Native Grid projection

Dear Ncl users,
I am a bit confused about Native grid projection (and after looking at the examples on NCL scripts, even a bit more).

My doubts are in two parts. Part 1: in general on native proj. Part 2: specific to my script

--------------------PART 1-------------------------

(a) On http://www.ncl.ucar.edu/Applications/list_maps.shtml , there are a few projections defined and then there is Native projection.

Suppose I have a model output file based on a certain UNKNOWN projection, with no coordinate variables eg. variable Temp is (z,y,x); where x/y are NOT lon/lat but model x/y grid points from 1 to 550. Would this be regarded as a Native grid projection?

(b) If I define "res_at_tfDoNDCOverlay = True", it effectively means that my model projection is used without any transformation. Then do I still need to specify "res_at_mpProjection = CylindricalEquidistant/Lambert/..." ? If yes, then wouldn't this cause a conflict with "res_at_tfDoNDCOverlay = True"

(c) If my projection is native and "res_at_tfDoNDCOverlay = True", what purpose does "res_at_mpLimitMode = LatLon/Corner" serve? I understand that this is just a way to zoom in. But your tutorial says that if it is Native projection, then "res_at_mpLimitMode = Corner" (ALWAYS!!).

(d) Is "res_at_gsnAddCyclic = False" absolutely necessary for regional data files ?

--------------------PART 2-------------------------

Attached is my WRF output (.nc) file. It is Regional data based on Rotated Lat-Lon (I assume the same as Cylindrical Equidistant Projection). The center (origin) is shifted to 42 deg N, 6 deg E. Below is my NCL script defining the Limit window using LatLon method and also giving a rotation of 6 deg (as the shifted pole would be 6.0 deg E)

Since there are no coordinate variables, I assume it is a Native Grid issue. By doing ncdump on the file, I have used the additional info to write the script (attached).

I have compared the plot with NCView and I can definitely say that the plot shifts a little (this is easily evident by looking at Italy where the contour should be within the boundary).

I have tried different things (such as using different LimitMode). The mpMinLatF / mpMinLonF etc are calculated using the grid spacing & the center of domain (42.5 deg N, 6.0 deg E) which is the shifted origin. This should mean that the shifted pole is 6.0 deg E.

(e) Is it necessary to know the exact origin / pole etc if this is a native projection?

Am not sure why the plot is still not correct. Hoping to understand this better....Thanks in advance.

Malcolm
(Barcelona Supercomputing Center)

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