Using data that spans multiple files.

Question:

How can I plot the time series of a variable which spans multiple data files?

Example:

> I have some netcdf-files with a lot of data. Each file represents the
> status of a certain model time. Now i want to plot one variable (say the
> instantanous mean) as a function of time. How can i do it?
>
> And can I save the calculated time series in a seperate (binary or netcdf)
> file from ferret?

Explanation:

The answer to this depends on how your existing NetCDF files are arranged. You say that each file has a single time step. Let us suppose there are 3 spatial dimensions for the variables in your NetCDF files. There are two ways that this could be encoded: either as 3D (time-independent) variables or as 4D variables on a single point time axis. "ncdump -h mynetcdf.cdf" will show if the variables have a 1-point time axis.

Solution:

==> If the files have a 1 point time axis there are two solutions:

Use ferret to concatenate the files:
```
        use file1
        save/file=concat.cdf var1, var2, var3
        use file2
        save/append/file=concat.cdf var1,var2,var3
.
.
.
```
note that the names "var1", "var2",... refer to the "current" data set since no explicit data set was specified as in "var1[d=file1]".

create an "MC" (multi-cdf) data set by writing a descriptor file that ties the files into a single data set. I will include an example of such a descriptor below.

==> If your files do not have a time word,

        ! *** set up
        define axis/t=1-jan-1980:10-jan-1980:1/unit=days t10days
        define grid/t=t10days gg

        let var1_with_T = t[g=gg]*0 + var1
        set variable/title="xxx"/units="uuu" var1_with_T

        ! *** loop over input files
        use file1.cdf
        save/file=concat.cdf var1_with_T[l=1]

        use file2.cdf
        save/file=concat.cdf/append var1_with_T[l=2]

        ...

--------------------------------------------------
Here is an example "MC" descriptorfile: my_mc_descriptor.des:

 $FORMAT_RECORD
   D_TYPE               = '  MC',
   D_FORMAT             = '  1A',
   D_SOURCE_CLASS       = 'MODEL OUTPUT',
   D_SOURCE             = 'PHIL/SIEG',
   D_SUBSOURCE          = 'TROPICAL PACIFIC',
 $END
 $BACKGROUND_RECORD
   D_EXPNUM             = '0068',
   D_MODNUM             = '  AA',
   D_TITLE              = 'MOM model output; Hindcast, Double Ramp; NCEP 79-95 twice daily winds'
   D_FORCING_SET        = 'NCEP 79-95 daily',
   D_T0TIME             = '14-JAN-1978 14:00:00',
   D_TIME_UNIT          = 3600.0,
   D_TIME_MODULO        = .TRUE.,
   D_ADD_PARM           = 15*' ',
 $END
 $MESSAGE_RECORD
   D_MESSAGE            = ' ',
   D_ALERT_ON_OPEN      = F,
   D_ALERT_ON_OUTPUT    = F,
 $END
*************************************************
*************************************************
 $EXTRA_RECORD
 $END

 $STEPFILE_RECORD
   s_filename           = 'mtaa068-nc.001',
   S_AUX_SET_NUM        = 0,
   S_START              = 87623.0,
   S_END                = 183599.0,
   S_DELTA              = 24.0,
   S_NUM_OF_FILES       = 400,
   S_REGVARFLAG         = ' ',
 $END
 **************************************************
 $STEPFILE_RECORD
   s_filename           = '**END OF STEPFILES**'
 $END
 **************************************************

Last modified: Aug 25, 1998