[gradsusr] Computation of Pressure Gradient

[sushant puranik]

Thank you Eric.

Sushant


On Wed, Aug 15, 2012 at 3:50 AM, Eric Altshuler <ela at cola.iges.org> wrote:

> 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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