Re: monthly in daily

[Mick Spillane <spillane@xxxxxxxxxxxxx>]

Hi Celine,
	Assuming that the monthly data are located near the center of
each month then two cases arise

1) you don't need the daily to monthly differences before mid-Jan
   and after mid-Dec : this is the easiest case
2) you need to compare the daily data with interpolated monthlies
   at all days of the year : this requires a modulo axis.

Both cases are demonstrated below.  There may be more elegant solutions
from users who do this more often than I but I believe this should work
for you.

A) generate a test monthly series by creating the files celine.cdl and
   celine_mod.cdl follows:
----------------------
netcdf celine {
dimensions:
         TIME =   12 ;
variables:
         double TIME(TIME) ;
                TIME:units = "days since 1900-01-01 00:00:00" ;
                TIME:time_origin = "1-JAN-1900" ;
                TIME:point_spacing = "uneven" ;
                TIME:modulo = " " ;
         float  V(TIME) ;
data:
  TIME =
    32887.5, 32917.0, 32946.5, 32977.0, 33007.5, 33038.0,
    33068.5, 33099.5, 33130.0, 33160.5, 33191.0, 33221.5;
  V =
    1, 3, 5, 7, 5, 3, 1, -1, -3, -5, -3, -1 ;
}
--------------------
netcdf celine_mod {
...
                TIME:modulo = " " ;
...
(celine_mod.cdl is identical to celine.cdl except for these two lines)

then create a netCDF file (celine.cdf and celine_mod.cdf) by the command
	ncgen -n celine.cdl
	ncgen -n celine_mod.cdl

B) Run the following in Ferret
        ! make a demo daily data series
	def axis/t=1-jan-1990:31-dec-1990:1/units=days tax
	def grid/t=tax grd
	let vday=5*sin(l[g=grd]*atan(1.)/45)
	use celine                    ! the non-modulo axis version
	set view upper ; plot v[d=1],vday
	let v1=v[d=1,g=grd,gt=@lin]
	plot/o v1-0.5               ! offset for clarity - notice that
                                      ! the ends of the year are missing
	use celine_mod                ! now use modulo version
	set view lower ; plot v[d=2],vday
        let v2=v[d=2,g=grd,gt=@lin]
        plot/o v2-0.5               ! offset for clarity - now the whole
                                    ! year is available
        list vday-v2
 01-JAN-1990 00 /   1:  0.121
 02-JAN-1990 00 /   2:  0.141
 03-JAN-1990 00 /   3:  0.162
 04-JAN-1990 00 /   4:  0.182
 05-JAN-1990 00 /   5:  0.202
...
 27-DEC-1990 00 / 361:  0.387
 28-DEC-1990 00 / 362:  0.408
 29-DEC-1990 00 / 363:  0.428
 30-DEC-1990 00 / 364:  0.449
 31-DEC-1990 00 / 365:  0.469

Hope this helps.  Depending on the format of your monthly data you will
need to edit in the "modulo" to its CDL file or define the grid axis
that reads it to be modulo.  See "modulo" in the user guide.

Mick

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