# [Wrf-users] interpolating to height above ground

Roberto Chavez roberchavez at hotmail.com
Tue Nov 23 13:40:50 MST 2010

```Sam,

As you said, the geopotential height is expected to be time-dependent.
Thus, in principle you have to make one fit for every file you want to

The main problem with the similarity theory is that the wind’s vertical
profile is related to the atmospheric stability condition, and these are
mostly non-linear equations, hence, you have to use either non-linear
least squares or maximum likelihood methods to get your parameters.
Furthermore, since the equations of the similarity theory are based on
empirical assumptions, there are differences among several authors. I've
been using the ones proposed by Simiu & Scanlan* with relatively
good results.

You are right, many similarity parameters such as the L* u* and z0 can
be obtained from the WRF, but I haven’t been able to get some good
results for the wind speed vertical interpolation so far.

On the other hand, the neutral logarithmic profile and the power law
sometimes tend to either under or over predict the wind speed (which is
expected for certain stability conditions) however, in average they
provide close predictions compared to those obtained from in-situ
measurements and more importantly,  you save a huge amount of time &
computational requirements needed to perform the non-linear fits from
the Monin-Obukvov theory.

Roberto

*Simiu E., Scanlan R.H. 1996: Wind Effects on Structures, 3rd ed.; John Wiley & Sons: New York, NY, USA.

> From: johannes.keller at psi.ch
> Date: Tue, 23 Nov 2010 07:42:03 +0100
> To: s.hawkins at ed.ac.uk
> CC: wrf-users at ucar.edu
> Subject: Re: [Wrf-users] interpolating to height above ground
>
> Hi Sam,
>
> I use the staggered geopotential in the WRF history file wrfout....
> Height in m asl is (PH+PHB) / 9.81. Subtract terrain height HGT to get height in m agl.
>
> Regards
>
> Hannes
>
>
> On Nov 22, 2010, at 12:20 PM, Sam Hawkins wrote:
>
> > Dear WRF users,
> >
> > I want to interpolate WRF output from eta levels to heights above the
> > model terrain height.  I need the code to be relatively fast, and
> > ideally write to netcdf files. My eta levels are quite closely spaced,
> > so I could probably get away with linear interpolation between levels.
> >
> > Does anyone know whether there is any existing tool which will do this
> > (it seems GEMpak might have this functionality), or (b) are there any
> > tools, such as the netcdf operators or climate data operators which
> > might be adaptable to do this.
> >
> > Sam.
> >
> > --
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>
> ---------------------------------------------------------------------
> Dr.Johannes Keller
> Paul Scherrer Institut (PSI)
> Laboratory of Atmospheric Chemistry (LAC)
> Building OFLA / 012
> CH-5232 Villigen PSI, Switzerland
> Phone: +41 56 310 20 65,  Fax: +41 56 310 45 25
> e-mail: johannes.keller(at)psi.ch
> http://www.psi.ch/
> http://lac.web.psi.ch
> ---------------------------------------------------------------------
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>
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