Subtracting pan-Arctic MM5 analysis from ERA40 global reanalysis

Hello,

I'm trying to subtract a monthly RMSD field (for the pan-Arctic MM5 from an ERA40 monthly RMSD field.
The MM5 data is only for the polar region, but the ERA40 reanalysis covers the whole globe. When I run
the program, I receive the error below. It may not make sense, since I'm subtracting a smaller field from
a larger one. However, I am only interested in the polar region (i.e. the smaller spatial scale). It just so
happens the ERA40 data is global and my MM5 data is polar.

I think my question would revolve around regridding the ERA40 data to the MM5 grid, so the arrays are
the same size.

fatal:Dimension sizes of left hand side and right hand side of assignment do not match
fatal:Execute: Error occurred at or near line 184

Below, I've printed the variable attributes,
========================================
Variable: rmsdTime_MM5
Type: double
Total Size: 95504 bytes
            11938 values
Number of Dimensions: 2
Dimensions and sizes: [icross | 94] x [jcross | 127]
Coordinates:
Number Of Attributes: 4
  units : Pa
  standard_name : MSLP
  long_name : RMSD: Sea Level Pressure
  rmsd_op_ncl : dim_rmsd applied to dimension: time
========================================
Variable: rmsdTime_ERA40
Type: double
Total Size: 84096 bytes
            10512 values
Number of Dimensions: 2
Dimensions and sizes: [latitude | 73] x [longitude | 144]
Coordinates:
            latitude: [90..-90]
            longitude: [ 0..357.5]
Number Of Attributes: 5
  _FillValue : -32767
  missing_value : -32767
  units : Pa
  long_name : RMSD: Mean sea level pressure
  rmsd_op_ncl : dim_rmsd applied to dimension: time
========================================
Variable: diff
Type: double
Total Size: 95504 bytes
            11938 values
Number of Dimensions: 2
Dimensions and sizes: [icross | 94] x [jcross | 127]
Coordinates:
Number Of Attributes: 6
  rmsd_op_ncl : dim_rmsd applied to dimension: time
  long_name : RMS field differences
  standard_name : MSLP
  units : Pa
  lat2d : <ARRAY of 11938 elements>
  lon2d : <ARRAY of 11938 elements>
(0) min=0 max=0
========================================

I assume this is occurring because the actual spatial expanse of each field is different? Am I right to
assume this? Each RMSD plot comes out fine for the respective input data (i.e. MM5 & ERA40), so I
pretty sure the error is in the actual subtraction.

; Now, take the difference of the MM5 and the ERA40.

    diff = rmsdTime_ERA40 ; trick to create array with coordinate info
    diff = rmsdTime_ERA40 - rmsdTime_MM5 ; over write values with differences
    diff_at_long_name = "RMS field differences"

Thank you,

Justin

___________________________
Justin Glisan
Doctoral Research Assistant
Geological and Atmospheric Sciences
Iowa State University
glisanj_at_iastate.edu
636.795.8859

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Received on Thu May 01 2008 - 00:52:44 MDT