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

[Russ Fiedler <russell.fiedler@xxxxxxxx>]

Hi Lev,

I take it that H and Hw are on different grids (space?,time?) and that 
is why you are using the set interpolate mode. I think the problem is 
that interpolations are being performed by ferret in places and at times 
where you don't expect it.

I would explicitly regrid variables to a common grid and avoid any 
ambiguity.

Changing your mask definition to

 LET mask_grounded = if H*rhoI/rhoO gt Hw[g=H@AVE] then 1 else 0
or
  LET mask_grounded = if H*rhoI/rhoO gt Hw[g=H@LIN] then 1 else 0
or maybe some other like @MIN may make sense.

That should allow you to run with interpolate mode off.


Cheers,
Russ



On 08/08/13 01:42, Lev Tarasov wrote:
> 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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