[gradsusr] Incompatible Grids in 20th Century Reanalysis (Version 2) Data

[Stephen McMillan]

Recently I wrote the Webmaster for the 20th Century Reanalysis V2 data
plotting pages, regarding grid incompatibility between the following two
data files:

Monthly means:

ftp://ftp.cdc.noaa.gov/Datasets/20thC_ReanV2/Monthlies/monolevel/air.sig995.mon.mean.nc
Monthly longterm means:

ftp://ftp.cdc.noaa.gov/Datasets/20thC_ReanV2/Derived/Monthlies/monolevel/air.sig995.mon.ltm.nc

I can plot the "air" variable from each without a problem, but when I
subtract the longterm mean from the mean to create an anomaly, I get a grid
incompatibility error.  The means file was sdfopened first, the longterm
means file second.  For example, doing Aug1969:

ga-> d air(time=aug1969)-air.2(t=8)
Operation error:  Incompatable grids
  Dimension ranges aren't equivalent
  Dimension = 0
  1st grid range = 1 181   2nd = 1 181
  Error ocurred at column 29
DISPLAY error:  Invalid expression
  Expression = air(time=aug1969)-air.2(t=8)

My workaround is to first regrid the longterm mean to a 2-deg linear grid,
but it would be preferable to do a simple difference as in example above.
 The Webmaster suggests it's a GrADS bug.  His response:

"The grids are identical.  This appears to be a GrADS bug.  No such
problem obtains in NCL.  GrADS appears to insist that the longitudes
in the LTM file don't wrap, even when they are manually set to do so.
We have run every test we can think of, including dumping the longitudes
from both files and comparing them.  They are the same."

I have tried this with OpenGrADS versions 2.0.a7.oga.3 and 2.0.a9.oga.3 with
equal results, but have not tried with the COLA version of either.  Can one
of the developers or other user shed some light on this?

I am using GrADS on a Windows XP.

Stephen McMillan

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