Internal Grads Grid

[Ben Quinn]

Hi all,

I'm downloading 1 degree (subregion) GFS forecast gribs from the NCEP FTP2U
servers and creating the control files with gribtoctl.pl.  Creating Grads
plots with the 1 degree grid is fine but i'm also running a script that
extracts vertical profiles from the gribs via Grads for use in vertical
sounding plots and i wanted Grads to interpolate to a finer resolution - ie
at the moment Grads rounds off any lat/lon figures i give it to the nearest
whole degree, but ideally i'd like to extract data in .2 degree increments
at least.  Reading the manual, it sounded easy enough to do :

" It is very important to point out that the internal GrADS grid can be any
grid as it is completely independent of the preprojected data grid. Thus,
there is nothing stopping you displaying preprojected data on a very high
res lon/lat grid (again, defined in the .ctl by xdef and ydef). In fact, you
could create and open multiple .ctl files with different resolutions and/or
regions which pointed to the same preprojected data file."

But for some reason when i alter xdef and ydef in the control file Gribmap
returns an error when trying to open the grib file.  I've tried a few
different combinations - running gribmap with the original control file,
then altering it and trying to open it all in Grads etc but to no avail.
I'm missing something somewhere.  A cut and paste from the original control
files gribtoctl is producing :

set ^gfs.t18z.pgrbf%f3
index ^gfs.t18z.pgrbf000.idx
undef 9.999E+20
title gfs.t18z.pgrbf000
*  produced by grib2ctl v0.9.12.5p34c
dtype grib 255
options template
options yrev
ydef 36 linear -45.000000 1
xdef 51 linear 110.000000 1.000000
tdef 61 linear 18Z14apr2006 3hr
zdef 21 levels
1000 975 950 925 900 850 800 750 700 650 600 550 500 450 400 350 300 250 200
150 100
vars 16

Altering ydef and xdef (for say a .5 degree grid) to

ydef 73 linear -45.000000 0.5
xdef 102 linear 110.000000 0.500000

Doesn't work for some reason... any help would be much appreciated!


Ben Quinn

The Brisbane Storm Chasers Homepage
http://www.bsch.au.com
The Australian Severe Weather Association
http://www.severeweather.asn.au