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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Received on Wed Oct 17 2018 - 05:44:26 BST
