[gradsusr] Finding the maximum value of Ri in a single grid point

Fri Nov 4 16:32:01 EDT 2011

Hi Jeff,

  This is a great explanation. This would make a very good recipe for the
cookbooks <http://cookbooks.opengrads.org/index.php?title=Main_Page>. This
issue has been a very common cause of confusion among new users of GrADS.

  (If you need any help getting through the cookbooks let me know.)

      Arlindo

On Thu, Nov 3, 2011 at 7:46 PM, Jeffrey Duda <jdduda at iastate.edu> wrote:

> The problem is that there is a difference between grids that can be
> displayed and script variables.  They can't be handled the same way.  You
> can't use if statements using displayed fields.  What you need to do is to
> save the output of each computation in your script to a script variable
> using the subwrd command.  Here is an example:
>
>
> 'define th1 = temp * (pow(100000 / press , (0.286)))'
> th1 = subwrd(result,4)
>
> Hopefully you've noticed this before, but when you enter a display command
> at the Grads prompt, there will be a small amount of text output.  If you
> display a variable with all of the dimensions in your environment fixed,
> the text will include the value of the field you are displaying.  What
> subwrd does is grab the fourth word in the string variable "result".
> "result" is automatically assigned each time a display command is run, so
> that's why you don't see it being assigned anywhere else.  NOTE: for some
> data sets and for some environment settings, Grads will interpolate the
> output before displaying, so the text output from the display command will
> read
>
> "Note: interpolating variable...
> Result value: _____"
>
> In that case you'll need to use the sublin command first:
> line = sublin(result,2)
> th1 = subwrd(line,4)
> This tells Grads to nab the second line of the output from the result
> variable (result includes all of the text output from the display
> command).  Then you use subwrd to nab the 4th word of the string variable
> "line".  When you do this, you'll have the fields you want as Grads
> scripting variables.  Then you can run if statements on them.  To output
> the value of a Grads script variable, use the say command:
> say th1 OR
> say "th1 = "th1
>
> ON THE OTHER HAND, you can do this without using any scripting variables.
> In this case use the max/min and maxloc/minloc functions.  Then your script
> would look something like this:
>
> 'set lev 'botlevel' 'toplevel
>
>
>  'define th1 = temp * (pow(100000 / press , (0.286)))'
> 'define u1 = u'
>  'define v1 = v'
> 'define z1 = geopot/9.81'
>
>  'define th2 = temp(z-1) * (pow(100000 / press(z-1) , (0.286)))'
> 'define u2 = u(z-1)'
>  'define v2 = v(z-1)'
> 'define z2 = geopot(z-1)/9.81'
>
> 'define
> ri=9.81/(0.5*th1+th2)*(th2-th1)/(pow((u2-u1),2)+pow((v2-v1),2))*(z2-z1)'
>
> 'd ri'
> say result
>
> 'd min(ri,lev='botlev',lev='toplev')'
> 'd minloc(ri,lev='botlev',lev='toplev')'
> 'd max(ri,lev='botlev',lev='toplev')'
> 'd maxloc(ri,lev='botlev',lev='toplev')'
>
> Carefully follow the location of the single quotes and note that there is
> no while loop here.  Also note the (z-1) appended to the *2 variables to
> represent the field one level below the current level.  The whole quote
> thing confuses some people, so that's understandable.  Try this and see if
> it works.  Ask if you have any questions.  Good luck.
>
> Jeff Duda
>
> On Thu, Nov 3, 2011 at 5:23 AM, Lippo Nester <liikkuvattaakse at gmail.com>wrote:
>
>> Hi!
>>
>> I've been trying to create my very first GrADS-script. The script should
>> compute values of the bulk Richardson number in a specific grid point
>> between predefined model levels and then find the minimum value of Ri and
>> probably the model level associated with it.
>>
>> I tried to do this using while and if loops, but for some reason it's not
>> working as I expected. Maybe I can't understand handling variables in GrADS?
>>
>> In this script the top and the bottom model layers are set to 40 and 50.
>> Ri is computed and I get a list of the Ri at different levels, and they
>> seem to be ok. However the variable "minri" gets always the value of ri at
>> the lowest model level (here: ri at level 50). Variable "minlevel" is
>> always the highest model level (here: level 40)!
>>
>> When count is 40 at the first step of the while-loop, the value of ri
>> should be saved into variable minri and the model level at minlev. After
>> that, if ri < minri at lower levels (count > 40), values of minri and
>> minlev should be varied by the second if-loop...
>>
>> Is there a more simple or more correct way to do this? I thought max()
>> can't make it...
>>
>> Any help is much appreciated!
>>
>> Best regards, Lippo
>>
>> (version of GrADS 2.0.a1)
>>
>> 'set lat 50.00'
>> 'set lon 10.00'
>> botlevel=50
>> toplevel=40
>>
>> count=toplevel
>>
>> while (count <= botlevel)
>>
>> 'set lev 'count
>>  'define th1 = temp * (pow(100000 / press , (0.286)))'
>> 'define u1 = u'
>>  'define v1 = v'
>> 'define z1 = geopot/9.81'
>>
>> 'set lev 'count - 1
>>  'define th2 = temp * (pow(100000 / press , (0.286)))'
>> 'define u2 = u'
>>  'define v2 = v'
>> 'define z2 = geopot/9.81'
>>
>> 'define
>> ri=9.81/(0.5*th1+th2)*(th2-th1)/(pow((u2-u1),2)+pow((v2-v1),2))*(z2-z1)'
>>
>> 'd 'ri
>> say result *THIS SEEMS TO WORK AS I GET 10 DIFFERENT VALUES OF RI
>>
>> if (count = toplevel) *THESE IF-LOOPS AREN'T OKAY, BUT WHY?
>> minri = ri
>>  minlevel = count
>> endif
>>
>> if (ri < minri)
>>  minri = ri
>> minlevel = count
>> endif
>>
>> count = count+1
>> endwhile
>>
>> 'd 'minri
>> say "Min Ri " result *ALWAYS THE VALUE OF RI AT LEVEL 50 (AT THE BOTTOM
>> LEVEL)
>> 'd 'minlevel
>> say "Model level " result *ALWAYS LEVEL 40 (TOP LEVEL)
>>
>>
>>
>>
>> _______________________________________________
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>>
>>
>
>
> --
> Jeff Duda
> Iowa State University
> Meteorology Graduate Student
> www.meteor.iastate.edu/~jdduda <http://www.meteor.iastate.edu/%7Ejdduda>
>
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>
>

-- 
Arlindo da Silva
dasilva at alum.mit.edu
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