Thank you very much Dr.Eric! <br>Can i find the geopotential finite differences pertaining to a single level by the formulae you suggest? <br><br><div class="gmail_quote">On Tue, May 8, 2012 at 6:22 PM, 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">Hello Aishwarya,<br>
<br>
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:<br>
<br>
0.5*(geopt(z+1)-geopt(z-1))/(100*(lev(z+1)-lev(z-1)))<br>
<br>
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:<br>
<br>
-R*T/(100*lev)<br>
<br>
where R=287 and T is temperature in K.<br>
<br>
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.<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: <a href="tel:%28301%29%20902-1257" value="+13019021257">(301) 902-1257</a><br>
Fax: <a href="tel:%28301%29%20595-9793" value="+13015959793">(301) 595-9793</a><br>
<div class="im HOEnZb"><br>
----- Original Message -----<br>
From: "Aishwarya" <<a href="mailto:spaceaish@gmail.com">spaceaish@gmail.com</a>><br>
To: "GrADS Users Forum" <<a href="mailto:gradsusr@gradsusr.org">gradsusr@gradsusr.org</a>><br>
</div><div class="HOEnZb"><div class="h5">Sent: Tuesday, May 8, 2012 7:17:07 PM<br>
Subject: Re: [gradsusr] Grads vertical derivative error with cdiff<br>
<br>
<br>
Hi,<br>
<br>
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.<br>
<br>
I followed the suggestion 0.5*(geopt(z+1)-geopt(z-1)) and it worked out. Although I'm not sure of my answer.<br>
<br>
Thank you all,<br>
Aishwarya.<br>
<br>
<br>
On Tue, May 8, 2012 at 4:10 PM, Eric Altshuler < <a href="mailto:ela@cola.iges.org">ela@cola.iges.org</a> > wrote:<br>
<br>
<br>
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.<br>
<br>
Eric<br>
<br>
<br>
<br>
----- Original Message -----<br>
From: "Arlindo da Silva" < <a href="mailto:dasilva@alum.mit.edu">dasilva@alum.mit.edu</a> ><br>
To: "GrADS Users Forum" < <a href="mailto:gradsusr@gradsusr.org">gradsusr@gradsusr.org</a> ><br>
Sent: Tuesday, May 8, 2012 2:21:41 PM<br>
Subject: Re: [gradsusr] Grads vertical derivative error with cdiff<br>
<br>
<br>
On Tue, May 8, 2012 at 1:42 PM, Andre Pattantyus < <a href="mailto:apattantyus2008@my.fit.edu">apattantyus2008@my.fit.edu</a> > wrote:<br>
<br>
<br>
<br>
you need to use geopotential on pressure surface you will never get an answer with z coordinate. This is a basic units concept people!<br>
<br>
<br>
<br>
<br>
In GrADS "z" is used to refer to the vertical dimension in index space, whatever that is: pressure or height.<br>
<br>
<br>
Arlindo<br>
--<br>
Arlindo da Silva<br>
<a href="mailto:dasilva@alum.mit.edu">dasilva@alum.mit.edu</a><br>
<br>
<br>
<br>
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<br>
<br>
--<br>
Aishwarya,<br>
Graduate Research Assistant,<br>
Atmospheric Sciences,<br>
University of Arizona,Tucson.<br>
<br>
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</div></div></blockquote></div><br><br clear="all"><br>-- <br><font style="font-family:times new roman,serif" size="2"><i><span style="color:rgb(153,0,0)">Aishwarya,</span><br style="color:rgb(153,0,0)"><span style="color:rgb(153,0,0)">Graduate Research Assistant,</span><br style="color:rgb(153,0,0)">
<span style="color:rgb(153,0,0)">Atmospheric Sciences,</span><br style="color:rgb(153,0,0)"><span style="color:rgb(153,0,0)">University of Arizona,Tucson.</span></i></font><br><br>