# [ncl-talk] divergence

Dennis Shea shea at ucar.edu
Fri Dec 19 16:04:44 MST 2014

```(Very) small values are reasonable for divergence:  -/+ 1e-5 or 1e-6
(1/sec).
d = (du/dx)+(dv/dy)

Basic meteorology. Small differences in the numerator eg:
[u(j,i-1)-u(j,i+1)] divided by
large numbers: spatial distance  (meters) between (i+1) and (i-1). Same for
v.

The ud and vd are divergent wind components  14.8m/s,  vd=-2.7 m/s. Very
reasonable!

Did you look at this site like I requested.
http://www.ncl.ucar.edu/Applications/wind.shtml

The divergence field here was arbitrarily scaled by 1e+6 for  nicer numbers.

Really, this is basic ....

On Fri, Dec 19, 2014 at 2:03 PM, Abdoulaye SY <sy.abdoulaye3 at ugb.edu.sn>
wrote:

> Thank you Dennis
>
> My problem is when calculate divergence with
>
> * div    =uv2dvF_Wrap(qu,qv) *I have result* div *is an array of small
> values like 1.427085e-05
>
> when i calculate the components with *dv2uvf (div, ud, vd) *i get
> ud=14.63604
>
> vd=-2.875391
>
> Why this difference?
>
> 2014-12-19 17:59 GMT+00:00 Abdoulaye SY <sy.abdoulaye3 at ugb.edu.sn>:
>>
>> Hi
>>
>> I want to calculate divergence of the water vapor flux with ncl.
>>
>> I use
>> * div    = uv2dvF_Wrap(qu,qv)            *
>>     and
>> *          dv2uvf (div, ud, vd) *where qu and qv are water vapor flux
>> zonal and meridional respectively.
>>
>> I want to understand the Principles of operation of these functions. I
>> have read the documentation but it is not clear.
>>
>> thanks
>>
>
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