From: Tom Kilpatrick <thomaski_at_nyahnyahspammersnyahnyah>

Date: Mon Mar 28 2011 - 19:40:48 MDT

Date: Mon Mar 28 2011 - 19:40:48 MDT

See below for the helpful response from wrfhelp...

On Mon, Mar 28, 2011 at 3:16 PM, wrfhelp <wrfhelp@ucar.edu> wrote:

*> The correct c grid should look like:
*

*>
*

*> v
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*> u T u
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*> v
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*>
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*> For the C grid, the T point is where all mass fields are located, and there
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*> is nothing at the corners of
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*> the grid. When you do compute vorticity and divergence, they should fall on
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*> the mass points
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*> since dx is the grid distance between u and u, and v and v. Your
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*> definitions of divergence and vorticity
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*> are correct.
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*>
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*> Right now, NCL doesn't have the function to compute divergence, and there
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*> is a function where
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*> you can obtain absolute vorticity (avo) - but I have a feeling that is not
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*> done this way and it seems
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*> that it is computed over two grid boxes.
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*>
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*> wrfhelp
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*>
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*>
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*> On Mar 27, 2011, at 10:34 PM, Tom Kilpatrick wrote:
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*>
*

*> I have a question about computing the divergence and curl of 2D vector
*

*>> fields (eg low-level winds) on the WRF lambert grid. After reading a bit of
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*>> Haltiner & Williams 1980 I am wondering if the following is correct. For a
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*>> horizontal vector field with components (A,B) in the x- and y-directions on
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*>> the lambert grid, then the divergence is given by
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*>>
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*>> m^2 * [ d/dx(A/m) + d/dy(B/m) ]
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*>>
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*>> and the curl is
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*>>
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*>> m^2 * [ d/dx(B/m) - d/dy(A/m) ],
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*>>
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*>> where m is the map factor. The above formulae could be used to compute the
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*>> divergence at the theta (mass) points and the curl at the '+' pts on the
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*>> C-grid:
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*>>
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*>> th u th
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*>> v + v
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*>> th u th
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*>>
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*>> One could then average the '+' pts to get the curl at the interior
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*>> mass-pts.
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*>>
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*>> If the above is correct it might be helpful if there were built-in NCL
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*>> functions to do this. My apologies if there already are, but I couldn't find
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*>> anything on the site, and searching through the ncl-talk archives, it looks
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*>> other users have similar questions. Note that uv2dv_cfd, uv2vr_cfd don't
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*>> work for the lambert grid.
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*>>
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*>> Thanks,
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*>> Tom Kilpatrick
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*>>
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*>>
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*>>
*

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Received on Mon Mar 28 19:40:55 2011

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