[Wrf-users] problem with "shifted" SST grid when using RTG_SST_HR

Zulauf, Michael Michael.Zulauf at iberdrolaren.com
Fri Mar 4 17:09:52 MST 2011

Hi WRF Community. . .

I'm attempting to use the 1/12 degree RTG_SST_HR data with WRF (from
http://polar.ncep.noaa.gov/sst/) in conjunction with 1/2 degree GFS
output, and I'm having some problems.  What I'm seeing is that the SST
data is not being properly mapped to the model grid - instead it is
being offset by a small (but significant) amount.

For example, my d03 domain covers the Pacific Northwest at 3km
resolution.  If I examine the met_em.d03 files and look at the SST
field, I see that the coastline is shifted west by approximately 1
degree (and by a smaller amount north) compared to the GFS and static
fields.  It appears to me as if the ungrib/metgrid combo isn't reading
the grid locations properly.

I'm guessing that might be because the grid definition in the RTG_SST_HR
files seems a bit wonky.  Using wgrib2 to output the grid information, I
get this:

% wgrib2 -grid /filer_data/tmp/rtgssthr_grb_0.083.grib2
        lat-lon grid:(4320 x 2160) units 1e-06 input WE:NS output WE:SN
res 48
        lat 89.958000 to -89.958000 by 0.083000
        lon 0.042000 to 359.958000 by 0.083000 #points=9331200

The number of points is correct (assuming a 1/12 degree spacing), but
the stated spacing and start/stop lat/lon values are only approximate.
It seems to me that the lat and lon should be defined as:
	lon = (i - 1/2) * ds            (i from 1 to 4320)
	lat = -90 + (j - 1/2) * ds      (j from 1 to 2160)

	ds = 1/12 degree

Is there any way to override the internal grid description in the grib
file?  Or rewrite the grid description in the grib file?  Has anybody
seen this problem before?



michael.zulauf at iberdrolaren.com

Mike Zulauf
Meteorologist, Lead Senior
Wind Asset Management 
Iberdrola Renewables
1125 NW Couch, Suite 700
Portland, OR 97209
Office: 503-478-6304  Cell: 503-913-0403

Please be advised that email addresses for Iberdrola Renewables personnel have changed to first.last at iberdrolaREN.com effective Aug. 16, 2010.  Please make a note.  Thank you.

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