[MITgcm-devel] exf: u/v-wind interpolation and cubed sphere

[Martin Losch]

Hi Dimitris,

I think that for now we should go back to version 1.11 (which is  
simply done by always having offset=0). However, if the exf_interp.F  
routines are as simple/straightforward as I think they are, then the  
wind velocities are likely to be wrong around the north pole (that is  
north of the norhernmost data grid points): If you imagine a perfect  
polar vortex (uwind = constant and vwind =0 along a latidude) and you  
simply interpolate uwind from a spherical lat-lon grid row an latMax  
(e.g. 88.xN for ncep) to grid points north of of this latitude, then  
these grid points will all have the values uwind,vwind(at latMax), so  
that you'll have discontinuity AT the pole (regardless of where the  
tile corners are). We should talk about this and my simple (and non- 
conservative, as Ed points out correctly) fix next week (I'll be in  
Rome and offline until Wednesday).

Martin
On Jun 23, 2006, at 10:34 PM, Dimitris Menemenlis wrote:

> Martin et al., I am back on e-mail contact.  I have verified that,  
> for both arctic40km and cs510, the North Pole is a corner (XG,YG)  
> point.  In that case the version of exf_set_uv.F <= Revision 1.11  
> should work fine, no?
>
> The reason for introducing the offset is that the pre-1.11  
> algorithm cannot accomodate situations where the North Pole is not  
> a corner point and because there was an anomaly in the North-Pole  
> grid definition of the arctic40km grid, now fixed.
>
> For time being, until Ed's interpolation algorithm is in place,  
> shall we go back to exf_set_uv.F Revision 1.11 with a warning if   
> North Pole is not a corner point (not sure how to check for this in  
> an elegant and foolproof way though)?
>
> A third option is to specify AngleCS and AngleSN a priori and to  
> avoid online agle computations alltogether.  These arrays did not  
> exist at time I wrote exf_set_uv and are not currently specified  
> for cs510 nor for arctic40km.
>
> Dimitris
>
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