[Wrf-users] terrain features in 850mbar geopotential height

[Hein Zelle]

Dear WRF users,

we're seeing an unexpected terrain-following behaviour in the
geopotential height field at 850 mbar in our WRF output.  In the mean
time I'm convinced that I'm computing things correctly, so perhaps
there's a more conceptual problem.  We are computing

1)  the model level at which P+PB = 85000 Pa   (interpolated)
2)  the geopotential height (PH + PHB)/9.81  interpolated to that level.

Attached is an image over western europe, clearly showing the alps and
other terrain features in this field.  The variable shown is
geopotential height in m at 850 mbar level.  At 500mbar the terrain
features are mostly gone.  I've checked for earlier mails on this
topic and found one, but that poster claimed the problem went away
after the first time steps.  Not so in our case - this behaviour is
consistent across a 48 hour run.  We have also tried WRF POST, it
shows the same terrain-following behaviour and does the same
computation as far as I can see from the source code.

Are we computing the correct variable?  Charts from
www.wetterzentrale.de show the 850 mbar geopotential height field even
above the alps, suggesting they are computing a virtual field.  How
should we compute this from WRF output files?  

Kind regards,

     Hein Zelle

-- 

  dr. ir. Hein Zelle

  Project manager

  ARGOSS: Atmospheric, marine & coastal information, systems and consultancy.

  ARGOSS BV
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