Re: NAM 6KM Alaska nest - rotation of wind components

From: David Brown <dbrown_at_nyahnyahspammersnyahnyah>
Date: Tue Sep 04 2012 - 14:37:50 MDT

Hi John,
If you read the data as GRIB2, then NCL provides a pseudo-varlable in the file that contains the rotation angles associated with each grid. I don't have a suitable Polar Stereographic example at hand, but here is what a file dump looks like for a GRIB 2 lambert conformal grid:

     float gridrot_0 ( ygrid_0, xgrid_0 )
         long_name : vector rotation angle
         GridType : Lambert Conformal (secant, tangent, conical or bipolar)
         units : radians
         formula_u : Uearth = sin(rot)*Vgrid + cos(rot)*Ugrid
         formula_v : Vearth = cos(rot)*Vgrid - sin(rot)*Ugrid
         note1 : u and v components of vector quantities are resolved relative to grid
         note2 : apply formulas to derive u and v components relative to earth

      float gridlat_0 ( ygrid_0, xgrid_0 )
         corners : ( 16.281, 17.34023, 55.48131, 54.17241 )
         long_name : latitude
         grid_type : Lambert Conformal (secant, tangent, conical or bipolar)
         units : degrees_north
         Latin2 : 25
         Latin1 : 25
         Dy : 13.545
         Dx : 13.545
         Lov : 265
         Lo1 : 233.862
         La1 : 16.281

      float gridlon_0 ( ygrid_0, xgrid_0 )
         corners : ( -126.138, -69.03799, -57.38108, -139.8561 )
         long_name : longitude
         grid_type : Lambert Conformal (secant, tangent, conical or bipolar)
         units : degrees_east
         Latin2 : 25
         Latin1 : 25
         Dy : 13.545
         Dx : 13.545
         Lov : 265
         Lo1 : 233.862
         La1 : 16.281

Hopefully the rotation angle attributes are self-explanatory. The underlying code that computes the rotation angle (as well as the lat and lon coordinates) is from NCEP. We do not currently have a direct interface to those routines.
 -dave

On Sep 4, 2012, at 11:48 AM, John Kelley wrote:

> I plan to use GRIB2 output from NCEP's NAM 6km Alaska NEST domain.
> The Alaska NAM Nest output is on Grid #198, a 6km polar stereographic projection grid
> (http://www.nco.ncep.noaa.gov/pmb/docs/on388/tableb.html#GRID198)
>
> I need to convert the u and v wind components from grid relative to Earth relative.
>
> Does anyone have NCL code for converting the u and v wind components from grid relative on the NAM 6km Alaska nest to Earth relative?
>
>
> Sincerely,
> John Kelley
> NOAA/NOS
>
>
>
>
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Received on Tue Sep 4 14:37:58 2012

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