[CF-metadata] CMIP6 Sea Ice MIP: Ice thickness
- Contemporary messages sorted: [ by date ] [ by thread ] [ by subject ] [ by author ] [ by messages with attachments ]
---- 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 <dirk.notz at mpimet.mpg.de> ----- >> >>> Date: Thu, 21 Jul 2016 15:32:37 +0200 >>> From: Dirk Notz <dirk.notz at mpimet.mpg.de> >>> To: Jonathan Gregory <j.m.gregory at reading.ac.uk> >>> 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 <dirk.notz at mpimet.mpg.de> ----- >>>> >>>>> Date: Thu, 21 Jul 2016 13:33:55 +0200 >>>>> From: Dirk Notz <dirk.notz at mpimet.mpg.de> >>>>> To: Jonathan Gregory <j.m.gregory at reading.ac.uk> >>>>> 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 <dirk.notz at mpimet.mpg.de> >>>>>> ----- >>>>>> >>>>>>> Date: Tue, 19 Jul 2016 09:53:02 +0200 >>>>>>> From: Dirk Notz <dirk.notz at mpimet.mpg.de> >>>>>>> To: cf-metadata at cgd.ucar.edu >>>>>>> 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 >>>>>>> CF-metadata at cgd.ucar.edu >>>>>>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata >>>>>> ----- End forwarded message ----- >>>>>> _______________________________________________ >>>>>> CF-metadata mailing list >>>>>> CF-metadata at cgd.ucar.edu >>>>>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata >>>>>> >>>> ----- End forwarded message ----- >>>> _______________________________________________ >>>> CF-metadata mailing list >>>> CF-metadata at cgd.ucar.edu >>>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata >>>> >> ----- End forwarded message ----- >> _______________________________________________ >> CF-metadata mailing list >> CF-metadata at cgd.ucar.edu >> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > > _______________________________________________ > CF-metadata mailing list > CF-metadata at cgd.ucar.edu > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadataReceived on Thu Jul 21 2016 - 14:43:02 BST
This archive was generated by hypermail 2.3.0 : Tue Sep 13 2022 - 23:02:42 BST