[ferret_users] Re: external function samplexy_curv and modulo

[Ansley Manke <ansley.b.manke@xxxxxxxx>]

Hi Patrick,
What you say is quite true.  For a general curvilinear grid, some of 
which are very non-rectangular, the concept of modulo longitudes is 
difficult to define. How will the algorithm know which lon,lat cell 
should wrap to which cell across the branch cut?

The function SAMPLEXY_CURV looks from one side only, for the "closest" 
location.

In fact your data shows a shortcoming of the algorighm in 
samplexy_curv.  For the grid in your file samplexy_curv.nc the latitudes 
in LAT are 0 at index j=74.  For j=74, and everywhere nearby, there are 
no longitudes in LON anywhere near 0.  So the function should not return 
a result for samplexy_curv(votemper, lon,lat, 0,0).  What the function 
does is it finds where on each J line of LON the values of LON crosses 
the xpoint requested, here 0.  It finds that LON crosses 0 only along 
the line x=51, j=149, at the top of the grid (do a SHADE LON to see what 
the LON coordinate variable is like), and it then looks in LAT and finds 
the location closest to (51,149) where LAT = ypt, again 0 for this 
example.  We need to implement a threshold, so that the function will 
not return a result from halfway across the globe.

Ansley

Brockmann Patrick wrote:
> Hi all,
> A question on the external function samplexy_curv
> which does not consider modulo aspect. If I sample at (lon,lat) (0,0) 
> or (360,0),
> the samplexy_curv gives different results.
>
> See scripts: 
> http://dods.ipsl.jussieu.fr/brocksce/ferret_sample/samplexy_curv_test.jnl
>
> list samplexy_curv(votemper, lon,lat, 0,0)
>             VARIABLE : SAMPLEXY_CURV(VOTEMPER, LON,LAT, 0,0)
>             FILENAME : samplexy_curv.nc
>             X        : 1
>             Z        : 5
>             T        : 1
>          0.01885
> list samplexy_curv(votemper, lon,lat, 360,0)
>             VARIABLE : SAMPLEXY_CURV(VOTEMPER, LON,LAT, 360,0)
>             FILENAME : samplexy_curv.nc
>             X        : 1
>             Z        : 5
>             T        : 1
>          30.29
>
>
> What do I have to believe ?
> Regards
> Patrick