Talk:Units

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UDUNITS have to be used for the standard_names according to the CF convention. This is however not always obvious for chemistry and aerosol variables, where also other units are common. This page provides a discussion on this matter. See also Unidata Supported Units || Unit Converter, UDUNITS

Important issues are marked in RED , please COMMENT!

Agreed items of former discussions on the "Units" page can be found here.


JGregory: Don't mix units with the Compound

However, kgC and kgS aren't SI units. I think the unit has to be kg. This implies that it's the standard name which must somehow indicate that it is the mass of C or mass of S which is being referred to, rather than the mass of the compound e.g. dry_deposition_flux_of_sulfur_as_sulfate_at_surface. --JonathanGregory 16:51, 22 May 2006 (EDT)

......CTextor:

Very good idea, I will change this. --Christiane Textor (Christiane) 17:11, 2 June 2006 (EDT)

CTextor: SI units

Try to use SI units if allowed within udunits (e.g. not hours but seconds) --Christiane Textor (Christiane) 12:11, 2 June 2006 (EDT)

RanjeetSokhi(RS) / CTextor (CT): remarks on version June 12 from RS and answers from CT

  • RS: Table 1 mass_concentration is usually stated in ug/m3 ie micrograms per cubic metre - this is conventional and used in all air quality literature (scientific and policy)
CT: --Christiane Textor (Christiane) 13 June 2006 (EDT)


CTextor (CT) / Martin Schultz (MS) / Sebastian Rast (RS): UDUNITS in atmospheric chemistry: concentration Christiane Textor (Christiane) 3 July 2006 (EDT)

CT: usually the concentration of chemical compound are not given in UDUNITS, see the remark of Ranjeet above for the air quality community, where aerosol mass_concentration is usually stated in ug/m3ich, or the global chemistry community where the tropospheric column of a gas is given in [molec/cm2], and the dry deposition in [g/m2/month]. This should be changed
cm2 -> m2
molec -> mole
g -> kg
We had a discussion (in german) that this would be difficult, because all papers are in the non-udunits, but at the same time it would be good to agree on one standard within GEMS and HTAP.

CTextor (CT) / Martin Schultz (MS) / Sebastian Rast (RS) / Jonathan Gregory (JG): UDUNITS in atmospheric chemistry: time issuesChristiane Textor (Christiane) 3 July 2006 (EDT)

  • MS: (translated from german by ct) The use of non-UDUNITS can be of advantage, if it concern a variable that is integrated over a certain period, e.g. a the dry deposition flux over a month would need to have mole/m2/month. This is not the same as a mean dry dep. which could be given in mole/m2/s.
  • CT: (corrected) I think the cell method of CF should be applied here, question asked to Jonathan Gregory
  • JG In other situations like this we have taken the view that the rate (X s-1) and the integral (X) are different quantities, with different standard names. They are related in a simple way, of course, but it's not quite as simple as a statistical relationship like mean or standard deviation, for which cell_methods are intended. Thus, for instance, we have different names for precipitation_flux (kg m-2 s-1) and precipitation_amount (kg m-2). In your case, we would define a standard name for surface_dry_deposition_mole_amount_of_sulfur_dioxide_due_to_turbulence (mol m-2). Does that make sense? Its cell_methods would be sum, as you say (the default for extensive quantities), and its time-bounds indicate the period of integration. (See discussion in CF 7.3.)
  • CT This would give a file like this for 2 months, Martin, Sebastian, what do you think?

dimensions:

lat = 18 ;
lon = 36 ;
pres = 15 ;
time = UNLIMITED ;//(2 currently

variables:

float drydep_SO2(time,pres,lat,lon) ;
ddep_SO2:standard_name = "surface_dry_deposition_due_to_turbulence_mole_amount_of_sulfur_dioxide" ;
ddep_SO2:units = "mole/m2" ;
ddep_SO2:cell_methods="time: sum (over month)"
float lon(lon) ;
lon:long_name = "longitude" ;
lon:units = "degrees_east" ;
float lat(lat) ;
lat:long_name = "latitude" ;
lat:units = "degrees_north" ;
float pres(pres) ;
pres:long_name = "pressure" ;
pres:units = "hPa" ;
double time(time) ;
time:long_name = "time" ;
time:units = "days since 1990-1-1 0:0:0" ;
time:bounds = "time_bnds";
double time_bnds(time,nv);

data:

time = 1., 2.;
time_bnds = 0.,1.,1.,2.;
  • CT We will however get the same kind of problem with the chemical production/destructions, which are given per months in GEMS-GRG. I guess we should use:;
chemical_net_production_rate_of_mole_fraction_of_ozone 1/s=mole/mole/s
chemical_gross_production_rate_of_mole_fraction_of_ozone 1/s=mole/mole/s
chemical_destruction_rate_of_mole_fraction_of_ozone 1/s=mole/mole/s
and for the integral over a month using the cell method:
chemical_net_production_of_mole_fraction_of_ozone mole/mole
chemical_gross_production_of_mole_fraction_of_ozone mole/mole
chemical_destruction_of_mole_fraction_of_ozone mole/mole