[gradsusr] Meridional gradient?

Tue Jun 28 01:25:44 EDT 2011

Thanks Jeff.

On Mon, Jun 27, 2011 at 9:45 PM, Jeffrey Duda <jdduda at iastate.edu> wrote:

> 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
>>>>>>>>>
>>>>>>>>> _______________________________________________
>>>>>>>>> gradsusr mailing list
>>>>>>>>> gradsusr at gradsusr.org
>>>>>>>>> http://gradsusr.org/mailman/listinfo/gradsusr
>>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> --
>>>>>>>> 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>
>>>>
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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 <http://www.meteor.iastate.edu/%7Ejdduda>
>
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