[gradsusr] mysterious bug with merra-2 data

[Muhammad Yunus Ahmad Mazuki]

Hi,

It is a guess, but from the description in
https://gmao.gsfc.nasa.gov/pubs/docs/Bosilovich785.pdf , the data you
downloaded may be 'packed'. If you would provide the output of ncdump
-h of the files, that would be great. Please look at the variable
attribute scale_factor and add_offset. If the values are different for
each file, you will encounter a little bug in GrADS template option.
The situation is where GrADS would only take the attribute from the
first file and apply it to all files. This does lead to incorrect
reading of the all the data except for the first data in the template
option.

There are few ways to overcome this. First is to use cdo mergetime
with option -b F32 or -b F64, refer
https://code.mpimet.mpg.de/projects/cdo/wiki#netCDF-with-packed-data,
this will lead to the output being unpacked and large in size. Since
the bug is old, I'm not sure of the current status of cdo in handling
packed data.
Second is to unpack all the data using nco, merge them all, and repack
them. This will creat one large size packed data.
Third is to unpack all the data using nco, find a common value of
scale_factor and add_offset (which you have to calculate yourself),
and repack each file using the new common value. This will retain the
original number of files with slight variation in the size.

Do note that nco can be quit RAM intensive, in a sense that its
suitable for a cluster job compared to cdo. Whether to use cdo or nco,
it depends on the overall size of the data being worked on and the
amount of RAM in your machine.

Yunus

On Fri, Mar 2, 2018 at 10:11 PM, Antoine Molin <amolin at plenr.fr> wrote:
> Hi,
>
>
>
> I am using merra-2 data U&V wind speeds at 50m, for wind energy studies in
> France ; very good data…..
>
>
>
> Getting « MERRA2_400.tavg1_2d_slv_Nx.%y4%m2%d2.SUB.nc4»  daily files, data
> are obtained with grads, using xdfopen and enclosed data descriptor files.
>
>
>
> I really don’t understand why i get different values with different
> descriptor files, for same point and time :
>
>
>
> As an example, at point (49°N ; 4.375°E) and time  2017-12-01 at 0h,  i get
> U50m=5.31 m/s using « merra2_dec2017.ctl »  (lat = 49, lon = 4.375, t = 1),
> and U50m=5.22 m/s using « merra2_2017.ctl »  (lat = 49, lon = 4.375,
> t=8017)…..
>
>
>
> Why ????????????????
>
>
>
> Thanks !
>
>
>
>
>
> Antoine Molin
>
> Etudes de vent PlenR
>
> Tel : 03 20 47 99 76
>
>
>
>
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