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 <[email protected]> > To: "CF-metadata ([email protected])" <[email protected]> > 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: > [email protected] > STFC Rutherford Appleton Laboratory > R25, 2.22 > Harwell Oxford, Didcot, OX11 0QX, U.K. > > > -----Original Message----- > From: CF-metadata <[email protected]> On Behalf Of Jonathan > Gregory > Sent: 18 October 2018 05:45 > To: [email protected] > 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 > <[email protected]> ----- > > > Date: Wed, 17 Oct 2018 15:05:06 +0000 > > From: Alison Pamment - UKRI STFC <[email protected]> > > To: Karl Taylor <[email protected]>, Martin Juckes - UKRI STFC > > <[email protected]>, "CF-metadata ([email protected])" > > <[email protected]> > > 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: > > [email protected] > > STFC Rutherford Appleton Laboratory > > R25, 2.22 > > Harwell Oxford, Didcot, OX11 0QX, U.K. > > > > > > -----Original Message----- > > From: CF-metadata <[email protected]> On Behalf Of > > Martin Juckes - UKRI STFC > > Sent: 17 October 2018 12:44 > > To: Taylor, Karl E. <[email protected]>; [email protected] > > 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 <[email protected]> on behalf of > > Taylor, Karl E. <[email protected]> > > Sent: 17 October 2018 05:38 > > To: [email protected] > > 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 <[email protected]> ----- > > > > > >> Date: Tue, 9 Oct 2018 11:44:53 -0700 > > >> From: Karl Taylor <[email protected]> > > >> To: "Nowicki, Sophie (GSFC-6150)" <[email protected]>, > > >> "[email protected]" <[email protected]> > > >> CC: Jonathan Gregory <[email protected]> > > >> 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." <[email protected]> > > >>> To: "[email protected]" <[email protected]> > > >>> Subject: Re: [CF-metadata] ice_sheet / land_ice confusion > > >>> Message-ID: <[email protected]> > > >>> 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 ----- > > > _______________________________________________ > > > CF-metadata mailing list > > > [email protected] > > > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > > > > _______________________________________________ > > CF-metadata mailing list > > [email protected] > > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > > _______________________________________________ > > CF-metadata mailing list > > [email protected] > > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > > > > _______________________________________________ > > CF-metadata mailing list > > [email protected] > > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > > ----- End forwarded message ----- > _______________________________________________ > CF-metadata mailing list > [email protected] > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > _______________________________________________ > CF-metadata mailing list > [email protected] > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
