[gradsusr] Meridional gradient?

Mon Jun 27 12:15:36 EDT 2011

Sushant,
I apologize.  I was wrong about what I said in my last email.  Your value of
dy is correct.  It should be a constant value unless you have a stretched
grid.  Thus you have both dx and dy correct.  Now you can use them in
computations of gradients and advection.

Jeff

On Mon, Jun 27, 2011 at 10:26 AM, Jeffrey Duda <jdduda at iastate.edu> wrote:

> Since dx is dependent on latitude, the contour plot for dx is correct.  The
> largest values of dx are going to be at the equator where there is the
> largest meridional distance between longitude lines.
>
> For your dy plot, I guess we need to make sure you actually have a latitude
> variable in your data.  Try just 'd lat'.  You should not get a constant
> value.  If that doesn't work, send me either the .ctl file or the result of
> ncdump -h if your data is coming from a netCDF file.
>
> Jeff
>
>
> On Mon, Jun 27, 2011 at 12:13 AM, sushant puranik <
> sushantpuranik at gmail.com> wrote:
>
>> Jeff
>> I tried with making changes asper your suggestion.
>> I got results as Constant field value =333533 for
>> dy=6.37e6*cdiff(lat,y)*(3.1416/180) command
>>
>> and also tried dx=6.37e6*cos(lat*(3.1416/180))*cdiff(lon,x)*(3.1416/180)
>> in this case it shows simply parallel lines on the plot.
>> Results are attached with this mail.
>>
>> Thank you
>>
>> Sushant
>>
>> On Sat, Jun 25, 2011 at 8:58 PM, Jeffrey Duda <jdduda at iastate.edu> wrote:
>>
>>> Put the literal argument "lat" in place of "19.5" in the cdiff command
>>> for dy and put "lon" for "30" in the cdiff for the dx definition.  There's
>>> nothing wrong with setting your dimensions first, as that will only affect
>>> the horizontal scope of the subsequent fields you define.
>>>
>>> Jeff
>>>
>>>
>>> On Sat, Jun 25, 2011 at 12:11 AM, sushant puranik <
>>> sushantpuranik at gmail.com> wrote:
>>>
>>>> Thanks Jeff.
>>>> But i am not able to plot the output. It gives error with the
>>>> latitudinal command
>>>>
>>>> ga-> define dy=6.37e6*cdiff(19.5,y)*(3.1416/180)
>>>> Error from CDIFF:  Specified dimension non varying
>>>> Operation Error:  Error from cdiff function
>>>>   Error ocurred at column 8
>>>> DEFINE error:  Invalid expression.
>>>>
>>>> and also tried with the longitudinal command
>>>>
>>>> ga-> define dx=6.37e6*cos(10*(3.1416/180))*cdiff(30,x)*(3.1416/180)
>>>> Error from CDIFF:  Specified dimension non varying
>>>> Operation Error:  Error from cdiff function
>>>>   Error ocurred at column 29
>>>> DEFINE error:  Invalid expression.
>>>> Segmentation fault
>>>>
>>>> i think i am have to set dimensions first?
>>>>
>>>> Thank you
>>>>
>>>> Sushant
>>>>
>>>>
>>>> On Fri, Jun 24, 2011 at 10:14 PM, Jeffrey Duda <jdduda at iastate.edu>wrote:
>>>>
>>>>> Sushant,
>>>>> Unfortunately, no.  Getting the longitudinal gradient is not as simple
>>>>> since the distance between longitude lines is a function of the latitude.
>>>>> However, the formula is
>>>>>
>>>>> dx = Re*cos(lat*(pi/180))*cdiff(lon,x)*(pi/180).
>>>>>
>>>>> The cos(lat*(pi/180)) term is the adjustment for dependence of
>>>>> longitude space on latitude.
>>>>>
>>>>> Jeff
>>>>>
>>>>>
>>>>> On Fri, Jun 24, 2011 at 12:55 AM, sushant puranik <
>>>>> sushantpuranik at gmail.com> wrote:
>>>>>
>>>>>> Hi Jeff
>>>>>> Can i use same command for longitudinal gradient computation by
>>>>>> replacing lat --> lon in the command
>>>>>> dy = Re*cdiff(lat,y)*(3.14159/180)
>>>>>>
>>>>>> thanks
>>>>>>
>>>>>> Sushant
>>>>>>
>>>>>>
>>>>>> On Tue, May 4, 2010 at 2:33 AM, Jeffrey Duda <jdduda at iastate.edu>wrote:
>>>>>>
>>>>>>> Li,
>>>>>>> If you want to find a temperature gradient, why not just use cdiff?
>>>>>>> Just realize that you'll have to use the following expression for dy (the
>>>>>>> incremental change in meridional distance between grid points, in meters):
>>>>>>>
>>>>>>> dy = Re*cdiff(lat,y)*(3.14159/180)
>>>>>>>
>>>>>>> where Re is the radius of the Earth (about 6.371e6 meters, I
>>>>>>> believe).   This is to account for the projection Grads uses to display your
>>>>>>> data.  Hope this helps.
>>>>>>>
>>>>>>> Jeff Duda
>>>>>>>
>>>>>>>
>>>>>>> On Mon, May 3, 2010 at 3:20 PM, Li Dong <ldong at unm.edu> wrote:
>>>>>>>
>>>>>>>> Hello list,
>>>>>>>>
>>>>>>>> I need to calculate the meridional temperature gradient across 20
>>>>>>>> degree
>>>>>>>> latitude belt using global gridded data. Below is what I did:
>>>>>>>>
>>>>>>>> 'open ta.ctl'
>>>>>>>> 'set t 1'
>>>>>>>> 'set lev 1000'
>>>>>>>> 'set lon 0 360'
>>>>>>>> 'set lat -80 80'
>>>>>>>> 'define Tgrad=ta(lat+10)-ta(lat-10)'
>>>>>>>>
>>>>>>>> However, Grads complained "Cannot use an offset value with a varying
>>>>>>>> dimension". If I set 'lat' at a fixed value, such as 'set lat 40',
>>>>>>>> the
>>>>>>>> above script did work, but the downside is that this only gives
>>>>>>>> temperature meridional gradient at the specified latitude instead of
>>>>>>>> over the entire latitude range. Any inputs would be highly
>>>>>>>> appreciated!
>>>>>>>>
>>>>>>>> Best,
>>>>>>>>
>>>>>>>> Li
>>>>>>>>
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>>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> --
>>>>>>> Jeff Duda
>>>>>>> Iowa State University
>>>>>>> Meteorology Graduate Student
>>>>>>> 3134 Agronomy Hall
>>>>>>> www.meteor.iastate.edu/~jdduda<http://www.meteor.iastate.edu/%7Ejdduda>
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> gradsusr mailing list
>>>>>>> gradsusr at gradsusr.org
>>>>>>> http://gradsusr.org/mailman/listinfo/gradsusr
>>>>>>>
>>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
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>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>> --
>>>>> Jeff Duda
>>>>> Iowa State University
>>>>> Meteorology Graduate Student
>>>>> 3134 Agronomy Hall
>>>>> www.meteor.iastate.edu/~jdduda<http://www.meteor.iastate.edu/%7Ejdduda>
>>>>>
>>>>> _______________________________________________
>>>>> gradsusr mailing list
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>>>>>
>>>>>
>>>>
>>>>
>>>> --
>>>>
>>>>
>>>>
>>>>
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>>>
>>>
>>> --
>>> Jeff Duda
>>> Iowa State University
>>> Meteorology Graduate Student
>>> 3134 Agronomy Hall
>>> www.meteor.iastate.edu/~jdduda <http://www.meteor.iastate.edu/%7Ejdduda>
>>>
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>>
>>
>>
>>
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>
>
> --
> Jeff Duda
> Iowa State University
> Meteorology Graduate Student
> 3134 Agronomy Hall
> www.meteor.iastate.edu/~jdduda <http://www.meteor.iastate.edu/%7Ejdduda>
>

-- 
Jeff Duda
Iowa State University
Meteorology Graduate Student
3134 Agronomy Hall
www.meteor.iastate.edu/~jdduda
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