ncl-talk 2010 archive: Re: Questions on irrotational and non-div

From: Xiaoming Sun <xiaoming.sun_at_nyahnyahspammersnyahnyah>
Date: Thu Feb 25 2010 - 20:56:33 MST

Dear Dr. Shea,

Thanks for your reply!

You said:
"
Then you would have to write a relaxation scheme to derive the
strean function and velovity potential.
Then interpolate to the WRF grid via rgrid2rcm
"
Could you please give more details on the meaning of relaxation scheme to
derive the stream function and velocity potential?

Also, since Wind = Wind_nondiv + Wind_irrotational
Could I just get Wind_nondiv firstly as the link
http://www.ncl.ucar.edu/Document/Functions/Built-in/uv2vr_cfd.shtml
and
http://www.ncl.ucar.edu/Applications/wind.shtml#ex3 (Example 2)
, then get Wind_irrotational by
Wind_irrotational = Wind - Wind_nondiv?

Thanks again for your help!

Best Regards,

Xiaoming

On Thu 02/25/10 12:05 PM , Dennis Shea wrote:

I think you have asked this question has been asked before.
In fact it has been asked often.

The WRf project does not provide NCL with any functions which
do what you want. Further, it seems that typical WRF output does
not include divergence and vorticity variables. Only U and V are
available.

On a sphere, to properly interpolate U and V *must* be interpolated
together. Interpolating U and then interpolating V is *not*
technically correct. They must be done together.
Appropriate 'rotation' must be applied.

uNew = U*cos(rot) + V*sin(rot)
vNew = -U*sin(rot) + V*cos(rot)

Still, you could probably use 'rcm2rgrid' to interpolate U and V
independently to a rectilinear grid. These could be used to get
vorticity.

http://www.ncl.ucar.edu/Document/Functions/Built-in/uv2vr_cfd.shtml

You could also get divergence.

http://www.ncl.ucar.edu/Document/Functions/Built-in/uv2vr_cfd.shtml

However, divergence is calculated
as the difference between numbers. You are not sure if the
calculated divergence is the 'real' divergence or a (partly)
a result of the interpolation.

Then you would have to write a relaxation scheme to derive the
strean function and velovity potential.

Then interpolate to the WRF grid via rgrid2rcm

> I am trying to get the irrotational and non-divergent wind component
> from WRF model output.
>
> However, the functions:
> uv2dvG
> dv2uvg
> uv2vrG
> vr2uvg
> (http://www.ncl.ucar.edu/Applications/wind.shtml)
> , which are used to do this work, are all for gaussian grid, while WRF
> output is on a cuvilinear grid.

There are versions for fixed [rectilinear] grids also. As stated
in the documentation, the grids are *required to be global*.
Typical WRF grids are not so these functions can *not* be used.

>
> Should I
> 1) Interpolate the WRF output to gaussian grid.

> 2) use the method as shown at
> http://www.ncl.ucar.edu/Applications/wind.shtml, to get irrotational
and
> non-divergent wind component.

No ... again the WRF grid is not global.

> 3) Interpolate the calculated wind component back to WRF curvilinear
grid/
> ?

>
> Is there any better way on doing this?
>
> Thanks in advance for your help!
>
> Best Regards,
>
> Xiaoming
>
>
>
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Received on Thu Feb 25 20:56:44 2010

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Re: Questions on irrotational and non-divergent wind plotfrom WRF, Thanks!