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[GEMPAK #FYW-889767]: Help with using dcgrib2 to decode AMPS model output

Robert,

I suspect that the problem you have is that since the central longitude of the 
grid is
on the date line, the decoder is missinterpreing the direction that the grid 
goes around the dateline.

The routine that would be used is $GEMPAK/source/gemlib/gb/gbpolr.c

I found a similar problem  with the LCC case where the code tries to add 360 to 
the longitude when it extends across 180
for the particular case when the central longitude was outside the grid. 
Can you provide one of your gribs?

Steve Chiswell
Unidata User Support



> CSBF is on Matt Lazzara's Antarctic IDD and I started trying to decode the 
> AMPS model from UCAR using dcgrib2.  All of the parameters decoded okay it 
> appears, but the navigation is incorrect.  GEMPAK placed the grid over 
> Africa/Indian Ocean:
> 
> GRID NAVIGATION:
> PROJECTION:          STR
> ANGLES:               -90.0   180.0     0.0
> GRID SIZE:          164 216
> LL CORNER:              35.48     37.57
> UR CORNER:             -25.16     38.41
> 
> Matt said that Tom Yoksas had helped him modify the McIDAS tables so it would 
> decode this GRIB data.  I wonder if GEMPAK needs a table mod as well.   Could 
> you help me out with that? I am afraid it's a bit over my head.  Kevin 
> Manning at UCAR senr me this.  Hopefullt it will help:
> 
> 
> Hi, Robert.
> 
> I *think* our grid description in the GRIB files is correct.  The
> parameters you listed make me a little suspicious of the GEMPAK
> decoding.  What you've listed as the coordinates of the upper-right
> corner are actually the lower left corner, and your lower-left
> corner is completely out of the ballpark.
> 
> I'm not sure why the "Starting Lat/Lon" should be interpreted as
> the upper-right corner in the GEMPAK decoding.  Maybe there's
> something with the sign of certain parameters for Southern
> Hemisphere grids that my encoding or GEMPAK's decoding is missing.
> 
> -------
> 
> Here's my reading of the grid parameters I've encoded in the GRIB
> file:
> 
> GRID DESCRIPTION SECTION (GRIB SECTION 2):
> Length of GDS                     : 32
> Number of V. Coordinate Parms     : 0
> List Starting point               : 255
> Data Representation type          : 5 : Polar Stereographic Grid.
> NI                           : 164
> NJ                           : 216
> Lat 1                        : -25.163002
> Lon 1                        : 38.406002
> Direction increments not given.
> Earth assumed Spherical.
> U and V relative to grid x and y.
> Lov                          : 180.000015
> Dx                           : 57.548004
> Dy                           : 57.548004
> Projection Center Flag       : 1
> Points scan in +i direction
> Points scan in +j direction
> Adjacent points in i direction are consecutive.
> 
> Thanks very much,
> 
> Robert Mullenax
> 
> 
> 


Ticket Details
===================
Ticket ID: FYW-889767
Department: Support GEMPAK
Priority: Normal
Status: Closed