20050705: 20041008: how to handle staggered U & V in IDV?

>From: Rich Signell <address@hidden>
>Organization: USGS
>Keywords: 200507051400.j65E0Bjo002656 IDV staggered grids

Hi Rich

>I still cannot look at any of the 3D fields of U and V
>in my model results using IDV.
>
>This solution given in your previous message below
>works for my 2D fields of U and V, but not for the 3D
>fields of U and V.  I believe the problem is that the
>vertical positions of the 3D U and V fields cannot be
>determined, since the "ocean_s_coordinate" formula
>which describes the vertical coordinate is only
>defined at grid cell "centers" where elevation exists,
>and the U and V points are on the "faces".  

Yes, this is basically the problem.

>Thus any attempt to display or regrid the 3D fields of
>U and V fails.  I think what we need is a function
>that *horizontally* averages the U and V fields at
>each "s level" (or sigma level) without doing anything
>to the vertical coordinate, and then assigning this
>new variable the vertical coordinate at the grid cell
>center.  

Even just trying to display is problematic.  The code
that computes the sizes of the ocean_s_coordinate variable
ends up being diffferent than the number of points for
the variable bases on the dimension.  This is a problem
before it ever gets to the IDV.

We do something like what you suggest to support WRF
Eta coordinates, and we need to look at generalizing 
this.

>The function would have three inputs: (1) the 3D U
>field, (2) the 3D V field, (3) a 3D Temp or Salt field
>(something that is at the grid cell centers, with an
>existing vertical coordinate).   All of these fields
>would have the same number of vertical levels, but not
>necessarily the same horizontal dimensions.  In our
>ROMS model, for example, the dimensions are
>
>     K  J   I
>U  (20,60,159)
>V  (20,59,160)
>T  (20,60,160)
>
>So with M=160 and N=60, we just want a function that
>does
>
>U_at_T=T*NaN;
>V_at_T=T*NaN;
>
>U_at_T(:,2:(N-1),2:(M-1))=0.5*(U(:,:,1:(M-1))+U(:,:,2:M))
>V_at_T(:,2:(N-1),2:(M-1))=0.5*(V(:,1:(N-1),:)+V(:,2:N,:))
>
>This should be possible to implement in Jython, right?

I don't think so.  The problem is happening upstream of
where you can access it in VisAD/Jython.  We'll have to
look at solving it in the vertical transform code.

>There is a ROMS output sample file at:
>
>http://stellwagen.er.usgs.gov/rps/test/roms_cf.nc
>
>that illustrates the problem.  Any attempt to do
>something with the 3D U and V fields fails.

Thanks for the sample.  That will help.

>Thanks for looking at this again,

We're currently in the process of preparing for the IDV
training workshop and I don't expect to get to look 
closely at this until after that. 

I've cc'd a couple of others here who might have some input
on this.

Don Murray
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