Thank you Eric.<br><br>Sushant<br><br><br><div class="gmail_quote">On Wed, Aug 15, 2012 at 3:50 AM, Eric Altshuler <span dir="ltr"><<a href="mailto:ela@cola.iges.org" target="_blank">ela@cola.iges.org</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">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,<br>
<br>
PGF = -del(phi)<br>
<br>
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:<br>
<br>
pi=3.14159265359<br>
dtr=pi/180<br>
a=6.371e6<br>
g=9.80665<br>
dx=a*cos(dtr*lat)*dtr*cdiff(lon,x)<br>
dy=a*dtr*cdiff(lat,y)<br>
pgfx=-g*cdiff(hgt,x)/dx<br>
pgfy=-g*cdiff(hgt,y)/dy<br>
<br>
The PGF is given by the vector (pgfx,pgfy). hgt is assumed to be in meters.<br>
<br>
Best regards,<br>
<br>
Eric L. Altshuler<br>
Assistant Research Scientist<br>
Center for Ocean-Land-Atmosphere Studies<br>
4041 Powder Mill Road, Suite 302<br>
Calverton, MD 20705-3106<br>
USA<br>
<br>
E-mail: <a href="mailto:ela@cola.iges.org">ela@cola.iges.org</a><br>
Phone: (301) 902-1257<br>
Fax: (301) 595-9793<br>
<div class="HOEnZb"><div class="h5"><br>
----- Original Message -----<br>
From: "Djordje Romanic" <<a href="mailto:djordje8@gmail.com">djordje8@gmail.com</a>><br>
To: "GrADS Users Forum" <<a href="mailto:gradsusr@gradsusr.org">gradsusr@gradsusr.org</a>><br>
Sent: Tuesday, August 14, 2012 7:21:47 AM<br>
Subject: Re: [gradsusr] Computation of Pressure Gradient<br>
<br>
<br>
Hi,<br>
<br>
I have asked a similar question a few days ago. Here is the answer provided by Jeff Duda:<br>
<br>
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------<br>
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.<br>
<br>
Jeff Duda<br>
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------<br>
<br>
Regards<br>
--<br>
Djordje Romanic<br>
M.Sc. in Meteorology<br>
<br>
<br>
<br>
<br>
On Tue, Aug 14, 2012 at 10:25 AM, sushant puranik < <a href="mailto:sushantpuranik@gmail.com">sushantpuranik@gmail.com</a> > wrote:<br>
<br>
<br>
Hi<br>
Is there any script or predefined function available in GrADS for the computation of pressure gradient force?<br>
<br>
<br>
<br>
<br>
Thanks<br>
<br>
<br>
<br>
Sushant<br>
<br>
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