20040921: Gempak - gdinfo and gdlist for eta 212 grids ...

>From: "Kevin Doty" <address@hidden>
>Organization: UCAR/Unidata
>Keywords: 200409211839.i8LId7V9000101

>Institution: University of Alabama at Huntsville
>Package Version: 5.7.1
>Operating System: Linux
>Hardware Information: i386-linix cluster
>Inquiry: Dear Colleague:
>
>I have a couple of simple questions about map projections related to using com
> mands like "gdinfo" and "gdlist".
>
>In the example below with the eta 212 grid it identifies the Lambert Conformal
>  Projection, a central latitude of 25 degrees, a central longitude of -95.0 d
> egrees, and a true latitude of 25 degrees (correct?).  

Kevin,
The LCC/25;-90;25 is the projection of the grid defined by NCEP as #212
and that which the grib data is interpolated to by NCEP for distribution.
See the common grids used by NCEP for distribution here:
http://my.unidata.ucar.edu/content/software/gempak/examples/models/grids/

This is not the staggered Arakawa grid that the model computational domain uses.
For more information see: http://www.emc.ncep.noaa.gov/modelinfo/


>I presume that is the 
> projection that GEMPAK has interpolated to and not the original model data - 
> correct?  Am I correct that the original eta model "raw" data was on a Lamber
> t Conformal with 2 true latitudes centered at -100.0 West and 40.0 North ?

No. GEMPAK is not interpolating. GDLIST provides the data at the row/colum
locations available in the grid file. No interpolation is done. Interpolation 
is done by NCEP on postprocessing from the native ETA grid to those used for 
dissemination.

GEMPAK grid programs allow the user to specify a GAREA/PROJ to define
a region for which grid points within the domain are displayed.

If you use GDMAP, (or plot points in GDPLOT2) and use the same projection
as the grid, you will see that the grid is rectangular. If you choose to
display the data in a projection other than what the grid is defined,
then the grid points are being projected to their correct location, so
the grid lines will curve relative to the projection on display. This is not
interpolation, rather it is projecting the grid to the displayed location.


>
>Am I correct that a utility like "gdlist" interpolates all variables to a comm
> on grid or does one still have to account for "staggering" for variables like
>  wind components?  When one dumps the variables "UREL" and "VREL" from the et
> a grid 212 with gdlist, does one get "true" wind components or are they still
>  cartesian components for the specified GEMPAK map projection?

No. No interpolation is being done by gdlist. The U,V and mass quantities have
already been postprocessed by NCEP to common grid 212 grid points from the 
staggered
computaional grid. GEMPAK provides UREL, VREL and UN, VN for grid and north
relative components respectively. When you use grid functions like VECR()
or VECN(), they use the map scale factors necessary for the grid projection.

You can obtain the native Arakawa grid from NCEP, known as the E-grid.
Decoding to GEMPAK from NAGRIB will fill in the staggers since computation 
of finite differences in GEMPAK requires that all the quantities be co-located.
The projection will remain that defined in the GRIB. No reprojection will be 
done.

Steve Chiswell
Unidata User Support


>
>Many thanks,
>Kevin Doty
>
>
>GRID FILE: 2004092012_eta212.gem                                              
>                                    
>
> GRID NAVIGATION:
>     PROJECTION:          LCC
>     ANGLES:                25.0   -95.0    25.0
>     GRID SIZE:          185 129
>     LL CORNER:              12.19   -133.46
>     UR CORNER:              57.29    -49.39
>
>
>
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