd.ucar.edu>
> Subject: Re: [CF-metadata] ice_sheet/land_ice confusion
>
> Dear Jonathan and Karl,
>
> I agree with Karl's suggestion to change "lying snow" to "surface snow" in the definitions of ice_on_land and ice_and_snow_on_land. That would be more consistent with standard names and their definitions.
>
> We seem to agree also that changes to area_type strings should be treated in the same way as changes to standard_names, i.e. using aliases. There is no formal description of the area_type table in the Conventions document. The XML schema would look very much like that of the standard name table, minus the units and the amip and grib tags. In fact, I think it has been agreed in principle to remove the AMIP and GRIB columns from the standard name table itself (CF trac #116). I will submit a GitHub issue to encompass trac #116 and some proper documentation of the area_type table. The CF-checker would need to cope with the possibility of aliases for area_types, so it probably needs to go in the conformance document too.
>
> Regarding Jonathan's assertion that "snow *is* ice", once again I am a little cautious. We certainly have standard names that *don't* regard them as being the same thing. For example, the two names I mentioned previously: change_over_time_in_amount_of_ice_and_snow_on_land and change_over_time_in_thermal_energy_content_of_ice_and_snow_on_land. I was under the impression that models treat ice and freshly fallen snow as different variables which sometimes co-exist in the same grid cell and sometimes don't. I haven't yet managed to track it down in the mailing list archives, but I also have a vague recollection of a discussion some years ago about the surface albedo being affected by factors such as the age of ice and snow, which would be important when considering an area_type. Perhaps I'm wrong about the last point, so I'd be interested to know what others think about the suggestion to turn ice_on_land into an alias of ice_and_snow_on_land.
>
> Best wishes,
> Alison
>
> ------
> Alison Pamment Tel: +44 1235 778065
> NCAS/Centre for Environmental Data Archival Email: alison.pamment at stfc.ac.uk
> STFC Rutherford Appleton Laboratory
> R25, 2.22
> Harwell Oxford, Didcot, OX11 0QX, U.K.
>
>
> -----Original Message-----
> From: CF-metadata <cf-metadata-bounces at cgd.ucar.edu> On Behalf Of Jonathan Gregory
> Sent: 18 October 2018 05:45
> To: cf-metadata at cgd.ucar.edu
> Subject: Re: [CF-metadata] ice_sheet/land_ice confusion
>
> Dear Alison
>
> I agree that an alias mechanism would be better than abolishing something we have introduced. Although it might sound inaccurate, "land ice" is a term that is used in the literature to mean ice sheets and glaciers, rather than all ice on land. It contrasts with sea ice, as has been remarked,
>
> I think that ice_on_land could be confused with land_ice. In addition, ice_on_land could be confusing because snow *is* ice; there isn't a clear distinction between snow and non-snow ice, and ice_and_snow_on_land could mean the same as ice_on_land. Therefore I suggest that we made ice_on_land into an alias of ice_and_snow_on_land.
>
> Best wishes
>
> Jonathan
>
> ----- Forwarded message from Alison Pamment - UKRI STFC <alison.pamment at stfc.ac.uk> -----
>
> > Date: Wed, 17 Oct 2018 15:05:06 +0000
> > From: Alison Pamment - UKRI STFC <alison.pamment at stfc.ac.uk>
> > To: Karl Taylor <taylor13 at llnl.gov>, Martin Juckes - UKRI STFC
> > <martin.juckes at stfc.ac.uk>, "CF-metadata (cf-metadata at cgd.ucar.edu)"
> > <cf-metadata at cgd.ucar.edu>
> > Subject: Re: [CF-metadata] ice_sheet/land_ice confusion
> >
> > Dear Karl et al.,
> >
> > Thank you all for the comments in this discussion, which I have been watching with interest.
> >
> > I think we can regard the three existing land ice area_types as nested:
> > ice_sheets = Grounded ice sheets + Floating ice shelves;
> > land_ice = ice_sheets + Glaciers + Ice caps;
> > ice_on_land = land_ice + River ice + Lake ice + Other ice on land, e.g frozen flood water.
> >
> > In addition we have:
> > ice_and_snow_on_land = snow overlying ice_on_land + snow overlying
> > bare ground or vegetation
> >
> > As Martin says, ice_on_land and ice_and_snow_on_land were designed to work with LS3MIP standard names. They include all frozen terrestrial water and are therefore wider than the other two categories. I can't comment on whether or not they are currently being used in the CMIP6 archive, but certainly that was the intention. The reason was to enable the use of the surface_albedo standard name along with specifying an area_type, instead of introducing lots of separate albedo standard names for different surface types. This approach received support in the mailing list discussions of LS3MIP names. We also introduced some standard names: change_over_time_in_amount_of_ice_and_snow_on_land and change_over_time_in_amount_of_ice_and_snow_on_land. The definition of "ice_and_snow_on_land" in these names follows that of the area_type.
> >
> > Martin has supported Karl's suggestion to modify the description of ice_sheet. In addition, Martin and Jonathan have suggested adding Greenland and Antarctica as examples rather than part of the basic definition so that the area_type can also be used for paleoclimate models. That seems like a good approach, hence I suggest:
> > 'An area type of "ice_sheet" indicates where ice sheets are present, for example, in the present climate this would refer to the Greenland and Antarctic ice sheets. It includes both the grounded portion of those ice sheets (i.e., the portion resting on bedrock either above or below sea level) and the portion that is floating as ice shelves. It excludes all other ice on land (in contrast to land_ice, which includes, for example, small mountain glaciers and in contrast to ice_on_land, which is comprehensively inclusive of all types of ice on land).'
> >
> > Karl has asked whether ice_on_land includes snow. I think it doesn't, because as already mentioned we have ice_and_snow_on_land as a separate area_type. Therefore, I support Karl's suggestion to modify the description of ice_on_land to make that point clear:
> > 'The area type "ice_on_land" means ice in glaciers, ice caps, grounded ice sheets (grounded and floating shelves), river and lake ice, and any other ice on a land surface, such as frozen flood water (but excluding snow). This is distinct from the area type 'land ice' which has a narrower definition. The area_type ice_and_snow_on_land is defined similarly, but includes lying snow.'
> >
> > It would also make sense to add a corresponding cross-reference in the description of ice_and_snow_on_land:
> > 'The area type "ice_and_snow_on_land" means ice in glaciers, ice caps, ice sheets (grounded and floating shelves), river and lake ice, any other ice on a land surface, such as frozen flood water, and snow lying on such ice or on the land surface. The area_type ice_on_land is defined similarly, but excludes lying snow.'
> >
> > I am cautious about Jonathan's suggestion to remove ice_on_land - it was introduced specifically to cope with CMIP6, so might it not be needed in due course? Also, I don't know that the Conventions have anything to say about simply removing an area_type once it's gone into the table. I have been managing the area_types vocabulary following a parallel procedure to standard names. It would be nice if we could think of a better term, so as to cause less confusion with land_ice. We could then turn ice_on_land into an alias, just as we would with a standard name.
> >
> > I agree with Martin that Evan will probably need to request some new area_types to work with his microwave data. Evan's suggestion of land_without_snow_or_ice sounds like a good starting point. Similarly we can discuss new area types for lakes with or without snow and/or ice. The key thing with all of these, as with standard names, is to describe them clearly. Where categories sound similar, or perhaps overlap, we need to be very clear about what is included or excluded in each area_type.
> >
> > Best wishes,
> > Alison
> >
> > ------
> > Alison Pamment Tel: +44 1235 778065
> > NCAS/Centre for Environmental Data Archival Email: alison.pamment at stfc.ac.uk
> > STFC Rutherford Appleton Laboratory
> > R25, 2.22
> > Harwell Oxford, Didcot, OX11 0QX, U.K.
> >
> >
> > -----Original Message-----
> > From: CF-metadata <cf-metadata-bounces at cgd.ucar.edu> On Behalf Of
> > Martin Juckes - UKRI STFC
> > Sent: 17 October 2018 12:44
> > To: Taylor, Karl E. <taylor13 at llnl.gov>; cf-metadata at cgd.ucar.edu
> > Subject: Re: [CF-metadata] ice_sheet/land_ice confusion
> >
> > Hello All,
> >
> >
> > I agree with Karl's suggestion that it is useful to mention Greenland and Antarctica to clarify the intended meaning of "ice_sheet", and also with with Jonathan point that there needs to be a caveat (perhaps "present era", rather than "modern world" -- the latter is often used to describe a much shorter timescale than we want here).
> >
> >
> > The CMIP approach to dividing the world is a little different from the approach Evan : the term "land_ice" has been introduced long ago and includes floating ice shelves. This could be described as a process driven approach: "land_ice" includes ice formed on land which has moved out to sea and has very different characteristics to "sea_ice", which is ice that has formed at sea.
> >
> >
> > In CMIP6 "land" is interpreted as including floating ice shelves when it refers to the surface. In CMIP5 the models did not include a physical representation of floating ice shelves, so areas such as the Ross Sea would generally be represented as grounded ice sheets, I believe. For CMIP6, we did discuss restricting "land" to exclude floating ice shelves and introducing a new area type for the broader meaning, but in the end opted for continuity with CMIP5. "land" is also taken to include lakes -- the fact that we have a small number of lakes and inland seas resolved in CMIP models is not yet reflected in the area types.
> >
> >
> > Consequently, Evan's requirements will need some new area types which will need to be named carefully to avoid confusion with existing ones.
> >
> > "ice_on_land" appears to have been introduced following a discussion of LS3MIP variables, one of which was originally an albedo of ice and snow on land but later got changed to an albedo of snow on land, hence this area type is not used.
> >
> > regards,
> > Martin
> > ________________________________
> > From: CF-metadata <cf-metadata-bounces at cgd.ucar.edu> on behalf of
> > Taylor, Karl E. <taylor13 at llnl.gov>
> > Sent: 17 October 2018 05:38
> > To: cf-metadata at cgd.ucar.edu
> > Subject: Re: [CF-metadata] ice_sheet/land_ice confusion
> >
> > Hi all,
> >
> > In CMIP5 only one of the three terms under discussion here was used:
> > "land_ice" (in the standard_name "land_ice_area_fraction"), which was described as "fraction of grid cell occupied by "permanent" ice (i.e., glaciers)." This was a "fixed" (time-independent) field.
> >
> > As far as I can tell, "ice_on_land" isn't needed by CMIP6 (and it wasn't needed or used in CMIP5). I don't know (or have forgotten) what led it to be introduced as a valid surface type.
> >
> > best regards,
> > Karl
> >
> > On 10/14/18 7:30 AM, Jonathan Gregory wrote:
> > > Reposting this, which didn't get to the list.
> > >
> > > Dear Karl, Sophie, Alison
> > >
> > > If we define ice_sheet to mean those of Greenland and Antarctica, it
> > > won't be applicable for palaeoclimate, so I think it's too
> > > restrictive. Although it's a continuum, there is a distinction between "ice sheet" and "glacier"
> > > that refers to size, with "ice-cap" being in the middle (and not
> > > used in IPCC to make things simpler). Ice sheets are big enough to
> > > bury the bedrock topography, so that the surface shape is determined
> > > by mass balance and dynamics. Glaciers are smaller, and confined
> > > within bedrock topography, which strongly influences their shape.
> > >
> > > If we want to mention Greenland and Antarctica explicitly, it would
> > > be a good idea to say "for example, in the modern world".
> > >
> > > No doubt it was discussed and I have forgotten, but being confronted
> > > with it now, I feel rather uncomfortable about there being distinct
> > > area_types of land_ice and ice_on_land. These types are not
> > > self-describing, in that the difference in wording does not convey anything about the difference in meaning.
> > >
> > > When and why was ice_on_land introduced?
> > >
> > > Best wishes
> > >
> > > Jonathan
> > >
> > > ----- Forwarded message from Karl Taylor <taylor13 at llnl.gov> -----
> > >
> > >> Date: Tue, 9 Oct 2018 11:44:53 -0700
> > >> From: Karl Taylor <taylor13 at llnl.gov>
> > >> To: "Nowicki, Sophie (GSFC-6150)" <sophie.nowicki at nasa.gov>,
> > >> "cf-metadata at cgd.ucar.edu" <cf-metadata at cgd.ucar.edu>
> > >> CC: Jonathan Gregory <j.m.gregory at reading.ac.uk>
> > >> Subject: Re: ice_sheet/land_ice confusion
> > >> User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:52.0)
> > >> Gecko/20100101 Thunderbird/52.9.1
> > >>
> > >> Thanks, Sophie, for your quick response. Given your clarification,
> > >> perhaps we might replace the description of ice_sheet, which
> > >> currently reads:
> > >>
> > >> > ice_sheet: An area type of "ice sheet" indicates where ice sheets are
> > >> > present. It includes both grounded ice sheets resting over bedrock and
> > >> > ice shelves flowing over the ocean, but excludes ice-caps and glaciers
> > >> > (in contrast to land_ice, which includes all components).
> > >>
> > >> with this description:
> > >>
> > >> ice_sheet: An area type of "ice_sheet" indicates where the
> > >> Greenland and Antarctic ice sheets are present. It includes both
> > >> the grounded portion of those ice sheets (i.e., the portion resting
> > >> on bedrock either above or below sea level) and the portion that is
> > >> floating as ice shelves. It excludes all other ice on land (in
> > >> contrast to land_ice, which includes, for example, small mountain
> > >> glaciers and in contrast to ice_on_land, which is comprehensively
> > >> inclusive of all types of ice on land).
> > >>
> > >> Also I think it should be clarified whether "snow" is considered to
> > >> be "ice_on_land". If not, I think the descriptive phrase "any
> > >> other ice on a land surface" should be modified to read "any other
> > >> ice on a land surface (except snow)".
> > >>
> > >> Best regards,
> > >> Karl
> > >>
> > >>
> > >>
> > >> On 10/9/18 11:03 AM, Nowicki, Sophie (GSFC-6150) wrote:
> > >>> Hi Karl,
> > >>>
> > >>> I am responding to your question about ice_sheet/land_ice (CF-metadata Digest, Message 2, Vol 186, Issue11), and deleted the other topics from the thread.
> > >>>
> > >>> ?ice_sheet would be the Greenland and Antarctic ice sheets. It contains both the grounded_ice_sheet (part of the ice sheet flowing over bedrock, and you are technically right that an ice sheet is a combination of many many glaciers) and floating_ice_shelf (the part that only flows on water).
> > >>>
> > >>> land_ice is much bigger as it includes the polar ice sheets, glaciers in non-polar regions (glaciers are considered small body of ice: for example in the Alps, or the US), and the small ice caps. The ice caps are also a large combinations of glaciers, but too small to be considered an ice sheets. For example the Svartissen Ice Cap in northern Norway.
> > >>>
> > >>> For ISMIP6, we are interested in ice_sheet, but some climate models may also include glaciers and ice caps (which ISMIP6 does not care about). Hence the use of both ice_sheet and land_ice in the ISMIP6 protocol (and I cant recall if land_ice was already present in CMIP5, but I think that it was).
> > >>>
> > >>> I don?t know the origin of ice_on_land.
> > >>>
> > >>> Jonathan: please help me make my answers less confusing...
> > >>>
> > >>> I hope that this helps,
> > >>>
> > >>> Sophie
> > >>> Message: 2
> > >>> Date: Tue, 9 Oct 2018 17:19:36 +0000
> > >>> From: "Taylor, Karl E." <taylor13 at llnl.gov>
> > >>> To: "cf-metadata at cgd.ucar.edu" <cf-metadata at cgd.ucar.edu>
> > >>> Subject: Re: [CF-metadata] ice_sheet / land_ice confusion
> > >>> Message-ID: <ec366da6-0f45-0c3a-0ebe-d7b20f7cfb55 at llnl.gov>
> > >>> Content-Type: text/plain; charset="utf-8"
> > >>> HI all,
> > >>> Can anyone provide any guidance on the difference between ice_sheet and
> > >>> land_ice (see below)?? It has a bearing on metadata to be stored with
> > >>> CMIP6 model output.
> > >>> thanks and best regards,
> > >>> Karl
> > >>> On 10/4/18 10:29 AM, Taylor, Karl E. wrote:
> > >>> > Hi all,
> > >>> >
> > >>> > I think there might be a mistake in the descriptions of "ice_sheet"
> > >>> > and/or "land_ice" in the "area type" table at
> > >>> > http://cfconventions.org/Data/area-type-table/current/build/area-type-table.html
> > >>> > .
> > >>> >
> > >>> > I find there the following definitions:
> > >>> >
> > >>> > ice_sheet: An area type of "ice sheet" indicates where ice sheets are
> > >>> > present. It includes both grounded ice sheets resting over bedrock and
> > >>> > ice shelves flowing over the ocean, but excludes ice-caps and glaciers
> > >>> > (in contrast to land_ice, which includes all components).
> > >>> >
> > >>> > land_ice: "Land ice" means glaciers, ice-caps, grounded ice sheets
> > >>> > resting on bedrock and floating ice-shelves.
> > >>> >
> > >>> > ice_on_land: The area type "ice_on_land" means ice in glaciers, ice
> > >>> > caps, grounded ice sheets (grounded and floating shelves), river and
> > >>> > lake ice, and any other ice on a land surface, such as frozen flood
> > >>> > water. This is distinct from the area type 'land ice' which has a
> > >>> > narrower definition.
> > >>> >
> > >>> > Are "ice-caps" and "glaciers" really excluded from "ice_sheet".? I would
> > >>> > have thought that "ice-cap" would be an ice_sheet located over a pole
> > >>> > (or something to that effect).? And i thought ice_sheets were just big
> > >>> > glaciers.
> > >>> >
> > >>> > ice_on_land is pretty clearly any frozen water, except sea ice,
> > >>> > icebergs, and ice particles in clouds, that is exposed to the atmosphere.
> > >>> >
> > >>> > So, I guess I'm trying to understand the difference between ice_sheet
> > >>> > and land_ice, and why do we need both of these?
> > >>> >
> > >>> > thanks and best regards,
> > >>> > Karl
> > >>> End of CF-metadata Digest, Vol 186, Issue 11
> > >>> ********************************************
> > >>>
> > > ----- End forwarded message -----
> > >
> > > ----- End forwarded message -----
> > > _______________________________________________
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> >
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