Dear Dirk Thank you for your understanding!
Best wishes Jonathan ----- Forwarded message from Dirk Notz <[email protected]> ----- > Date: Thu, 21 Jul 2016 22:43:02 +0200 > From: Dirk Notz <[email protected]> > To: [email protected] > Subject: Re: [CF-metadata] CMIP6 Sea Ice MIP: Ice thickness > User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 > Thunderbird/45.2.0 > > Dear Karl, dear Jonathan, > > thanks for your helpful feedback. I in particular appreciate the point > that these issues arise for a rather large number of additional > variables, too, and that we hence must be able to expect users to read > the cell_measures in addition to just the variable name. > > In the light of this discussion I hence withdraw the proposed name > "equivalent_sea_ice_thickness" and we will describe it simply through > the cell_methods as for CMIP5. > > Best, > > Dirk > > > > ---- > Dr. Dirk Notz > http://www.mpimet.mpg.de/~notz.dirk > > Am 21.07.2016 um 18:44 schrieb Karl Taylor: > > Dear all, > > > > I agree that standard name should describe the quantity itself, but not > > how it's computed. > > > > I note that for CMIP5 data we tried to make it clear how the quantity > > identified by "sea_ice_thickness" was calculated, first by including the > > cell_measures: > > > > time: mean area: mean where sea > > > > and then by including a "comment" attribute that reads: > > > > "the mean thickness of sea ice in the ocean portion of the grid cell > > (averaging over the entire ocean portion, including the ice-free > > fraction). Reported as 0.0 in regions free of sea ice." > > > > I think users of this data could easily avoid misinterpreting it simply > > by reading the attributes describing it. > > > > best regards, > > Karl > > > > > > > > On 7/21/16 9:12 AM, Jonathan Gregory wrote: > >> Dear Dirk > >> > >> I understand your concern, but there are plenty of other instances where > >> important distinctions are made by coordinates and cell_methods rather > >> than > >> by standard_name. For example, maximum, minimum, mean and instantaneous > >> temperatures are distinguished by cell_methods only. I can't think of > >> another > >> instance where a given geophysical variable has more than one standard > >> name. > >> I think that the users of the data have to be aware that the standard > >> name > >> alone is not sufficient. In practice people often also identify > >> variables by > >> the variable name, although CF does not standardise these, so that is > >> not a > >> reliable method in general but could work in a particular project. > >> > >> Best wishes > >> > >> Jonathan > >> > >> ----- Forwarded message from Dirk Notz <[email protected]> ----- > >> > >>> Date: Thu, 21 Jul 2016 15:32:37 +0200 > >>> From: Dirk Notz <[email protected]> > >>> To: Jonathan Gregory <[email protected]> > >>> Subject: Re: [CF-metadata] CMIP6 Sea Ice MIP: Ice thickness > >>> User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 > >>> Thunderbird/45.2.0 > >>> > >>> Dear Jonathan, > >>> > >>> I appreciate that a CF variable is not solely defined by its variable > >>> name, but also by its cell_methods. > >>> > >>> I hence believe that the question of introducing > >>> "sea_ice_equivalent_thickness" boils down to a question of the > >>> overarching CF philosophy: > >>> > >>> I agree that usually a new variable name should only be introduced if a > >>> certain quantity cannot be described by existing variables. > >>> > >>> However, I believe that in some cases, introduction of a new variable > >>> name can also be warranted simply to prevent significant errors that can > >>> occur because the actual data does not describe the quantity given by > >>> the variable name because of applied cell_methods. > >>> > >>> Even if we can create "equivalent_sea_ice_thickness" from the existing > >>> variable "sea_ice_thickness", its physical usefulness does not primarily > >>> relate to the actual sea-ice thickness implied by the variable name. > >>> Instead, equivalent_sea_ice_thickness is primarily of interest for > >>> oceanographers to easily appreciate the volume of sea water in the > >>> vertical column that is on average frozen to sea ice. Given the > >>> sometimes significant differences in magnitude between actual thickness > >>> and equivalent thickness, I am worried that by keeping the same variable > >>> name for such different quantities, significant confusion might arise. > >>> > >>> Best, > >>> > >>> Dirk > >>> > >>> > >>> > >>> > >>> ---- > >>> Dr. Dirk Notz > >>> http://www.mpimet.mpg.de/~notz.dirk > >>> > >>> Am 21.07.2016 um 15:05 schrieb Jonathan Gregory: > >>>> Dear Dirk > >>>> > >>>> I think I get your point, but nonetheless I would say that the > >>>> "equivalent > >>>> sea ice thickness" can be correctly described by existing > >>>> conventions. If the > >>>> standard_name is sea_ice_thickness and the cell_methods says "area: > >>>> mean", > >>>> the interpretation is that the sea_ice_thickness is integrated over > >>>> the area > >>>> of the grid box and then divided by the grid box area. That is what > >>>> you want, > >>>> isn't it? By contrast the local thickness of sea-ice, at a point, > >>>> would have > >>>> cell_methods with "area: point", and the thickness averaged over the > >>>> sea-ice > >>>> area is "area: mean where sea_ice". Thus the cases are distinguished > >>>> by the > >>>> cell_methods. In the CF convention, the standard_name is only part > >>>> of the > >>>> description of the data variable. > >>>> > >>>> Best wishes > >>>> > >>>> Jonathan > >>>> > >>>> ----- Forwarded message from Dirk Notz <[email protected]> ----- > >>>> > >>>>> Date: Thu, 21 Jul 2016 13:33:55 +0200 > >>>>> From: Dirk Notz <[email protected]> > >>>>> To: Jonathan Gregory <[email protected]> > >>>>> Subject: Re: [CF-metadata] CMIP6 Sea Ice MIP: Ice thickness > >>>>> User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 > >>>>> Thunderbird/45.2.0 > >>>>> > >>>>> Dear Jonathan, > >>>>> > >>>>> thanks a lot for your guidance. I understand that > >>>>> equivalent_sea_ice_thickness can be constructed from existing > >>>>> mechanisms. > >>>>> > >>>>> However, the definition of equivalent_sea_ice_thickness is not related > >>>>> to actual sea-ice thickness, but simply defined as sea-ice volume per > >>>>> grid area. This then only happens to have the same units as > >>>>> sea_ice_thickness. We hence initially proposed to have > >>>>> equivalent_sea_ice_thickness recorded as sea_ice_volume, but Alison > >>>>> pointed out that units of "m" are not possible for any variable > >>>>> that is > >>>>> called "volume". > >>>>> > >>>>> However, given the substantial confusion that arose in some papers > >>>>> published with CMIP5 data, where people assumed that > >>>>> "sea_ice_thickness" > >>>>> is actual thickness, we believe that it would be worthwhile to add a > >>>>> variable with a distinct name to avoid such confusion in the future. > >>>>> > >>>>> Best, > >>>>> > >>>>> Dirk > >>>>> > >>>>> > >>>>> > >>>>> ---- > >>>>> Dr. Dirk Notz > >>>>> http://www.mpimet.mpg.de/~notz.dirk > >>>>> > >>>>> Am 20.07.2016 um 15:21 schrieb Jonathan Gregory: > >>>>>> Dear Dirk > >>>>>> > >>>>>> I believe that this distinction can be recorded by the existing > >>>>>> mechanisms > >>>>>> of "where" and "over" in cell_methods, using the existing > >>>>>> standard_name of > >>>>>> sea_ice_thickness. Please see Section 7.3.3 of the CF standard. > >>>>>> The grid-box- > >>>>>> mean sea-ice thickness is "area: mean where all_area_types" and > >>>>>> the thickness > >>>>>> meaned over sea-ice only is "area: mean where sea_ice", I think. > >>>>>> > >>>>>> Best wishes > >>>>>> > >>>>>> Jonathan > >>>>>> > >>>>>> ----- Forwarded message from Dirk Notz <[email protected]> > >>>>>> ----- > >>>>>> > >>>>>>> Date: Tue, 19 Jul 2016 09:53:02 +0200 > >>>>>>> From: Dirk Notz <[email protected]> > >>>>>>> To: [email protected] > >>>>>>> Subject: [CF-metadata] CMIP6 Sea Ice MIP: Ice thickness > >>>>>>> User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 > >>>>>>> Thunderbird/38.8.0 > >>>>>>> > >>>>>>> Dear CF community, > >>>>>>> > >>>>>>> traditionally, the variable sea_ice_thickness from CMIP-type model > >>>>>>> output was calculated by dividing the entire volume of sea ice in > >>>>>>> a grid > >>>>>>> cell by the entire area of the grid cell, independent of the area > >>>>>>> fraction of the grid cell that was actually covered by sea ice. > >>>>>>> > >>>>>>> This gave rise to substantial confusion for users who expected that > >>>>>>> sea_ice_thickness as stored within CMIP simulations refers to the > >>>>>>> actual > >>>>>>> sea-ice thickness that is used in the sea-ice model code to > >>>>>>> calculate > >>>>>>> heat fluxes, for example, rather than the average ice thickness > >>>>>>> that the > >>>>>>> ice would have if it were to cover the entire area of the grid cell > >>>>>>> while conserving its volume. > >>>>>>> > >>>>>>> To prevent such confusion in the future, we would like to add the > >>>>>>> following variable to the CF convention: > >>>>>>> > >>>>>>> 1. equivalent_sea_ice_thickness (new variable with units 'm3 m-2' > >>>>>>> or 'm') > >>>>>>> to describe sea-ice volume per grid-cell area > >>>>>>> > >>>>>>> The term "Equivalent sea-ice thickness" is known within the sea-ice > >>>>>>> community to refer to "sea-ice volume per grid-cell area". > >>>>>>> Ideally, we > >>>>>>> would have liked to suggest a variable name containing the term > >>>>>>> "volume", but this seems difficult within the CF convention as then > >>>>>>> units couldn't be 'm'. > >>>>>>> > >>>>>>> Thank you very much for any feedback, help and guidance. > >>>>>>> > >>>>>>> Best, > >>>>>>> > >>>>>>> Dirk Notz > >>>>>>> > >>>>>>> > >>>>>>> > >>>>>>> -- > >>>>>>> ---- > >>>>>>> Dr. Dirk Notz > >>>>>>> http://www.mpimet.mpg.de/~notz.dirk > >>>>>>> _______________________________________________ > >>>>>>> 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 > >>>>>> > >>>> ----- End forwarded message ----- > >>>> _______________________________________________ > >>>> 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 ----- End forwarded message ----- _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
