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
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