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20010703: Converting RUC hybl coords to pressure



Chris,

FSL uses hybrid levels (HYBL) for the internal model surfaces
(those are the nggrib files on the NCEP server). The pggrib files
on the server are grids on pressure levels. sgrib are the surface/boundary
kevel grids.

Hybrid levels are not a constant coordinate, so you won't be able to interpolate
them using gdvint because nothing in the hybl coordinate tells you what the
level is. It is a user defined coordinate.

The HYBL model levels are levels like 1,2,3....40 etc. The lower levels of the
RUC are on sigma surfaces, while the upper levels are theta surfaces.
You need top consult the FSL documentation on the RUC coordinate system to
identify what the levels are for the data you have. That is, there is no
fixed definition that states that HYBL is always .995 sigma. FSL has a list of
what each HYBL equates to. 

Gdvint knows how to convert defined coordinates like PRES, THTA, SGMA where the
definition of the coordinate is known. Where SGMA = P/Ps, etc, you can get
PRES levels by knowing Ps, and with THTA, you can invert P when you know T.
Since nothing in the HYBL definition gives a relationship to a state variable,
all you can do is rename the grid to the appropriate SGMA or THTA level
given the RUC lookup values, and then interppolate to pressure- or get the
pggrib grids.

Steve Chiswell
Unidata User Support

On Tue, 3 Jul 2001, Chris E. Halcomb wrote:

> Steve,
> 
> We have been trying to solve this problem for a while now, so its time to 
> turn 
> to you.  We have RUC grib files that we obtained from NCEP.  We were able to 
> successfully convert (using GEMPAK 5.4) those files to a gempak format, but 
> have 
> had no luck in converting into pressure coords using GDVINT.  PRESS and HGHT 
> fields are listed
> among the listed HYBL data and they plot using GDCNTR at a single HYBL level.
> Here is an example of what we have tried in GDVINT:
> 
> GDFILE   = 1999120506_ruc.gem
>  GDOUTF   = 1999120506_ruc.gem
>  GDATTIM  = 991205/06f00
>  GVCORD   = hybl/pres
>  GLEVEL   = 1000-100-50
>  MAXGRD   = 2000
>  GAREA    = grid
>  VCOORD   = hybl;none;sgma
>  
> Do you have any ideas?  Thank you for your time.
> 
> Chris Halcomb
> Graduate Research Assistant
> 201 Gentry Hall
> Columbia, MO 65211
> 
>