[gradsusr] Specialized Display Question

[Rowell, Mason D.]

Apparently it won't take lev in my potential temp expression, even though it is set to the right lev I need, but If I define hgt = 850 for example, use that to set lev, then use hgt in my expression, it takes it. The issue with defining potential temp. from NARR data is solved then (I knew it shouldn't have been challenging). Now I just need to confirm that Jeff's second method for cdiff (given it still doesn't take the first method provided) does indeed approximate the vertical deriv. for the central level, as I would assume it does if I know the method like I think I do.

Mason
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From: gradsusr-bounces at gradsusr.org [gradsusr-bounces at gradsusr.org] on behalf of Rowell, Mason D. [Mason.D.Rowell-1 at ou.edu]
Sent: Tuesday, November 22, 2011 10:58 PM
To: GrADS Users Forum
Subject: Re: [gradsusr] Specialized Display Question

Jeff,

Well the expression given second will work with lev set statically before entering the command (doesn't make sense to me to enter it after given the level about which to compute a central difference would have to be known first), but the first expression (w/ lev first set) gives 'Error from CDIFF:  Specified dimension non varying', which is what I thought I would get out of cdiff given lev is set statically. Though the second command may suffice for my purposes. As I said I was able to get it to work, after having set lev, so I assume the display is appropriate for the lev I choose and no other (I'd imagine it would have to be if the expression is working as it appears, that is the central difference about the set lev, with this approximation valid for the state of the derivative at the center, ie the level about which the central difference is computed). The syntax is new to me so it is good to learn. Now I just need to get a potential temperature expression to display for me.

Mason
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From: gradsusr-bounces at gradsusr.org [gradsusr-bounces at gradsusr.org] on behalf of Jeff Duda [jeffduda319 at gmail.com]
Sent: Tuesday, November 22, 2011 8:41 PM
To: GrADS Users Forum
Subject: Re: [gradsusr] Specialized Display Question

Mason,
You can use the cdiff command in the z direction to display variables (unfortunately, you can't use it to define variables).  However, you can approximate the vertical derivative by copying the method that cdiff uses.  If you have geopotential height, then you have all you need to compute a vertical derivative with respect to height coordinates.  You can display the vertical derivative of theta by either of the following:

'd cdiff(theta,z)'
'd (theta(z+1)-theta(z-1)) / (hgtprs(z+1)-hgtprs(z-1))

If you want to just view the gradient on one pressure surface, set the level that surface, then enter the command.

Jeff

On Tue, Nov 22, 2011 at 7:54 PM, Rowell, Mason D. <Mason.D.Rowell-1 at ou.edu<mailto:Mason.D.Rowell-1 at ou.edu>> wrote:
All,

Is it possible to display the vertical derivative of some quantity (like say temp or potential temp) on a pressure surface? I know I have height in my NARR data, but it is specific to a pressure level (i.e. HGTprs). What I would need is the height just above the current pressure level and below it in order to get cdiff of say theta and z. I'm not sure how to set the dimensions to do this...I would be okay with just getting this quantity for display at some level height, it doesn't have to be on a pressure surface.

Mason
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--
Jeff Duda
Grad student - PhD, Meteorology
University of Oklahoma School of Meteorology - Center for Analysis and Prediction of Storms

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