[gradsusr] Computation of Pressure Gradient

[Eric Altshuler]

The formula for pressure gradient force depends on the vertical coordinate. For data on constant z (height) surfaces, the PGF is given by Jeff's formula below. If you have data on constant pressure surfaces, which is often the case for reanalysis products,

PGF = -del(phi)

where phi is geopotential, which is (to a good approximation) equal to g*hgt where hgt is geopotential height. The following calculations in grads give the PGF using centered finite difference approximation:

pi=3.14159265359
dtr=pi/180
a=6.371e6
g=9.80665
dx=a*cos(dtr*lat)*dtr*cdiff(lon,x)
dy=a*dtr*cdiff(lat,y)
pgfx=-g*cdiff(hgt,x)/dx
pgfy=-g*cdiff(hgt,y)/dy

The PGF is given by the vector (pgfx,pgfy). hgt is assumed to be in meters.

Best regards,

Eric L. Altshuler
Assistant Research Scientist
Center for Ocean-Land-Atmosphere Studies
4041 Powder Mill Road, Suite 302
Calverton, MD 20705-3106
USA

E-mail: ela at cola.iges.org
Phone: (301) 902-1257
Fax: (301) 595-9793

----- Original Message -----
From: "Djordje Romanic" <djordje8 at gmail.com>
To: "GrADS Users Forum" <gradsusr at gradsusr.org>
Sent: Tuesday, August 14, 2012 7:21:47 AM
Subject: Re: [gradsusr] Computation of Pressure Gradient


Hi, 

I have asked a similar question a few days ago. Here is the answer provided by Jeff Duda: 

------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 
The pressure gradient is simply 1/rho * del(p), where del(p) is the gradient operator, which can be displayed in Grads using the mathematical decomposition of the gradient operator (i.e., derivatives in the x- and y-directions). Use cdiff for those derivatives. Note, you'll have to define your x- and y- grid spacing values as well. See the examples on the cdiff help page for how to do that. 

Jeff Duda 
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 

Regards 
-- 
Djordje Romanic 
M.Sc. in Meteorology 




On Tue, Aug 14, 2012 at 10:25 AM, sushant puranik < sushantpuranik at gmail.com > wrote: 


Hi 
Is there any script or predefined function available in GrADS for the computation of pressure gradient force? 




Thanks 



Sushant 

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