Dear Martin
Your points are good ones and have been raised before. More than once we have
talked about maintaining a CF version of the udunits definition to include dB
and sverdrup, or ask udunits to add them (if they're not there). dB is a dimen-
sionless unit, equivalent to 1. I suggest that dBZ should be changed to dB,
as I don't think we ought to have several of them. I believe that the default
reference levels are mostly conventional and stated in the definitions of the
standard name, as you say. They can be overridden by supplying a size-one or
scalar coordinate variable. You have previously suggested an xml table to
contain more information about the definition of standard names, haven't you?
It seems to me that an arrangement like that would be the right place to store
the default reference levels and scale factor in a machine-readable way.
Best wishes
Jonathan
----- Forwarded message from Martin Juckes - UKRI STFC
<martin.juc...@stfc.ac.uk> -----
> Date: Tue, 30 Oct 2018 13:42:13 +0000
> From: Martin Juckes - UKRI STFC <martin.juc...@stfc.ac.uk>
> To: "CF-metadata (cf-metadata@cgd.ucar.edu)" <cf-metadata@cgd.ucar.edu>
> Subject: [CF-metadata] Decibel units in CF standard names
>
> Hello All,
>
>
> The CF standard names have several variables using decibels ("dB") as units,
> and one using "decibels of Z (dBZ)":
>
> sound_intensity_level_in_air 1e-12 W m-2 10 log10(I/I0)
> sound_intensity_level_in_water 6.7e-19 W m-2 10 log10(I/I0)
> sound_pressure_level_in_air 2e-5 Pa 20 log_10(p/p0)
> sound_pressure_level_in_water 1e-6 Pa 20 log_10(p/p0)
> equivalent_reflectivity_factor 1 mm6 m-3 10 log_10(Z/Z0)
>
>
> Each has a different reference level, and two use an additional factor two in
> the definition of the decibel level.
>
>
> There are a few issues here, the main one is that "dB" is not a valid Udunits
> string. There is a secondary point that the details of the definitions are
> not easily available to software reading the files.
>
>
> Where Udunits does support decibels, it is units such as dBZ, dBW for which a
> specific reference value is defined. As all these variables have different
> reference values, that would require 4 new units.
>
>
> The reference values are currently specified within the standard name
> description. It maattry make sense to add explicit attributes so that this
> information can be made more accessible to users and software reading the
> file. E.g. "decibel_reference_level" with the name of a scalar variable
> holding the reference value (specifying standard name, units and value) and
> "decibel_scale_factor" set to "10" or "20". With these modifications it would
> be possible to compute the power/intensity etc from the decibel parameter.
>
>
> regards,
>
> Martin
>
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----- End forwarded message -----
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