If the time axes are in different years, then the kind of regridding suggested above won't work. You will get all missing data as the result.
use "model_output_year_1.nc"If the time axes for the two variables show different years, then you need to do something different. Some model output might have some kind of generic time which doesn't refer to years, and in that case what Paul suggested would work.
Regridding generally requires the same region of time (or space). Say the data are in 2001 and 2002. The regridding
LET var_yr2_new_t = var_yr2[gt=var_yr1]is saying "take all the data from 2002 that is in 2001 and put it onto the 2001 time axis, then take the difference." There is no data in var_yr2 that is in 2001, so the result will be all missing. (What you can do with this regridding syntax is regrid say, a monthly time axis to a daily time axis for the same time ranges, or a regularly-spaced axis to an irregularly spaced one.)
If the time axes for the two years have the same number of points, say if they're both daily axes, then you could use the @ASN regridding transformation to just copy the data from one time axis to the other axis. Using Paul's example:
use "model_output_year_1.nc"It would be more accurate to define a time axis that has no calendar information, and use @ASN to regrid both years to that. If it were daily data, define a time axis but without the /T0= qualifier that tells Ferret what year and date the time coordinates relate to.
DEFINE AXIS/T=1:365:1/UNITS=days tdays
Another possibility is to regrid both variables to a climatological axis. There is lots of documentation about climatologies in the Users Guide, and also these two FAQ's which talk about particular examples with climatologies. The first uses monthly climatologies, the second shows how to define a daily climatological axis.
Paul Young wrote: