[ferret_users] question about replace a value on a grid

[Liya Jin <lj@xxxxxxxxxxxxxxx>]

Dear Ferreters,

There is a data file (sst.nc) like this:

(ncdump -h sst.nc)

dimensions:
      x = 182 ;
      y = 149 ;
      deptht = 31 ;
      time = UNLIMITED ; // (73 currently)
variables:
      float nav_lon(y, x) ;
              nav_lon:long_name = "Longitude" ;
              nav_lon:units = "degrees_east" ;
              nav_lon:standard_name = "longitude" ;
      float nav_lat(y, x) ;
              nav_lat:long_name = "Latitude" ;
              nav_lat:units = "degrees_north" ;
              nav_lat:standard_name = "latitude" ;
      float deptht(deptht) ;
              deptht:long_name = "Vertical T levels" ;
              deptht:units = "m" ;
      double time(time) ;
              time:units = "days since 999-01-03 12:00" ;
    float sosstsst(time, y, x) ;
              sosstsst:long_name = "Sea Surface temperature" ;
              sosstsst:units = "C" ;
              sosstsst:coordinates = "nav_lon nav_lat" ;
              sosstsst:valid_min = 1.00000002004088e+20 ;
              sosstsst:valid_max = -1.00000002004088e+20 ;
              sosstsst:short_name = "sosstsst" ;
              sosstsst:online_operation = "ave(x)" ;
              sosstsst:interval_operation = 5760. ;
              sosstsst:interval_write = 432000. ;

I would like to replace the values of some grids of the variable 
(sosstsst) with new values, e.g., replace sosstsst(time, 88,160) with 
sosstsst(time, 89,160), or with the interpolated values of surrounding 
grids (e.g., (sosstsst(time, 89,160)+sosstsst(time, 90,160))/2). Then 
the sosstsst(time, 88,160) will have a new value in sst.nc.

How can I do this with Ferret? Thanks for your help!

Regards,
Liya