[ncl-talk] sub: netcdf write 2dlat/2dlon

[Dave Allured - NOAA Affiliate]

Dale, Rick:

Please ignore my previous message, I hit send by accident.

The scheme for storing 2-D coordinates in Netcdf files is described by an
external convention, and is not part of the core Netcdf structure.  A full
description is in CF Conventions (1.8) section 5.1.  By conforming to the
convention, you enable the 2-D coordinates to be properly understood by a
wide range of generic software, not just NCL.  This also overcomes the
structural problem that 2-D attributes are allowed in NCL memory variables,
but not in Netcdf files.

Here is an example of the bare minimum Netcdf structure needed to conform.
There is a more complicated example in the CF document.

dimensions:
>   j = 384 ;
>   i = 320 ;
> variables:
>   integer VRH(j,i) ;
>     VRH:coordinates = "lon lat" ;
>   float lon(j,i) ;
>     lon:units = "degrees_east" ;
>   float lat(j,i) ;
>     lat:units = "degrees_north" ;


All variable names and dimension names are arbitrary.  You can use any
names you want.  But notice the special "coordinates" attribute which lists
the names of the two 2-D coordinate variables.  This is required by the CF
convention.

1-D coordinates are handled automatically by NCL, but not 2-D coordinates.
It is always necessary to have a little extra code to separately read and
write the 2-D coordinate variables.


On Fri, May 8, 2020 at 8:33 AM Rick Brownrigg via ncl-talk <
ncl-talk at ucar.edu> wrote:

> Hi Dale,
>
> You seem to be doing the proper things to attach coordinate variables to a
> variable. But by NetCDF convention (not due to NCL), "coordinate variables"
> are by definition !D arrays with a dimension name the same as the variable
> name.
>
> If you have 2D coordinates, the technique is generally to assign those to
> special lat2d/lon2d *attributes* of the variable.  Check out the quote
> below and the web page it was taken from.
>
> Rick
>
> "To plot data on a curvilinear grid, you must read these 2D lat/lon
> variables from the file and attach them to the data variable as special
> attributes called "lat2d" and "lon2d":"
> ,,,
>
>   temp at lat2d = a->gridlat_236    ;     113 x 151
>   temp at lon2d = a->gridlon_236    ;     113 x 151
>
> http://ncl.ucar.edu/Applications/plot_data_on_map.shtml
>
>
> On Fri, May 8, 2020 at 3:33 AM dale zuri via ncl-talk <ncl-talk at ucar.edu>
> wrote:
>
>> Hi NCL user,
>> I have been trying to write a variable that is function of 2dlat/lon to
>> the netcdf file.
>> I'm getting following error. I would appreciate any help to fix this
>> error.
>>
>> Thanks
>>
>> VRH!0    = "lat2d"                      ; assign named dimensions
>>
>> VRH!1    = "lon2d"
>>
>>
>> VRH&lat2d  = lat2d                     ; copy lat from T to ORO
>>
>> VRH&lat2d  = lon2d
>>
>>
>> ncdf->VRH = VRH
>>
>>
>> Variable: VRH
>>
>> Type: integer
>>
>> Total Size: 491520 bytes
>>
>>             122880 values
>>
>> Number of Dimensions: 2
>>
>> Dimensions and sizes: [nj | 384] x [ni | 320]
>>
>> Coordinates:
>>
>> Number Of Attributes: 3
>>
>>   lat2d : <ARRAY of 122880 elements>
>>
>>   lon2d : <ARRAY of 122880 elements>
>>
>>   _FillValue : -2147483647
>>
>> fatal:Coordinate variables must have one dimension only
>>
>> fatal:No coordinate variable exists for dimension (lat2d) in variable
>> (VRH)
>>
>
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