Dear Sebastien

I would favour your alternative
> Alternatively, we could move the last sentence of each description to the 
> respective "ocean_mixed_layer_thickness_defined_by_*" standard names to make 
> that specific usage more explicit. We then keep a more general description 
> for the new "*_difference" standard names.
for the reason you give, which appeals to me.

Also, instead of "auxiliary coordinate" I would say "coordinate variable or
scalar coordinate variable". A scalar coord var is formally like a size-one
auxiliary coord var. However the threshold variable could in principle be
multi-valued; then you'd have a dimension for it, and it wouldn't be an aux
coord var.

Best wishes

Jonathan


----- Forwarded message from Sebastien Villaume <sebastien.villa...@ecmwf.int> 
-----

> Date: Tue, 10 Apr 2018 10:36:21 +0000 (GMT-00:00)
> From: Sebastien Villaume <sebastien.villa...@ecmwf.int>
> To: cf-metadata@cgd.ucar.edu
> Cc: "Lowry, Roy K." <r...@bodc.ac.uk>, Jonathan Gregory
>       <j.m.greg...@reading.ac.uk>
> Subject: Re: [CF-metadata] how to     use
>       ocean_mixed_layer_thickness_defined_by_*
> X-Mailer: Zimbra 8.6.0_GA_1200 (ZimbraWebClient - FF52 (Linux)/8.6.0_GA_1200)
> 
> 
> Dear CF list,
> 
> I took a bit longer to write down the proposed definitions of these 3 
> standard names.
> 
> 
> standard name: sea_water_temperature_difference 
> units: K
> description:
> Sea water temperature is the in situ temperature of the sea water. The 
> standard name "sea_water_temperature_difference" can be used (as an auxiliary 
> coordinate) together with the standard name 
> "ocean_mixed_layer_thickness_defined_by_temperature" to define its 
> temperature criterion.
> 
> 
> standard name: sea_water_sigma_theta_difference
> units: kg/m3
> description:
> Sigma-theta of sea water is the potential density (i.e. the density when 
> moved adiabatically to a reference pressure) of water having the same 
> temperature and salinity, minus 1000 kg m-3. The standard name 
> "sea_water_sigma_theta_difference" can be used (as an auxiliary coordinate) 
> together with the standard name 
> "ocean_mixed_layer_thickness_defined_by_sigma_theta" to define its 
> sigma_theta criterion.
> 
> standard name: sea_water_sigma_t_difference
> units: kg/m3
> description:
> Sigma-t of sea water is the density of water at atmospheric pressure (i.e. 
> the surface) having the same temperature and salinity, minus 1000 kg m-3. The 
> standard name "sea_water_sigma_t_difference" can be used (as an auxiliary 
> coordinate) together with the standard name  
> "ocean_mixed_layer_thickness_defined_by_sigma_t" to define its sigma_t 
> criterion.
> 
> 
> Alternatively, we could move the last sentence of each description to the 
> respective "ocean_mixed_layer_thickness_defined_by_*" standard names to make 
> that specific usage more explicit. We then keep a more general description 
> for the new "*_difference" standard names.
> 
> /Sébastien
> 
> ----- Original Message -----
> > From: "Jonathan Gregory" <j.m.greg...@reading.ac.uk>
> > To: "Sebastien Villaume" <sebastien.villa...@ecmwf.int>
> > Cc: cf-metadata@cgd.ucar.edu, "Lowry, Roy K." <r...@bodc.ac.uk>
> > Sent: Friday, 6 April, 2018 14:24:18
> > Subject: Re: [CF-metadata] how to   use 
> > ocean_mixed_layer_thickness_defined_by_*
> 
> > Dear Sebastien and Roy
> > 
> > Thanks. I agree that it's a good idea to include sea_water_.
> > 
> > Best wishes
> > 
> > Jonathan
> > 
> > On Thu, Apr 05, 2018 at 07:04:26PM +0000, Sebastien Villaume wrote:
> >> Date: Thu, 5 Apr 2018 19:04:26 +0000 (GMT-00:00)
> >> From: Sebastien Villaume <sebastien.villa...@ecmwf.int>
> >> To: cf-metadata@cgd.ucar.edu
> >> Cc: Jonathan Gregory <j.m.greg...@reading.ac.uk>, "Lowry, Roy K."
> >>  <r...@bodc.ac.uk>
> >> Subject: Re: [CF-metadata] how to  use
> >>  ocean_mixed_layer_thickness_defined_by_*
> >> X-Mailer: Zimbra 8.6.0_GA_1200 (ZimbraWebClient - FF59
> >>  (Linux)/8.6.0_GA_1200)
> >> 
> >> Dear Jonathan and Roy,
> >> 
> >> thank you for your suggestions.
> >> 
> >> I am happy to go with a set of general standard names if it is fine with
> >> everyone. I find it actually useful to make the standard names reusable by 
> >> not
> >> hard-coding one of the reference. It is pretty clear from the mixed layer
> >> definition that it is a difference with respect to the sea surface.
> >> 
> >> I am also happy to prefix sigma_theta and sigma_t with sea_water:
> >> 
> >> sea_water_temperature_difference (K)
> >> sea_water_sigma_theta_difference (kg/m3)
> >> sea_water_sigma_t_difference (kg/m3)
> >> 
> >> I will draft some definitions for tomorrow.
> >> 
> >> thanks!
> >> 
> >> /Sébastien
> >> 
> >> ----- Original Message -----
> >> > From: "Lowry, Roy K." <r...@bodc.ac.uk>
> >> > To: cf-metadata@cgd.ucar.edu, "Jonathan Gregory" 
> >> > <j.m.greg...@reading.ac.uk>
> >> > Sent: Thursday, 5 April, 2018 18:34:48
> >> > Subject: Re: [CF-metadata] how to        use     
> >> > ocean_mixed_layer_thickness_defined_by_*
> >> 
> >> > Dear Jonathan and Sebastien,
> >> > 
> >> > 
> >> > 
> >> > 
> >> > I was initially thinking of not including ' _defining_mixed_layer' in my
> >> > suggestion and am certainly happy with leaving it out. However, I still 
> >> > think
> >> > sigma_t and sigma_theta should be prefixed with 'sea_water'. This would 
> >> > give:
> >> > 
> >> > 
> >> > 
> >> > 
> >> > 
> >> > sea_water_temperature_difference
> >> > sea_water_sigma_theta_difference
> >> > sea_water_sigma_t_difference
> >> > 
> >> > Cheers, Roy.
> >> > 
> >> > 
> >> > 
> >> > 
> >> > 
> >> > 
> >> > 
> >> > 
> >> > Please note that I partially retired on 01/11/2015. I am now only 
> >> > working 7.5
> >> > hours a week and can only guarantee e-mail response on Wednesdays, my 
> >> > day in
> >> > the office. All vocabulary queries should be sent to 
> >> > enquir...@bodc.ac.uk.
> >> > Please also use this e-mail if your requirement is urgent.
> >> > 
> >> > 
> >> > 
> >> > From: CF-metadata <cf-metadata-boun...@cgd.ucar.edu> on behalf of 
> >> > Jonathan
> >> > Gregory <j.m.greg...@reading.ac.uk>
> >> > Sent: 05 April 2018 18:28
> >> > To: cf-metadata@cgd.ucar.edu
> >> > Subject: Re: [CF-metadata] how to use 
> >> > ocean_mixed_layer_thickness_defined_by_*
> >> > Dear Sebastien
> >> > 
> >> > It's interesting that this question hasn't been raised before. Thanks 
> >> > for doing
> >> > so now. I agree that new standard names would be appropriate. There are 
> >> > already
> >> > some standard names containing "difference" in various ways. I would 
> >> > suggest
> >> > that for your purpose the names don't have to be so specific. You could 
> >> > use
> >> > sea_water_temperature_difference
> >> > sigma_theta_difference
> >> > sigma_t_difference
> >> > because in the context of your use of them it is obvious what difference 
> >> > you
> >> > mean. That would make these names perhaps useful for other purposes too. 
> >> > This
> >> > is like the rather generic name
> >> > air_temperature_threshold
> >> > which doesn't indicate what the threshold can be used for. I would also 
> >> > suggest
> >> > that the definitions of the three kinds of mixed layer depth should have 
> >> > a
> >> > sentence added to say that the threshold should be stored in a 
> >> > coordinate var,
> >> > e.g. like the sentence for 
> >> > number_of_days_with_air_temperature_above_threshold.
> >> > 
> >> > Best wishes
> >> > 
> >> > Jonathan
> >> > 
> >> > ----- Forwarded message from Sebastien Villaume 
> >> > <sebastien.villa...@ecmwf.int>
> >> > -----
> >> > 
> >> >> Date: Thu, 5 Apr 2018 13:55:50 +0000
> >> >> From: Sebastien Villaume <sebastien.villa...@ecmwf.int>
> >> >> To: CF Metadata <cf-metadata@cgd.ucar.edu>
> >> >> Subject: Re: [CF-metadata] how to use
> >> >> ocean_mixed_layer_thickness_defined_by_*
> >> >> X-Mailer: Zimbra 8.6.0_GA_1200 (ZimbraWebClient - FF52 
> >> >> (Linux)/8.6.0_GA_1200)
> >> >> 
> >> >> Hi all,
> >> >> 
> >> >> It seems that my question did not attract much response. :(
> >> >> 
> >> >> Is it because no one knows how to use these standard names and properly 
> >> >> encode
> >> >> the "quantity that differs from its surface value by a certain amount"?
> >> >> 
> >> >> Giving it more thoughts I feel that I need new standard names to 
> >> >> describe this
> >> >> properly.
> >> >> 
> >> >> Would it make sense to describe it this way:
> >> >> 
> >> >> float swtd ;
> >> >> swtd:standard_name = "sea_water_temperature_difference_wrt_sea_surface" 
> >> >> ;
> >> >> swtd:units = "degC" ;
> >> >> swtd:axis = "Z" ;
> >> >> swtd:positive = "down" ;
> >> >> swtd:long_name = "temperature difference wrt its surface value"
> >> >> 
> >> >> float omlt(t, j, i) ;
> >> >> omlt:standard_name = 
> >> >> "ocean_mixed_layer_thickness_defined_by_temperature" ;
> >> >> omlt:units = "m" ;
> >> >> omlt:coordinates = "time swtd1 latitude longitude" ;
> >> >> omlt:long_name = "ocean mixed layer defined by difference of 
> >> >> temperature from
> >> >> surface"
> >> >> 
> >> >> and the scalar value swtd can be 0.5 degC, 1.0 degC, etc.
> >> >> 
> >> >> In this case, I need standard names working with temperature, 
> >> >> sigma_theta and
> >> >> sigma_t:
> >> >> 
> >> >> sea_water_temperature_difference_wrt_sea_surface
> >> >> sigma_theta_difference_wrt_sea_surface
> >> >> sigma_t_difference_wrt_sea_surface
> >> >> 
> >> >> 
> >> >> any quick suggestions are very welcome!
> >> >> 
> >> >> thanks
> >> >> /Sébastien
> >> >> 
> >> >> ----- Original Message -----
> >> >> > From: "Sebastien Villaume" <sebastien.villa...@ecmwf.int>
> >> >> > To: "CF Metadata" <cf-metadata@cgd.ucar.edu>
> >> >> > Sent: Wednesday, 4 April, 2018 14:04:42
> >> >> > Subject: [CF-metadata] how to use 
> >> >> > ocean_mixed_layer_thickness_defined_by_*
> >> >> 
> >> >> > Dear list,
> >> >> > 
> >> >> > I have a question regarding the use of the standard names of the form
> >> >> > ocean_mixed_layer_thickness_defined_by_*
> >> >> > 
> >> >> > the definition from the official standard name table is:
> >> >> > 
> >> >> > "The ocean mixed layer is the upper part of the ocean, regarded as 
> >> >> > being
> >> >> > well-mixed. The base of the mixed layer defined by temperature, sigma 
> >> >> > or
> >> >> > sigma_theta is the level at which the quantity indicated differs from 
> >> >> > its
> >> >> > surface value by a certain amount."
> >> >> > 
> >> >> > From what I understand, in the case of
> >> >> > ocean_mixed_layer_thickness_defined_by_temperature, I should specify 
> >> >> > some
> >> >> > quantity as a scalar (Z axis), but it is actually a temperature 
> >> >> > difference that
> >> >> > I need to encode, not a plain temperature : temp_surf - 
> >> >> > temp_base_level =
> >> >> > temp_delta_defining_mixed_layer_thickness
> >> >> > 
> >> >> > if it was simply a temperature I would maybe use:
> >> >> > 
> >> >> > float swpt ;
> >> >> > swpt:standard_name = "sea_water_potential_temperature" ;
> >> >> > swpt:units = "degC" ;
> >> >> > swpt:axis = "Z" ;
> >> >> > swpt:positive = "down" ;
> >> >> > 
> >> >> > but what I really want to encode is a temperature difference. How can 
> >> >> > I specify
> >> >> > that? Is "sea_water_potential_temperature" the correct standard name 
> >> >> > to use?
> >> >> > 
> >> >> > 
> >> >> > same question for ocean_mixed_layer_thickness_defined_by_sigma_theta, 
> >> >> > how do I
> >> >> > encode a "difference" in sigma_theta?
> >> >> > 
> >> >> > thanks,
> >> >> > 
> >> >> > ____________________________________
> >> >> > 
> >> >> > Dr. Sébastien Villaume
> >> >> > 
> >> >> > M.A.R.S. Analyst
> >> >> > ECMWF Data Governance facilitator
> >> >> > 
> >> >> > ECMWF
> >> >> > Shinfield Park,
> >> >> > Reading RG2 9AX, UK
> >> >> > +44 (0)118 949 9301
> >> >> > +44 (0)7825 521592
> >> >> > sebastien.villa...@ecmwf.int
> >> >> > ____________________________________
> >> >> > _______________________________________________
> >> >> > CF-metadata mailing list
> >> >> > CF-metadata@cgd.ucar.edu
> >> >> > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
> >> > 
> >> >  
> >> > CF-metadata Info Page - mailman.cgd.ucar.edu
> >> > mailman.cgd.ucar.edu
> >> > This is an unmoderated list for discussions about interpretation, 
> >> > clarification,
> >> > and proposals for extensions or change to the CF conventions.
> >> > 
> >> > 
> >> >> _______________________________________________
> >> >> CF-metadata mailing list
> >> >> CF-metadata@cgd.ucar.edu
> >> >> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
> >> > 
> >> > ----- End forwarded message -----
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