[gradsusr] Grads vertical derivative error with cdiff

[Aishwarya]

Thank you very much Dr.Eric!
Can i find the geopotential finite differences pertaining to a single level
by the formulae you suggest?

On Tue, May 8, 2012 at 6:22 PM, Eric Altshuler <ela at cola.iges.org> wrote:

> Hello Aishwarya,
>
> Assuming your geopotential data is on equally spaced pressure levels
> specified in millibars (hPa) in the zdef line, you can use the following
> centered difference formula:
>
> 0.5*(geopt(z+1)-geopt(z-1))/(100*(lev(z+1)-lev(z-1)))
>
> The factor of 100 is necessary to convert the pressure levels (lev) from
> hPa to Pa. Because the atmosphere is nearly in hydrostatic balance on
> scales > 10 km, the above expression should be fairly close to:
>
> -R*T/(100*lev)
>
> where R=287 and T is temperature in K.
>
> The centered difference above is not valid for unequally spaced vertical
> levels. Also, if the pressure surface(s) below ground, you'll get missing
> or meaningless results.
>
> 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: "Aishwarya" <spaceaish at gmail.com>
> To: "GrADS Users Forum" <gradsusr at gradsusr.org>
> Sent: Tuesday, May 8, 2012 7:17:07 PM
> Subject: Re: [gradsusr] Grads vertical derivative error with cdiff
>
>
> Hi,
>
> Follow up on WRF vertical levels. Yes, WRF uses model levels. I used ARW
> post to convert my model levels to pressure and height levels. So, now I
> have geopotential and other parameters with height and pressure levels. I
> was trying to plot them in grads because ARW post gives the output in .ctl
> and .dat format.
>
> I followed the suggestion 0.5*(geopt(z+1)-geopt(z-1)) and it worked out.
> Although I'm not sure of my answer.
>
> Thank you all,
> Aishwarya.
>
>
> On Tue, May 8, 2012 at 4:10 PM, Eric Altshuler < ela at cola.iges.org >
> wrote:
>
>
> As I recall, the person who started this thread is using WRF. The vertical
> coordinate in raw WRF history output is neither pressure nor height, but a
> hybrid coordinate. In grads, z levels would correspond to constant hybrid
> coordinate surfaces. Taking vertical derivatives with respect to this
> hybrid coordinate would not be very useful. The available postprocessing
> tools for WRF can interpolate fields onto pressure or height levels.
>
> Eric
>
>
>
> ----- Original Message -----
> From: "Arlindo da Silva" < dasilva at alum.mit.edu >
> To: "GrADS Users Forum" < gradsusr at gradsusr.org >
> Sent: Tuesday, May 8, 2012 2:21:41 PM
> Subject: Re: [gradsusr] Grads vertical derivative error with cdiff
>
>
> On Tue, May 8, 2012 at 1:42 PM, Andre Pattantyus <
> apattantyus2008 at my.fit.edu > wrote:
>
>
>
> you need to use geopotential on pressure surface you will never get an
> answer with z coordinate. This is a basic units concept people!
>
>
>
>
> In GrADS "z" is used to refer to the vertical dimension in index space,
> whatever that is: pressure or height.
>
>
> Arlindo
> --
> Arlindo da Silva
> dasilva at alum.mit.edu
>
>
>
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>
> --
> Aishwarya,
> Graduate Research Assistant,
> Atmospheric Sciences,
> University of Arizona,Tucson.
>
>
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-- 
*Aishwarya,
Graduate Research Assistant,
Atmospheric Sciences,
University of Arizona,Tucson.*
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