Dear Jonathan and Siri,
Thanks for trying to help.
----- Original Message -----
> > In my understanding, sea_ice_area_fraction refers to sea ice
> > concentration as seen "from below" (where area(ice)+area(ocean
> > water) = area(cell)).
>
> Yes, I think so too. Sea ice with melt ponds is still sea ice. In
> terms of area types, sea = ice_free_sea + sea_ice.
Agreed. But where is this written? Should it enter the "help string" in the table of Area Types?
> You need a new area_type. I would say that sea_ice_melt_pond would be
> clear. I believe they are pretty much always called "melt ponds", aren't
> they, rather than "meltwater ponds".
Although Siri rightly mentions that some meltponds re-freeze at the surface, I would advocate for something simple like you propose, Jonathan, "sea_ice_melt_pond". Future applications needing re-frozen melt-ponds would ask for a new area type. So I adopt "sea_ice_melt_pond" for now.
> Given a new area_type, it is not necessary to
> have a new standard_name as well, since you could use the generic area_fraction.
This is interesting, I was not fully aware of that construct. I fear however that many application still expect a standard name as a string with underscores, maybe with a standard name modifier. I would not like to be the first one out with this construct...
I looked in the latest CF 1.7 draft1 and did not find a description of using area_fraction. Could someone propose an example, for example a (time, x, y) variable holding ice concentration (sea_ice_area_fraction) but using standard_name = "area_fraction"? Just as a thought experiment?
Independent of this newer construct, and if I would go for proposing a new standard name: "sea_ice_melt_pond_area_fraction", how would it be defined? It is supposedly the relative areal extent of sea_ice_melt_pond, but I always have difficulties figuring out what the normalizing area is: is it the whole cell area, or the area covered by sea (as in ice_free_sea + sea_ice), or by sea_ice? sea_ice_melt_pond_area/sea_ice_area or sea_ice_melt_pond_area/sea_area? Depending on the answer, this could agree with what Siri is using for the dataset at NSIDC.
For example, in my understanding, sea_ice_area_fraction is the area of sea_ice divided by the area of sea (again as ice_free_sea + sea_ice). So that a coastal cell with partial land cover of 0.6 could still have a sea_ice_area_fraction of 1.0 if the 0.4 sea area was fully ice covered. Correct? How can one tell from just "sea_ice_area_fraction"?
Finally, I also need the area fraction of "melt pond free sea ice", so that calls for another surface type: "sea_ice_melt_pond_free_sea_ice".
For the area_fraction, would something like sea_ice_melt_pond_free_sea_ice_area_fraction be the solution, and would it be computed as (sea_ice_area - sea_ice_melt_pond_area)/sea_area ? or divided by sea_ice_area?
Sorry for this lengthy email, hopefully I am not the only one confused about this, and it can help others.
Regards,
Thomas
>
> Best wishes
>
> Jonathan
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--
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Thomas Lavergne
Norwegian Meteorological Institute
P.O.BOX 43, Blindern, NO-0313 OSLO, Norway
Phone: (+47) 22963364 Fax: (+47) 22963380
Email: t.lavergne at met.no
OSISAF HL Portal: http://osisaf.met.no
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Received on Tue Feb 04 2014 - 14:23:10 GMT
