# Re: center_finite_diff_n for relative vorticity

From: Bold Khan <gkhan100_at_nyahnyahspammersnyahnyah>
Date: Sun Dec 19 2010 - 15:28:28 MST

Dennis, Thank you! This was an oversight on my part. I made this correction.
So the script, the way it's written, works. But I am not sure if its
actually right. I don't feel that it is.
May I have a little conceptual help?

If I want to calculate vorticity: (i.e. rv = (dv/dy) - (du/dx)
Do dv/dx and dvdy need to be in the same loop or can they be
separate as I have done?
And, since I yearn to be an efficient NCLer, and avoid do loops where
possible, is there a way not
to use loops for this calculation?

Again, thank you for your time.
Still trying to wrap my brain around this.
-Bold

05:12:16 ] > ncl tester.ncl
University Corporation for Atmospheric Research
NCAR Command Language Version 5.2.1
The use of this software is governed by a License Agreement.
See http://www.ncl.ucar.edu/ for more details.
(0)
(0) min=-4.3992e-05 max=4.80107e-05

CODE

;======================== some contants
pi = 4.0*atan(1.0)
FILEIN = "era_uv.nc"
;======================== Read in file and variables
U=a->U
u=U(0,{700},:,:)
V=a->V
v=V(0,{700},:,:)

lat = a->lat
lon = a->lon
nlat = dimsizes(lat)
mlon = dimsizes(lon)
dlon = (max(lon)-min(lon))/(mlon-1)
dlat = (max(lat)-min(lat))/(nlat-1)
;===================================== Arrays
dvdx = new ( (/nlat,mlon/), typeof(v), v@_FillValue)
dudy = new ( (/nlat,mlon/), typeof(v), v@_FillValue)
;===================================== loops
do nl=0,nlat-1
dvdx(nl,:) = center_finite_diff_n (v(nl,:), dX, False, 0, 0)
end do
do ml=0,mlon-1
dudy(:,ml) = center_finite_diff_n (v(:,ml), dY, False, 0, 0)
end do
vor = dvdx - dudy

printMinMax(vor,True)

On Sun, Dec 19, 2010 at 4:44 PM, Dennis Shea <shea@ucar.edu> wrote:
> You did not define the variables correctly.
>
> You created the following arrays
>
>> dvdx = new ( (/nlat,mlon/), typeof(v), v@_FillValue)
>> dudx = new ( (/nlat,mlon/), typeof(v), v@_FillValue)
>
> Then you used
>
>>    dvdx(nl,:) = center_finite_diff_n (v(nl,:), dX, False, 0, 0)
>                :
>>    dudy(:,ml) = center_finite_diff_n (v(:,ml), dY, False, 0, 1)
>
> Where did you preallocate space for dudy??
> The error messages told you exactly what the problem is:
>
>> fatal:Undefined identifier: (dudy) is undefined, can't continue
>> fatal:Execute: Error occurred at or near line 30 in file tester.ncl
>>
>> fatal:Variable (dudy) is undefined
>> fatal:Execute: Error occurred at or near line 33 in file tester.ncl
>>
>> fatal:Variable (vor) is undefined
>
> vor is undefined because the dudy is undefined
>
> ==
>
> I think waht you want is:
>
> dudy = new ( (/nlat,mlon/), typeof(v), v@_FillValue)
>
>
> On 12/19/10 12:55 PM, Bold Khan wrote:
>>
>> Dear ncl users,
>> Could I have some help? I am still learning ncl.
>> I am getting confused with using the center_finite_diff_n command to
>> calculate relative vorticity (RV) with non-cyclic data.
>> I want to calculate (RV) using center_finite_diff_n, instead of just
>> resorting to using the uv2vr_cfd command that is available, because I
>> want to calculate RV and then shear vorticity separately.
>> I am following example 6 at
>>
>> http://www.ncl.ucar.edu/Document/Functions/Built-in/center_finite_diff_n.shtml.
>>
>> 1) Do dv/dx and dvdy need to be in the same loop or can they be
>> separate as I have done?
>> 2) Since we want to try and avoid do loops in ncl, is there a way not
>> to use loops for this calculation
>> Any amount of advice, would be greatly appreciated.
>> Sincerely,
>> Bold
>>
>>
>> [ 02:42:16 ]>  ncl tester.ncl
>>  University Corporation for Atmospheric Research
>>  NCAR Command Language Version 5.2.1
>>  The use of this software is governed by a License Agreement.
>>  See http://www.ncl.ucar.edu/ for more details.
>> fatal:Undefined identifier: (dudy) is undefined, can't continue
>> fatal:Execute: Error occurred at or near line 30 in file tester.ncl
>>
>> fatal:Variable (dudy) is undefined
>> fatal:Execute: Error occurred at or near line 33 in file tester.ncl
>>
>> fatal:Variable (vor) is undefined
>> fatal:Execute: Error occurred at or near line 35 in file tester.ncl
>>
>> [ 02:48:25 ]>
>>
>>
>>
>> ;;; some contants
>> pi    = 4.0*atan(1.0)
>> FILEIN = "era_uv.nc"
>> ;=========================variables
>> U=a->U
>> u=U(0,{700},:,:)
>> V=a->V
>> v=V(0,{700},:,:)
>>
>> lat = a->lat
>> lon = a->lon
>> nlat = dimsizes(lat)
>> mlon = dimsizes(lon)
>> dlon = (max(lon)-min(lon))/(mlon-1)
>> dlat = (max(lat)-min(lat))/(nlat-1)
>> ; Arrays
>> dvdx = new ( (/nlat,mlon/), typeof(v), v@_FillValue)
>> dudx = new ( (/nlat,mlon/), typeof(v), v@_FillValue)
>> ; loops
>> do nl=0,nlat-1
>>   dvdx(nl,:) = center_finite_diff_n (v(nl,:), dX, False, 0, 0)
>> end do
>>  do ml=0,mlon-1
>>    dudy(:,ml) = center_finite_diff_n (v(:,ml), dY, False, 0, 1)
>>  end do
>> vor = dvdx - dudy
>>
>> printMinMax(vor,True)
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>
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Received on Sun Dec 19 15:28:37 2010

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