Re: ECMWF ds117.1 data reading

Tianle Yuan wrote:
> Hi, Folks
>
> I need to read and convert temperature and specific humidity from the
> ERA40 ds.117.1 data set. There was a nice message on how to convert
> the 117.0 data by Dennis Shea:
>
> http://www.ncl.ucar.edu/Support/talk_archives/2006/0690.html
>
> It used the function 'g2fsh_Wrap' which requeires that latitude is in
> ascending order while the data in ds117.1 is organized in descending
> latitude order. Does it really matter?
>
> if so does a simple reverse of the data solve the problem?
>
> Thanks.
>
> TL
> _______________________________________________
Hello,

I had an NCL script that reads the ERA40 data and created
a netCDF file where the data go North-to-South. I modified it
slightly from the original.

       http://www.cgd.ucar.edu/~shea/ds1171.ncl

[1]
Note Q on the original file is in N->S order and is 4-dimensional.

Q_GDS4_ISBL(initial_time0_hours, lv_ISBL1, g4_lat_2, g4_lon_3)

[2]
T is 5D because the 'real_imaginary' spectral coef represent an extra
dimension.

T_GDS50_ISBL(initial_time0_hours, lv_ISBL1, real_imaginary, g50_lat_5, g50_lon_6)

[3]
NCL use spherepack ( a fortran library developed here at NCAR ).
The shsgC synthesis function expects the coef to have been generated
by spherepack. It turns out that the ECMWF spectral coef, when input
to the spherepack synthesis functions generate data that are S->N.
These should be reordered to match Q ... hence the reorder below.

         T_S2N = shsgC(TC(nt,kl,:,:,:), mlon) ; synthesize [S->N]
         T(nt,kl,:,:) = T_S2N(::-1,:) ; reorder [N->S}

[4]
Because the data variables are rather large

         T_GDS4_ISBL(124,23,2,160,160) ==> T(124,23,160,320)

I decided to synthesize in a loop to minimize memory. [Spherepack has
many temporary arrays.] This results in slower execution.

[5]
I added a very simple plot code.

====

The separate issue of regridding via (say) g2fsh.

NCL's ability to read/synthesize GRIB [both GRIB and the newer GRIB2]
means that intermediate files need *not* be created. (The reason for
generating
the netCDF was so that it could be read by someon's IDL code.) You
can take the ds1171.ncl code and remove the netCDF and add regridding
as you desire.

OK ... your question about S-N ordering "Does it really matter? "
Well, there are historical reasons for the following answers

[a] No .... for g2fsh
[b] Yes .... for g2fsh_Wrap
     This is because the _Wrap functions create coordinate arrays.
     These are created using functions which generate latitudes that run
S->N
[c] simple code to demonstrate this
      http://www.cgd.ucar.edu/~shea/ds1171.ncl_regrid

Good luck
D
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Received on Wed May 30 2007 - 16:11:02 MDT