[IDV #MLX-680484]: Problems with MM5 Netcdf Sigmalevel an IDV

Hi Florian-

> pstarcrs is the surface pressure minus the top pressure,
> The top pressure for this file is 10 mb.

Thanks.

So here's where this stands.  I was able to create an
NcML file that wraps your netCDF file and adds the necessary
metadata for converting sigma to pressure.  It looks like
(also attached):

<?xml version="1.0" encoding="UTF-8"?>
<netcdf xmlns="http://www.unidata.ucar.edu/namespaces/netcdf/ncml-2.2"; 
location="MMOUT_DOMAIN1_02_interp.nc" >
  <attribute name="Conventions" type="String" value="CF-1.0" />
  <variable name="sigma_level">
    <attribute name="standard_name" type="String" 
value="atmosphere_sigma_coordinate"/>
    <attribute name="formula_terms" type="String" value="sigma: sigma_level 
ps:pstarcrs ptop: ptop"/>
    <attribute name="units" type="String" value="" />
    <values>.025 .075 .125 .175 .225 .275 .325 .375 .420 .460 .500 .540 .580 
.620 .659 .700 .740 .780 .820 .855 .885 .914 .945 .965 .975 .985 .995</values>
  </variable>
</netcdf>


Essentially, it changes the convention to CF-1.0 and adds in
the formula terms for the conversion.  The CF sigma formula
is:

p = ptop + sigma(k)*(ps-ptop)

where in your file, ptop is ptop and ps is pstarcrs.   Effectively,
the pressure is the percentage of the range between the top and
bottom pressure.  For  reference on this, see:

http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.1/cf-conventions.html#dimensionless-v-coord

The problem is that your sigma values are not percentages, but
range from 25 to 995:

sigma_level = 25, 75, 125, 175, 225, 275, 325, 375, 420, 460, 500, 539.9999,
   580.0001, 620, 659.9999, 700.0001, 740, 780, 820, 855, 885, 914.9999,
   945, 965.0001, 975, 985, 995 ;

when I would expect them to range from 0 to 1 (.025 to .995).  So, the
equation does not give the correct values, unless we override the
values with the <values> tag.

The problem with the latter is that if your sigma values change
for another file, you'd need to change the <values> tag accordingly.

Place the attached file in the same directory as your netCDF file
and open the NcML file in the IDV.

If you have questions, on this, let me know.

Don

> 2008/4/11, Unidata IDV Support <address@hidden>:
> >
> > Hi Florian-
> >
> > > and thank you for the quick response,
> > > I put a copy of the file on:
> >
> > For sigma, the formula needs the base pressure and the
> > top  pressure (PTOP).  I didn't see a PTOP variable in
> > the file and I'm guessing that pstarcrs is the base
> > pressure.  What is PTOP for this file?
> >
> > Don
> >
> > > >2008/4/4, Unidata IDV Support <address@hidden>:
> > > >
> > > > >Hi Florian-
> > > >
> > > > > I would like to view my MM5 model output with IDV 2.4. I converted
> > the
> > > > MM5
> > > > > output to netcdf Format with a tool available on MM5 Homepage. Its
> > > > possible
> > > > > to open the file in IDV and create 2D plots but IDV can't create 3D
> > > > displays
> > > > > because it doesn't recognize the sigma coordinate:
> > > > >
> > > > > "Unkown vertical coordinate with Unit null, using linear scale to
> > > > Altitude"
> > > >
> > > > >The problem is that the netCDF format that gets produced does
> > > > >not have the necessary information to convert from sigma to
> > > > pressuer/height.
> > > > >It needs to use something like the CF convention to specify what
> > > > >parameters are needed for the conversion.
> > > >
> > > > > I read in the support email archive that other users had similar
> > > > problems
> > > > > but i didn't find a solution for this problem. It would be nice if
> > you
> > > > could
> > > > > tell me if there is a way to tell IDV how to use the file correctly.
> > I
> > > > > attached  a copy of the file to this email.
> > > >
> > > > >The attachment wasn't there or didn't come through.  Can you resend
> > > > >it?  I'll see if this can be done through NcML.
> > > >
> > > > >Don Murray
> > > >
> > > >
> > > > Ticket Details
> > > > ===================
> > > > Ticket ID: MLX-680484
> > > > Department: Support IDV
> > > > Priority: Normal
> > > > Status: Open
> > > >
> > > >
> > >
> > >
> >
> >
> > Ticket Details
> > ===================
> > Ticket ID: MLX-680484
> > Department: Support IDV
> > Priority: Normal
> > Status: Open
> >
> >
> 
> 


Ticket Details
===================
Ticket ID: MLX-680484
Department: Support IDV
Priority: Normal
Status: Open

Attachment: MM5.ncml
Description: Binary data