[gradsusr] Meridional gradient?

[Jeffrey Duda]

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>
>>
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>>
>>
>
>
>
>
>
>
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>


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