[ncl-talk] Bug (?) in filled contours since version NCL 6.5.

[Michael Langguth]

Dear NCL-community,
  
we already posted this issue on the NCL GitHub repo (as
#95 on 11th April), but would like to bring it to a
probably broader attention via the ncl-talk. We hope not
to annoy some of you with a double posting, but we still
could not solve the problem and would like to update to
the newest NCL version in the near future.
  
Starting with NCL 6.5.0, a random-like bug in the filled
contour plots being produced routinely at our institute
(Institute of Geoscience - Department Meteorology of the
University of Bonn) arose.
  
We plot GFS data obtained from the NCEP data server over
a domain spanning from the West Atlantic to Eastern
Europe, i.e. our plot domain crosses the prime meridian.
  While we had never problems using NCL 6.3.0, the
contours sometimes aren't filled correctly neither with
NCL 6.5.0 nor with NCL 6.6.2.
  More specifically, uniformly colored segments over
several levels (which should have different fill colors)
are drawn. These segments are likely to be confined by
the prime meridian.
  The problem seems to occur randomly since a small change
in the plotting domain may yield a correct visualization
and since the plotting process may succeed using slightly
different data sets (i.e. another GFS data file).
  
For a visualisation of this issue we would like to refer
to our GitHub post: https://github.com/NCAR/ncl/issues/95
  
  The exemplary script and the data set (cropped data of
GFS run 2019/01/23 00 UTC +087 forecast) can be accessed
from:
  
  link to script:
https://uni-bonn.sciebo.de/s/I9dXjEAz8twDAtX
  link to data :
https://uni-bonn.sciebo.de/s/5lctA71qie3TSEp
  
  Our system runs on Linux 4.12.14 (OpenSuse 15.0) and NCL
was installed by using the binary from NCAR.
  
  Do you have any ideas how to fix this problem or is it
related to a deep bug in the NCL plotting routine?
  
  Many thanks in advance and kind regards!
  M. Langguth and A.Demers
  

-------------------------------
------------------------------------
M. Sc. Michael Langguth
Institute for Geoscience - Department Meteorology
University of Bonn
Auf dem Hügel 20, Bonn
phone: +49 228 73 5186
email: mlangguth at uni-bonn.de