Streamfunction/velocity potential: using fish with libbjt

[Arlindo da Silva]

See et al,

    One of the main issues with the fish() extension is that it requires
globally defined vorticity/divergence fields and the GrADS intrinsic
functions (hcurl, hdivg) tend to place undefined values at the boundaries. I
just realized that B.-J. Tsuang's function collection includes a set of
"advection functions" which can be used as a replacement for hcurl/hdivg.
B.-J.'s functions are documented here:

   http://opengrads.org/doc/udxt/libbjt/

(This extension library includes over 30 very useful new functions besides
the advection ones used here).

 The attached example script, which should be run under gradsdods,
demonstrates how you can use B.-J.'s functions to compute psi, chi as well
as the rotational and divergent wind.  Please notice the restrictions:

    - global and uniform grid; use re() if grid is not uniform
    - first and last latitudinal grid points cannot be at the poles

 Of course, you must have GrADS v1.9.0-rc1 and the OpenGrADS extensions
installed; these extensions do not yet work with GrADS v2. More information
here:

       http://opengrads.org/wiki/index.php?title=User_Defined_Extensions

 Good Luck,

     Arlindo

--
Arlindo da Silva
dasilva at alum.mit.edu
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