[gradsusr] Diagnostics in Ctl File?

Fri Nov 11 16:17:41 EST 2011

Mason,
I preferred to derive the formulas for spacings myself.  Simple
trigonometry reveals the following that you should use for dx and dy (i.e.,
meridional and zonal grid spacing):
dx = Re*cos(phi)*d(theta)
dy = Re*d(phi)

where Re is the radius of the Earth (6371 to 6374 km), phi is latitude,
theta is longitude, and the d operator means "change in".  To convert this
to Grads scripting code, you use the cdiff command.  Also note that all
angle measures need to be in radians, so you'll have to convert them as you
do the operation.  Here are the literal Grads command lines I use for this:

re = 6.371e3
'define dx = 're'*cos(lat*(3.1419/180))*cdiff(lon*(3.14159/180))'
'define dy = 're'*cdiff(lon*(3.14159/180))'

Obviously you can assign the conversion factor pi/180 to a script variable
to simplify the symbolism there:

conv_factor = 3.14159/180
e.g., 'define dy = 're'*cdiff(lon*'conv_factor')'

If you try to display dx or dy, you should get a nice little image that
contours or colors dx and dy (dx should decrease towards the poles and not
change along a line of constant latitude, and dy should only be a function
of latitude as well).  Now use this to define temperature advection
(assuming the temperature variable is tmpprs and wind is ugrdprs/vgrdprs):

'define tadv = -ugrdprs*cdiff(tmpprs,x)/(2*dx) -
vgrdprs*cdiff(tmpprs,y)/(2*dy)'

The 98cbar.gs is indeed just a script that defines color levels and sets a
color scheme for when you plot temperature advection.  Changing the color
tables is rather easy in Grads.  You only need to define a color using 'set
rgb # R G B', where # is anything between 16 and 255 (just a reference
number) and R, G, and B are between 0 and 255.  Then you set the contour
intervals manually using 'set clevs , , , , , ...' where you insert
literally the levels you want to use (separated by SPACES, not COMMAS).
Then set the colors for those levels using 'set ccols , , , , , , ,+1'
again using the reference numbers for each color and separating by SPACES.
The "+1" is for the fact that you need one additional color for the number
of levls you put.  You can consult the controlling
colors<http://www.iges.org/grads/gadoc/colorcontrol.html>page for full
info on what I just said.  Also, I have a number of very
simple color bar scripts that I've used in the past.  I can provide them to
all if anyone is interested.

You can also create a simple color bar using the cbar.gs script which is
provided in the script library<http://www.iges.org/grads/gadoc/library.html>
.

Let me know if you need any more help.

Jeff

On Fri, Nov 11, 2011 at 2:50 PM, Rowell, Mason D.
<Mason.D.Rowell-1 at ou.edu>wrote:

>  Jeff and others,
>
> Strange. This is exactly what was suggested in the ctl file near the
> comments at the bottom (that which I pulled from nomads). The only
> difference is that they have also the inclusion of a 98cbar.gs script
> right after the other expressions like that in this link you gave me, whose
> utility is a mystery since it is not necessary, at least suggested as so by
> the cdiff documenation given that it is not included. Perhaps it is just
> some coloring scheme. At any rate, the script fails at the first line
> (which attempts to define dtx), with each line coded exactly as it is shown
> (with the appropriate NARR temp as TMPprs and wind as UGRDprs and VGRDprs
> of course). I assume it will do so again at dty, and so forth. I originally
> didn't include define with these since I thought they weren't necessary but
> adding that in didn't solve the problem anyways. I suspect it is because
> cdiff calls x, and since I define my domain with lat long, it doesn't have
> a x variable to use? There definitely isn't a spacing issue this time...
>
> Mason
>  ------------------------------
> *From:* gradsusr-bounces at gradsusr.org [gradsusr-bounces at gradsusr.org] on
> behalf of Jeff Duda [jeffduda319 at gmail.com]
> *Sent:* Thursday, November 10, 2011 11:11 AM
> *To:* GrADS Users Forum
> *Subject:* Re: [gradsusr] Diagnostics in Ctl File?
>
>  Mason,
> There is a good example of how to code temperature advection in the page
> for the cdiff command: http://www.iges.org/grads/gadoc/gradfunccdiff.html
> The reason for all of the crazy manipulations is because the data points
> on your grid are not uniformly spaced on the globe.  Thus corrections have
> to be made to get the correct distance between individual grid points.
>
> Jeff
>
> On Thu, Nov 10, 2011 at 11:00 AM, Wesley Ebisuzaki <
> Wesley.Ebisuzaki at noaa.gov> wrote:
>
>> Mason,
>>
>> I think that you've discovered the "private" commands used by nomads
>> plot software for defining new variables
>> and custom plots.   The nomads plot package is a wrapper for GrADS.  It
>> has the ability to run grads .gs files that
>> are embedded within the ctl files.
>>
>>
>>
>> http://www.cpc.ncep.noaa.gov/products/wesley/ncep_data/plot_doc.htmlhttp://www.cpc.ncep.noaa.gov/products/wesley/ncep_data/plot_doc.html
>>
>> Ask NCDC for a copy of the .gs files.
>>
>> > Has anyone done this before with grads?
>>  How do you expect the developers to debug the scripts?
>>
>>     Wesley
>>
>>
>>
>> Rowell, Mason D. wrote:
>> > All,
>> >
>> > I noticed ample comments toward the end of the ctl files available on
>> the nomads servers from the NARR page, with a description of how to
>> implement these in grads scripting. I tried to take this to plot
>> temperature advection but it didn't work, b/c the last line of the comments
>> calls for some function never provided. Has anyone done this before with
>> grads?
>> >
>> > Mason
>> > _______________________________________________
>> > gradsusr mailing list
>> > gradsusr at gradsusr.org
>> > http://gradsusr.org/mailman/listinfo/gradsusr
>> >
>>
>> _______________________________________________
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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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>

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