[ferret_users] how to control domain of mode INTERP without awkward file writing/reading

[Lev Tarasov <lev@xxxxxx>]

I have a routine to figure out the time when a location last becomes free 
of grounded ice (ie for comparing model results to data for the last 
glacial cycle Antarctic ice sheet). To figure out whether the ice is 
floating, we need to use mode INTERP. However, this ends up messing up the 
@EVNT algorithm for finding out when the last crossing (in this case 
ungrounding of ice) occurs., ie: in the below, with mode interp on, 
mask_grounded (= if H*rhoI/rhoO gt Hw then 1 else 0) ends up with 
fractional values.

set region/x=143.14 /y=-66.48
! H and Hw are 2D space + 1D time fields
LET rhoI = 910  ![kg/m3]
LET rhoO = 1028 ![kg/m3]
LET mask_grounded = if H*rhoI/rhoO gt Hw then 1 else 0
LET grounded_event=mask_grounded[T=-120:0@EVNT:1]      ! find all the
        instances when ice grounded (this starts at -120, incrementing for each event)
LET last_event = grounded_event[T=@max]                ! find the value of the last event
LET mask_last = IF grounded_event EQ last_event THEN 1 ! make a mask based on last event value, ie domain with event value = last value
LET idx= mask_last[T=@LOC:1]                           ! get the index of the first value in the mask \ie the last grounded event
LET out = idx + 0.05 ! the 0.05 is to adjust the idx to output the time between grounded and floating

Now we can work around this by setting mode INTERP, saving the timeseries 
for each site, CAN DATA/ALL, can var/all, opening the saved timeseries and 
then doing the above masking operation. But for our large datasets, this
becomes awkward/slow. Anyway around this?

To be concrete, with the above work around for one data site and model, I 
get out= -31.75
without the work around I get out=-119.9 with CAN MODE 
INTERP and -70.57 with SET MODE INTERP

Cheers,

Lev

Lev Tarasov -   Dept of Physics and Physical Oceanography,
		Memorial University of Newfoundland.
                email: lev@xxxxxx
                http://www.physics.mun.ca/~lev/
		Sabbatical Tel (519)-821-3555
		Fax (709)-864-8739

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