[gradsusr] Specialized Display Question

Wed Nov 23 12:22:14 EST 2011

Mason,
I "corrected" your script file and reattached it here.  Basically you were
a little quote-happy with your definitions in the first half of the
script.  Other than that you coded it in a way that would work.  Hopefully
you understand why certain variables don't need to be in quotes.

Jeff

On Wed, Nov 23, 2011 at 11:17 AM, Jeff Duda <jeffduda319 at gmail.com> wrote:

> Mason,
> I haven't looked at your script, but here are the answers to your
> questions.
>
> You got the error from CDIFF when setting lev statically because it needs
> the vertical dimension to be varying in order to work.  On the other hand,
> typing the expression in manually does not require the vertical dimension
> to vary.  To see why this works, look through the top section of the
> variables page <http://www.iges.org/grads/gadoc/variable.html#names> on
> the Grads site.  Basically, you can use parentheses to tag a field with a
> spatiotemporal (and/or ensemble) offset or specification.  For example, 'd
> tmpprs(z+1)' tells Grads to plot tmpprs one vertical level away from where
> the current vertical dimension is set (this works for both fixed and
> varying vertical dimension settings).  So if you have two pressure levels
> at 850 and 800 mb, if your lev is set to 850, 'd tmpprs(z+1)' will plot
> temperatures at 800 mb.  'd tmpprs(z-1)' plots tmpprs one level down.  You
> can specify the level, too.  Say you want to plot 500 mb heights and MSLP.
> The following lines of code to do that are equivalent:
> 'set lev 500'
> 'd hgtprs'
> 'set lev 1000'
> 'd mslmamsl'
>
> OR
>
> 'set lev 500'
> 'd hgtprs'
> 'd mslmamsl(lev=1000)'
>
> OR
> (assume lev is set to something other than 500 or 1000)
> 'd hgtprs(lev=500)'
> 'd mslmamsl(lev=1000)'
>
> Thus, this command: 'd (theta(z+1)-theta(z-1)) /
> (hgtprs(z+1)-hgtprs(z-1))', tells grads to plot an approximation of the
> vertical derivative of theta in the same way that CDIFF would do it.  It's
> just that CDIFF requires a varying vertical dimension if you use Z as the
> argument, which means you pretty much can't use it to display a vertical
> derivative on a single pressure surface in a horizontal cross section
> (sorry, that just came to me, so I apologize for the confusion).
>
> To compute potential temperature using the formula you have, use the pow()
> function instead of the carrot.  Thus, potprs = tmpprs*pow(1000/lev,2/7)
> (with your vertical dimension varying over all the levels you want to have
> potential temperature defined) will give you a grid of potential
> temperatures.
>
> Here's my suggestion for how to plot the vertical temperature gradient at
> 850 mb given NARR data (assuming I remember the variable names):
>
> 'set x 1 (whatever last index is)'
> 'set y 1 (whatever last index is)'
> 'set lev 1000 (however high you want, but make sure it includes the level
> above the one you want to view the gradient on)'
> 'define theta = tmpprs*pow(1000/lev,2/7)'
> 'set lev 850'
> 'd (theta(z+1)-theta(z-1)) / (hgtprs(z+1)-hgtprs(z-1))'
>
> Jeff
>
>
> On Wed, Nov 23, 2011 at 12:44 AM, Rowell, Mason D. <
> Mason.D.Rowell-1 at ou.edu> wrote:
>
>>  All,
>>
>>  Well I appear to still have issues. I can define theta like I
>> discovered I can, but then I can't just use this in Jeff's vertical
>> derivative expression (the one that will work), given that its value at z+1
>> and z-1 is not defined. I thought I was being clever getting around that,
>> but it claims I am not setting lev right, and I don't know why. Script is
>> attached. Any ideas?
>>
>>  Mason
>>  ------------------------------
>> *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 11:21 PM
>>
>> *To:* GrADS Users Forum
>> *Subject:* Re: [gradsusr] Specialized Display Question
>>
>>   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
>>  ------------------------------
>> *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
>>  ------------------------------
>> *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> 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
>>> _______________________________________________
>>> gradsusr mailing list
>>> gradsusr at gradsusr.org
>>> http://gradsusr.org/mailman/listinfo/gradsusr
>>>
>>
>>
>>
>> --
>> Jeff Duda
>> Grad student - PhD, Meteorology
>> University of Oklahoma School of Meteorology - Center for Analysis and
>> Prediction of Storms
>>
>>
>> _______________________________________________
>> gradsusr mailing list
>> gradsusr at gradsusr.org
>> http://gradsusr.org/mailman/listinfo/gradsusr
>>
>>
>
>
> --
> Jeff Duda
> Grad student - PhD, Meteorology
> University of Oklahoma School of Meteorology - Center for Analysis and
> Prediction of Storms
>
>

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