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

[Lyndz]

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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