[gradsusr] Fwd: How to calculate the pressure gradient and geopotential height gradient?

[Davide Sacchetti]

I am afraid that
  gphgrad = cdiff(gheight,x)/dx + cdiff(gheight,y)/dy
was wrong ... (you are computing horizontal divergence)

correct definition should be:
  gphgrad = mag(cdiff(gheight,x)/dx, cdiff(gheight,y)/dy)

hoping not to add confusion ...
Davide

On Thu, 2013-04-04 at 10:05 -0500, Jeff Duda wrote:
> To calculate the horizontal pressure gradient, you will need pressure
> on a constant height surface.  If you have that, then the command to
> compute the pressure gradient is
> 
> 
> 'define pgrad = cdiff(pres,x)/dx + cdiff(pres,y)/dy',
> 
> 
> 
> where the array holding pressure values is "pres" and you have defined
> dx and dy as
> 
> 'define dx = 6.371e6*cos(lat*3.14159/180)*cdiff(lon*3.14159/180,x)'
> 
> 'define dy = 6.371e6*cdiff(lat*3.14159/180)'
> 
> 
> where the 3.14159/180 term is used to translate degrees to radians.
> 
> 
> To calculate the geopotential height gradient, you need height on a
> constant pressure surface.  If you have that, then the command to
> compute the geopotential height gradient is
> 
> 
> 'define gphgrad = cdiff(gheight,x)/dx + cdiff(gheight,y)/dy'
> 
> 
> where the same commands as above were used to define dx and dy.
> 
> Jeff Duda
> 
> 
> 
> On Thu, Apr 4, 2013 at 7:02 AM, Simona Skyraite <skyraite at gmail.com>
> wrote:
>         
>         
>         Hello,
>         
>         
>         How to calculate pressure gradient (hPa/100 km) and
>         geopotential height gradient? Could someone explain step by
>         step how to do that?"
>         
>         
>         
>         Thanks in advance! 
>         
>         Simona
>         
>         
>         
>         
>         
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>         
> 
> 
> 
> -- 
> Jeff Duda
> Graduate research assistant
> University of Oklahoma School of Meteorology
> Center for Analysis and Prediction of Storms
> 
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-- 
--> Attenzione cambio indirizzo: davide.sacchetti at arpal.gov.it
--
Davide Sacchetti
Centro Funzionale Meteo Idrologico di Protezione Civile della Regione Liguria
ARPAL Unità Tecnica Complessa di livello Regionale
V.le Brigare Partigiane 2 16121 Genova (I)
tel: +39 010 6437535                    fax: +39 010 6437520
mail: davide.sacchetti at arpal.gov.it     web: www.meteoliguria.it