Dennis,
Thank you very much. As you suggested, I reordered the latitude,
and used the routines for fixed grid. Now, everything is perfect.
> u = u(::-1,:) ; reorder the latitudes
> v = v(::-1,:)
> div = uv2dvF(u,v) ; divergence
> chi = ilapsF(div , 0) ; inverse laplacian ==> vel. potential
> dv2uvf(div,ud,vd)
Again, thank you very much.
Sang-ki
>>I am a ferret user, but I just started using NCL from today because
>>I need to compute the velocity potential from the monthly ncep
>>reanalysis. I just did that using "wind_4.ncl" (which can be
>>downloaded from the ncl webpage). The wind_4.ncl script uses
>>the follwing four ncl routines:
>>
>>uv2dvG(u,v) for divergence
>>ilapsG(div,0) for velocity potential
>>dv2uvg(div,ud,vd) divergent wind
>>
>>But, the computed velocity potential is very different from that
>>provided from CDC (velocity potential is included if you download
>>ncep reanalysis from CDC). Furthermore, the divergent wind is not
>>perpendicular to the velocity potential.
>>
>>Does it mean that the velocity potential computaion using NCL
>>is not reliable? Or am I doing something wrong? Only modification
>>I made to wind_4.ncl is the file reading statement (I am reading
>>grib data).
>
>
> The example you refer to is just that... an example.
> The example was for data on a gaussian grid. I don't think you
> want to use functions for gaussian grids on data that
> are not gaussian :-)
>
>
> [1] GRIB data from NCEP or ECMWF is typically ordered
> North-to-South. NCL's spherical harmonic routines
> require that the data be ordered from South-to-North.
>
> Note the documentation for the functions variables u/v
> stares that " input values must be in ascending latitude order"
> ^^^^^^^^^^^^^^^^^^^^^^^^
> http://ngwww.ucar.edu/ngdoc/ng/ref/ncl/functions/uv2dvF.html
>
> This can readily be accomplished via NCL's array syntax [like f90].
>
> Consider: u(lat,lon) and v(lat,lon) ordered N->S
>
> u = u(::-1,:) ; reorder the latitudes
> v = v(::-1,:)
>
> printVarSummary(u) ; verify that the latitudes are S->N
>
> [2] The functions the example [and you] are using are for
> data on a Gaussian grid.
>
> Most [all?] ranalysis data are on 2.5 degree grids. In NCL
> parlance, this means a "Fixed" grid.
>
> Thus, you want to use functions appropriate for a "Fixed" grid
> not a Gaussian grid.
>
> div = uv2dvF(u,v) ; divergence
> chi = ilapsF(div , 0) ; inverse laplacian ==> vel. potential
> dv2uvf(div,ud,vd) ; div ==> divergent wind components
>
> [3] Also, for plotting, the use of
>
> res@vcGlyphStyle = "CurlyVector" ; turn on curly vectors
>
> may not be the best choice here. Just comment it out (using ;)
> to get classic straight vectors.
>
> ====
>
> good luck
>
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-- ---------------------------------------------------------------------- Sang-ki Lee RSMAS/CIMAS University of Miami 4600 Rickenbacker Causeway Miami, FL 33149 USA Tel) 305-361-4521 Fax) 305-361-4412 E-mail) sang-ki.lee@noaa.gov ---------------------------------------------------------------------- Mailing-address: Sang-ki Lee AOML/PHOD 4301 Rickenbacker Causeway Miami, FL, 33149 USA ----------------------------------------------------------------------_______________________________________________ ncl-talk mailing list ncl-talk@ucar.edu http://mailman.ucar.edu/mailman/listinfo/ncl-talk
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