# Re: center_finite_diff_n missing values

From: Nicole Schiffer <nschiff2_at_nyahnyahspammersnyahnyah>
Date: Thu Dec 01 2011 - 16:17:47 MST

My goal is to calculate the 1-D moisture flux (mfu, mfv)and then use
that to figure moisture convergence.

I was trying to follow Example 6 from the documentation
(http://www.ncl.ucar.edu/Document/Functions/Built-in/center_finite_diff_n.shtml).
I switched the dimension of n1,n2 because I want to look at divergence
rather than vorticity, as in an earlier post
(http://www.ncl.ucar.edu/Support/talk_archives/2010/2835.html).

I also tried it using lat and lon without the looping or conversion from
degrees to distance just to get the function to work (see code below).
NCL still sees missing values in the u data and not in the v data (and
hence tries to calculate the gradient between real values and 1e+20).

mfu(:,:,:,n2) =
center_finite_diff_n((hus(time|:,plev|:,lat|:,lon|:)/g)*ua(:,:,:,:),lon,False,0,2)
mfv(:,:,:,:) =
center_finite_diff_n((hus(time|:,plev|:,lat|:,lon|:)/g)*va(:,:,:,:),lat,False,0,2)

-Nicole

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
* Nicole Schiffer
* Graduate Research Fellow (Dept. of Energy)
* Department of Atmospheric Sciences
* University of Illinois, Urbana-Champaign
* Email: nschiff2 [at] illinois [dot] edu
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

On 12/1/11 4:56 PM, Mary Haley wrote:
> Hi Nicole,
>
> Why are dx and dy calculated the way they are?
>
>> dx = 6378388*cos(0.0174533*lat(n2))*dlon
>> dy = 6378388.*0.0174533*dlon
>
>
>
> Did you mean to subscript "lat" with n2? You use "n2" as a longitude
> subscript
> in the center_finite_diff_n call, so I'm wondering if there's an error
> there...
>
> Something doesn't look quite right with the above code. I'm guessing
> one or
> both of dx or dy are incorrect.
>
> --Mary
>
> On Dec 1, 2011, at 10:31 AM, Nicole Schiffer wrote:
>
>> I'm using center_finite_diff_n on two 4D array (time, pressure, lat,
>> lon) with missing values. With one array, the u-wind, it handles missing
>> values fine. With the v-wind array, it doesn't. Both arrays have the
>> same metadata (missing and fill values). Any ideas why v-wind is a
>> problem?
>>
>> Code snippet:
>> dlat = 2.5*0.0174533
>> dlon = dlat
>> do n2=0,nlon-1
>> dx = 6378388*cos(0.0174533*lat(n2))*dlon
>> mfu(:,:,:,n2) =
>> center_finite_diff_n((hus(time|:,plev|:,lat|:,lon|n2)/g)*ua(:,:,:,n2),dx,False,0,2)
>>
>> end do
>> dlon = 2.5*0.0174533
>> do n1=0,nlat-1
>> dy = 6378388.*0.0174533*dlon
>> mfv(:,:,n1,:) =
>> center_finite_diff_n((hus(time|:,plev|:,lat|n1,lon|:)/g)*va(:,:,n1,:),dy,False,0,2)
>>
>> end do
>>
>> Thanks,
>> Nicole
>>
>> --
>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> * Nicole Schiffer
>> * Graduate Research Fellow (Dept. of Energy)
>> * Department of Atmospheric Sciences
>> * University of Illinois, Urbana-Champaign
>> * Email: nschiff2 [at] illinois [dot] edu
>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>
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>
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Received on Thu Dec 1 16:18:21 2011

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