[Met_help] Re: MET package question -- water vapor

Lacey Holland lholland at ucar.edu
Wed Nov 28 08:09:12 MST 2007

Hi, Brian!

I'm cc'ing this to met_help, since I think it may be of interest to 
others around here.  I hope you don't mind.  ;)

You are right - if the units are different, then it wouldn't make sense 
to verify until you get them into the same units.  Right now, I don't 
see an easy way around it, but I'll check on ways around it and see if 
others around here can think of solutions.  It may be a conversion we 
have to build into MET in a future release.  Currently, we're planning 
on building in conversations for relative humidity, dewpoint temperature 
and other calculated quantities into the next release, but I'll see what 
can be done about specific humidity.

Thanks for the heads up!  We certainly appreciate knowing about this 

Brian Gaudet wrote:
> Hi Lacey,
> I have run into some issues with regard to creating statistics for 
> water vapor
> in the atmosphere (some of which may be WRF postprocessor issues).
> Specifically:
> 1)  It appears that the only water-vapor related variables the WRF-
> postprocessor handles for the GRIB output format are specific
> humidity and relative humidity.  Since relative humidity involves
> temperature as well, that leads us to verify specific humidity
> (as is specified in the default Config files provided with MET).
> 2)  During the verification of specific humidity, however,
> I've noticed that the units of this parameter in the
> forecast field (i.e., ultimately from WRF via the
> postprocessor) appear to be in units of kg/kg
> (with a sample FBAR  value of 0.002 near the surface
> this time of year)
> but the units of the observational field (i.e., from PrepBufr
> via pb2nc) appear to be in units 10^6 times smaller
> (i.e., the corresponding sample OBAR value is 1918.4).
> Obviously, comparing quantities this many orders of magnitude
> apart does not produce meaningful statistics.
> The situation would be consistent with the observational-oriented
> code trying to convert g/kg observations into kg/kg but accidentally
> multiplying instead of dividing by 10^3.   But I have not yet
> been able to figure out how to modify the code in order to correct
> (or test) this.
> Do you know if I am doing anything wrong, or if not do you know
> how this problem may be corrected?
> Thanks,
> Brian Gaudet

Lacey Holland

UCAR/NCAR, Research Applications Laboratory
P.O. Box 3000
Boulder, CO  80307
(303) 497-8396

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