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

Dennis Shea shea at ucar.edu
Tue May 7 15:02:47 MDT 2019

```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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