Re: regridding emissions data

AFAIK, there is no standard interpolation function in any language
that is mass conserving. You could calculate mass before and after,
then apply some correction factor to the interpolated grid values.
Of course, this is rather simplistic. It assumes that the correction be
applied globally.

[1]
The f2gsh (or, better, f2gsh_Wrap) will perform the interpolations.

    x42 = f2gsh_Wrap (x, (/64,128/), twave)

These are not mass-conserving. Because these use spherical harmonics,
they will have a smoothing effect. Think in terms of an FFT along
a specific latitude: the fft will return 180 (=360/2) harmonics.
When the T42 grid (64 latitudes and 128 longitudes) is reconstructed
with twave=0, only 64 (=128/2) will be used. All the info above harmonic
(wave) 64 is lost. If you want to truncate at wave number 42 (twave=42)
even more high wave number info would be lost.

Also, if the data are bounded at (say) zero, then spherical harmonic
interpolation should be used with caution.

[2] I suggest simple bilinear interpolation:

      load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl"

      printVarSummary( x )
      printMinMax(x, True)
      print("======================================")

   nlat = 64
   LAT = *latGau*(nlat, "lat", "latitude", "degrees_north")
   mlon = 128
   LON = *lonGlobeF*(mlon, "lon", "longitude", "degrees_east")
     

       x42 = linint2_Wrap (lon,lat,x, True, LON,LAT, 0)

      printVarSummary( x42 )
      printMinMax(x42, True)

[3] SCRIP[ http://climate.lanl.gov/Software/SCRIP/ ]
      has area conserving interpolation as an option.
      I looked here at NCAR for weight files of the form:

          map_1x1d_to_T42_aave_da_yymmdd.nc

      but did not find any. :-(

      You could use the SCRIP provided fortran software to generate
       a weight file. NCL could be used to create the 64x128 grid.
      
Good luck
D

Rob Nicholas wrote:
> I have a large datset that I'd like to regrid from a fixed grid
> (360x180) to a Gaussian grid (T42). The tricky part is that this is
> emissions data, so the transformation needs to be mass-conserving. The
> functions f2gsh() and f2gsh_Wrap() almost look like the right things,
> but it doesn't look like they'll handle the mass conservation
> correctly.
>
> Can anyone suggest a straightforward way to handle this?
>
> Thanks,
> Rob
> _______________________________________________
> ncl-talk mailing list
> ncl-talk_at_ucar.edu
> http://mailman.ucar.edu/mailman/listinfo/ncl-talk
>

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Received on Fri Dec 14 2007 - 21:09:36 MST