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[netCDF #UBD-124546]: question on dBz

Hi Li,

> I have another unit usage to confirm. A field of "return power" with units of 
> "dB" is defined and given to us. But this cannot pass CF checker.
> 
> I looked at the latest version of uunit and found BW and Bm are defined. I 
> assume the base power of field above is watt, so my correct units will be
> :units="dBW";
> 
> Am I defining it correctly? Thanks for your help.

Yes, according to the udunits2 program:

  $ udunits2
  You have: BW
  BW
  You want: dBW
  dBW
      1 BW = 10 dBW
      x/dBW = 10*(lg((pow(10, (x/BW)))))

--Russ

> Xiang
> 
> -----Original Message-----
> From: Unidata netCDF Support [mailto:address@hidden]
> Sent: Wednesday, December 10, 2014 4:05 PM
> To: Li, Xiang
> Cc: address@hidden; address@hidden
> Subject: [netCDF #UBD-124546]: question on dBz
> 
> > Hi Russ,
> >
> > Thanks for your response of my question. Now I have two questions.
> >
> > 1. I checked ununits2-common.xml file for both 2.2.17 and 2.1.24 version 
> > and there is definition of BZ = lg(re (1e-6 m)^3) instead of dbz.
> 
> Right.
> 
> > If dBZ is not there, Should we use BZ as unit and use scaling factor of 0.1 
> > to make it equivalent to dBZ?
> 
> That's not necessary, "d" is a prefix understood by udunits, so "dBZ" is 
> already understood by udunits as 0.1 BZ.  You can verify this by using the 
> "udunits2" interactive command-line utility that comes with the udunits 
> software installation:
> 
> $ udunits2
> You have: dBZ
> You want: BZ
> 1 dBZ = 0.1 BZ
> x/BZ = lg((1e+18*(1e-18*(pow(10, 0.1*(x/dBZ))))))
> 
> Similarly, you should be able to use any of the SI prefixes in the 
> udunits2-prefixes.xml file in front of BZ, such as n, m, c, d, etc., and have 
> it understood.
> 
> > 2. thanks for pointing me to standard_name of 
> > equivalent_reflectivity_factor. Does this suggests that "radar 
> > reflectivity" measurement from Lidar or NAXRAD should use this as standard 
> > name attribute?
> 
> Since it's in the CF standard name table, if it means what you intend, as 
> given in its definition there:
> 
> "Equivalent reflectivity factor" is the radar reflectivity factor
> that is calculated from the measured radar return power assuming the
> target is composed of liquid water droplets whose diameter is less
> than one tenth of the radar wavelength, i.e., treating the droplets
> as Rayleigh scatterers. The actual radar reflectivity factor would
> depend on the size distribution and composition of the particles
> within the target volume and these are often unknown.
> 
> then I would use it as the value of the "standard_name" attribute for your 
> variable.  If you mean something else instead, you'll need to apply for a new 
> name by following the CF process described here:
> 
> http://cfconventions.org/faq.html#stdnames_ask
> 
> Also note that "dBZ" is merely the canonical unit for variables that have the 
> attribute
> 
> var:standard_name=equivalent_reflectivity_factor
> 
> You could use any compatible unit, such as BZ, or even the definition in 
> terms of SI units that's used in udunits2.
> 
> --Russ
> > -----Original Message-----
> > From: Unidata netCDF Support [mailto:address@hidden]
> > Sent: Tuesday, December 09, 2014 1:57 PM
> > To: Li, Xiang
> > Cc: address@hidden
> > Subject: [netCDF #UBD-124546]: question on dBz
> >
> > Hi,
> >
> > > I am working on constructing metadata for a satellite data set
> > > packaged using netCDF format. We want it CF-compliant.
> > >
> > > One of the variable is reflectivity profile with unit of 'dBz'.
> > > However, I could not find proper unit definition in UDUNITS.dat for
> > > this variable. Would you like to suggest what we should do to make
> > > the netCDF file CF-compliant and also describe the field correctly?
> >
> > The original udunits package did not support logarithmic units such as dBZ, 
> > but the current udunits (and all versions since 2.0.4) includes such 
> > support:
> >
> > http://www.unidata.ucar.edu/software/udunits/
> >
> > "dBZ" is defined there as equal to "0.1 lg(re (1e-6 m)^3)", so the 
> > reference level is included in the definition.  There is a CF standard name 
> > for the quantity "equivalent_reflectivity_factor"
> > which uses dBZ as its canonical units.
> >
> > > Thanks for your help,
> >
> > Your welcome!
> >
> > > Xiang Li
> >
> > --Russ
> >
> > Russ Rew                                         UCAR Unidata Program
> > address@hidden                      http://www.unidata.ucar.edu
> >
> >
> >
> > Ticket Details
> > ===================
> > Ticket ID: UBD-124546
> > Department: Support netCDF
> > Priority: Normal
> > Status: Closed
> >
> >
> Russ Rew                                         UCAR Unidata Program
> address@hidden                      http://www.unidata.ucar.edu
> 
> 
> 
> Ticket Details
> ===================
> Ticket ID: UBD-124546
> Department: Support netCDF
> Priority: Normal
> Status: Closed
> 
> 

Russ Rew                                         UCAR Unidata Program
address@hidden                      http://www.unidata.ucar.edu



Ticket Details
===================
Ticket ID: UBD-124546
Department: Support netCDF
Priority: Normal
Status: Closed