# [ncl-talk] Question about the sign of shear deformation

Lyndz olagueralyndonmark429 at gmail.com
Wed May 8 01:46:46 MDT 2019

```Dear Sir Dennis,

Thank you so much for the help.

One more point though in the documentation. I think this line should be
corrected?

Less commonly used are the
shear-deformation (SHD) and stretch-deformation (STD).

SHD = du/dx - dv/dy
STD = dv/dx + du/dy

to

STD = du/dx - dv/dy

SHD = dv/dx + du/dy

Sincerely,

Lyndz

On Wed, May 8, 2019 at 6:02 AM Dennis Shea <shea at ucar.edu> wrote:

> This could well be a matter of definition.  NCL does not include any
> explicit negative (or positive) sign or coefficient (eg:  0.5).
>
>
>
> *  shear  = dvdx + dudy  stretch= dudx - dvdy   deform = sqrt(shear^2 +
> stretch^2)*
>
> where the gradients are determined by spherical harmonics.
>
> You can associate signs/coefficients to these terms  as you wish.
>
> ====
> You have
> ************************************************
> ; Stretch, deform, and total
> ;************************************************
>   grdType = 1
>   *cyclic  = False       ; <========  This should be True*
>   opt_ssd = 0
>
>   ssdList = shear_stretch_deform(u,v,u&lat,grdType,opt_ssd)
>
>   stretch_sph = ssdList[0]
>   shear_sph   = ssdList[1]
>   deform_sph  = ssdList[2]
>   delete(ssdList)
>
> ======================================================================
> I don't think anyone wants to download a 600+MB file from the cloud.
>
> Use the netCDF operator the extract the exact subset
>
> *%>* *ncks* -O -d Time,tStrt,Tlast ncep_6h_jan.nc  NECP.deform.nc
>
> where tStrt and tLast are the temporal index values of the specified time
> interval.
>
> ===
> The following is 'ok'.
>
>   u   = f->U_GRD_2_ISBL(:,:,:,:)       ; pull u off file
>   v   = f->V_GRD_2_ISBL(:,:,:,:)       ; pull v off file
>
> However, when reading an entire array, it is more efficient to use the
> following. You can indicate dimensionalit in comment
>
>   u   = f->U_GRD_2_ISBL            ;         (:,:,:,:)       ; pull u off
> file
>   v   = f->V_GRD_2_ISBL             ;         (:,:,:,:)       ; pull v off
> file
>
> ===
> The following is superfluous
>
>   lat = f->lat
>   lon = f->lon
>
> These coordinates are part of each variable's structure
>
>    u&lat     ===> lat
>
> ===
> What order are the variables? N-S or S-N ? The following reverses the
> order.
>
>   u   = u(:,:,::-1,:)
>   v   = v(:,:,::-1,:)
>
> On Mon, May 6, 2019 at 1:34 AM Lyndz <olagueralyndonmark429 at gmail.com>
> wrote:
>
>> Dear NCL experts,
>>
>> I am calculating the shear_stretch deformation routine in NCL (
>> https://www.ncl.ucar.edu/Document/Functions/Contributed/shear_stretch_deform.shtml
>> ).
>>
>> According to the description:
>>
>> *"When the stretch-deformation is negative/positive the flow is
>> confluent/diffluent."*
>>
>> However, I encounter some inconsistensies in the results.
>>
>> Problems:
>>
>> [1] When I plot the stretch deformation term, I see positive values
>> (diffluence) along a region of confluence (encircled region in red in the
>> attached figure).
>> This plot is for timestep 62. Shading is for the 925-hPa stretch
>> deformation, while the streamline is for the 925-hPa winds.
>>
>> I am not sure if I implemented this routine correctly. I am attaching my
>> script in this email. The file that I am using can be accessed here:
>>
>> https://www.dropbox.com/s/m4sy7n76piw2mwn/ncep_6h_jan.nc?dl=0
>>
>> I would like to ask for suggestions on how to do this correctly in NCL.
>>
>> [2] I would like to draw line along the regions of positive stretch or
>> total deformation. If yes, how can I do this in NCL?
>>
>> Thank you in advance for the help.
>> Sincerely,
>>
>> *Lyndz*
>>
>> _______________________________________________
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>
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