[gradsusr] Grads vertical derivative error with cdiff

Tue May 8 23:33:50 EDT 2012

Yes, you can set Z to a fixed value and use the same formulas. Also, the formula for the vertical derivative I gave earlier has an error. The factor of 0.5 should be removed, so the correct formula is:

(geopt(z+1)-geopt(z-1))/(100*(lev(z+1)-lev(z-1)))

Eric

----- Original Message -----
From: "Aishwarya" <spaceaish at gmail.com>
To: "GrADS Users Forum" <gradsusr at gradsusr.org>
Sent: Tuesday, May 8, 2012 9:54:14 PM
Subject: Re: [gradsusr] Grads vertical derivative error with cdiff

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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