modulo regridding

[DELCLAUX Francois <francois.delclaux@xxxxxxxxxxxxxxxxxxx>]

Hi, ferret users,
I would like to create a monthly climatology using modulo regridding.
As my variable is defined on the 1rst of each month, I have to center
it in the middle of the month using interpolation regriding first.
So, as described is the documentation (p74 on ferret 4.4 user guide),
here are the different steps:

1) the variable:
 div         Divergence                       1:144     1:73      1:2      
1:540
             s**-1 on grid GNL1 with -0.00004415594 & -32767 for missing
data
             X=1.2W(-1.2):1.2W  Y=91.2S:91.2N  Z=-125:1375  

2) the grid:
yes? show g/t gnl1
    GRID GNL1
 name       axis              # pts   start                end
 LONGITUDE LONGITUDE          144mr   0E                   2.5W
 LATITUDE  LATITUDE            73 r   90S                  90N
 LEVELIST  HEIGHT (millib       2 r-  250                  1000
 TIME      TIME               540 i   01-SEP-1957 12:00    01-AUG-2002 12:00
 
       L     T                   TBOX      TBOXLO                TSTEP (hours)
       1>  01-SEP-1957 12:00:00  720       17-AUG-1957 12:00:00   505500
       2>  01-OCT-1957 12:00:00  732       16-SEP-1957 12:00:00   506220
       3>  01-NOV-1957 12:00:00  732       17-OCT-1957 00:00:00   506964
   etc...

3) direct modulo regriding on div variable
let divclim1=div[gt=month_reg@mod]

4) centering div variable by interpolation regridding on month_reg
let divc = div[gt=month_reg]

5) modulo regridding
let divclim2 = divc[gt=month_reg@mod]


Now, when I list  divclim1, divc and divclim2 on 1960-1961 period, I get
the following
results:

yes? list/i=10/j=10/k=1/T="1-jan-1960":"1-dec-1961" divclim1, divc,divclim2
             DATA SET: ./atm_era40.nc
             TIME: 01-JAN-1960 00:00 to 01-DEC-1961 00:00
             LONGITUDE: 22.5E
             LATITUDE: 67.5S
             HEIGHT (millibars): 250
 Column  1: DIVCLIM1 is DIV[GT=MONTH_REG@MOD]
 Column  2: DIVC is DIV[GT=MONTH_REG]
 Column  3: DIVCLIM2 is DIVC[GT=MONTH_REG@MOD]
                   DIVCLIM1 DIVCDIVCLIM2 
17-DEC-1959 / 23520:  -3.646 ....    ....
16-JAN-1960 / 23521:  -2.322 ....    ....
15-FEB-1960 / 23522:   2.363 ....    ....
17-MAR-1960 / 23523:   1.639 ....    ....
16-APR-1960 / 23524:   4.567 ....    ....
17-MAY-1960 / 23525:   2.185 ....    ....
16-JUN-1960 / 23526:   4.941 ....    ....
17-JUL-1960 / 23527:   1.758 ....    ....
16-AUG-1960 / 23528:   0.212 ....    ....
16-SEP-1960 / 23529:   2.182 ....    ....
16-OCT-1960 / 23530:   0.431 ....    ....
15-NOV-1960 / 23531:  -2.144 ....    ....
16-DEC-1960 / 23532:  -3.646 ....    ....
15-JAN-1961 / 23533:  -2.322 ....    ....
15-FEB-1961 / 23534:   2.363 ....    ....
17-MAR-1961 / 23535:   1.639 ....    ....
17-APR-1961 / 23536:   4.567 ....    ....
17-MAY-1961 / 23537:   2.185 ....    ....
16-JUN-1961 / 23538:   4.941 ....    ....
17-JUL-1961 / 23539:   1.758 ....    ....
16-AUG-1961 / 23540:   0.212 ....    ....
16-SEP-1961 / 23541:   2.182 ....    ....
16-OCT-1961 / 23542:   0.431 ....    ....
16-NOV-1961 / 23543:  -2.144 ....    ....

So I can't center my original
variable using month_reg interpolation regridding.

I trid the example as described in user guide (test_var_centered and so...)
and it's OK.

So, why does it run in user guide but not with my variables ?

Thank's in advance... 
-- 
Francois DELCLAUX 
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