Hi,<br><br>I wanted to expand on what Hannes contributed about converting geopotential height to actual height so that newcomers do not get the wrong impression:<br><br>PH and PHB are staggered in the Z direction, meaning that they are at the tops and bottoms of grid cells. That is fine for the heights of vertical winds, but for horizontal winds, other prognostic variables, and most diagnostic variables, the center of the cell should be used. You can average the top and bottom to get the center. For the cell at vertical level 'k,' height above sea level is:<br>
<br> <span style="font-family: courier new,monospace;"> <b>(((PH(k)+PH(k+1)) / 2) + ((PHB(k)+(PHB(k+1)) / 2) / 9.81</b></span><b><br></b><br>Call me lazy if you like (everyone else does), but WRF already makes part of that calculation and puts it in an unstaggered variable for the geopotential height at the cell center. That variable is PHP, so height above sea level for middle of cell(j,i) at level 'k' is PHP(k,j,i) / 9.81. In the subroutine calc_php, it uses this code:<br>
<br><span style="font-family: courier new,monospace;"> <b>php(i,k,j) = 0.5*(phb(i,k,j)+phb(i,k+1,j)+ph(i,k,j)+ph(i,k+1,j))</b></span><br><br>But by default, WRF does not output PHP, so in Registry.EM, you would have to change the line:<br>
<br> <b><span style="font-family: courier new,monospace;">state real php ikj dyn_em 1 - r "php" "geopotential" "m2 s-2"</span></b><br>
<br>to<br><br> <b><span style="font-family: courier new,monospace;">state real php ikj dyn_em 1 - rh "php" "geopotential" "m2 s-2"</span></b><br style="font-family: courier new,monospace;">
<br>But don't forget to divide by 9.81. And if you hadn't noticed, WRF internally holds variables with a different index order than it outputs them. Hope that helps whichever folks need to know such things (I happen to know Sam already knew that).<br>
<br>Kevin<br><br><br><div class="gmail_quote">On Mon, Nov 22, 2010 at 11:42 PM, Keller Johannes <span dir="ltr"><<a href="mailto:johannes.keller@psi.ch">johannes.keller@psi.ch</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">
Hi Sam,<br>
<br>
I use the staggered geopotential in the WRF history file wrfout....<br>
Height in m asl is (PH+PHB) / 9.81. Subtract terrain height HGT to get height in m agl.<br>
<br>
Regards<br>
<br>
Hannes<br>
<div><div></div><div class="h5"><br>
<br>
On Nov 22, 2010, at 12:20 PM, Sam Hawkins wrote:<br>
<br>
> Dear WRF users,<br>
><br>
> I want to interpolate WRF output from eta levels to heights above the<br>
> model terrain height. I need the code to be relatively fast, and<br>
> ideally write to netcdf files. My eta levels are quite closely spaced,<br>
> so I could probably get away with linear interpolation between levels.<br>
><br>
> Does anyone know whether there is any existing tool which will do this<br>
> (it seems GEMpak might have this functionality), or (b) are there any<br>
> tools, such as the netcdf operators or climate data operators which<br>
> might be adaptable to do this.<br>
><br>
> Sam.<br>
><br>
> --<br>
> The University of Edinburgh is a charitable body, registered in<br>
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<div><div></div><div class="h5"><br></div></div></blockquote></div>