mkscripgrid.ncl generates corners in CW rather than CCW order

[billsacks]

Brief summary of bug

It looks like https://github.com/ESCOMP/CTSM/blob/master/tools/mkmapgrids/mkscripgrid.ncl generates corners in clockwise rather than counter-clockwise order. According to ESMF documentation it is a requirement that corners be in counter-clockwise order.

General bug information

CTSM version you are using: master

Does this bug cause significantly incorrect results in the model's science? Probably not

Configurations affected: N/A

Details of bug

Both from examining the code and from running NCL's scrip_check_input on a generated grid, it appears that corners are in the wrong order. I'm not sure whether this actually causes problems with the generated mapping files.

[slevis-lmwg]

Shall I link this issue to #823 and resolve from there, or do you prefer that I open a new PR?

[billsacks]

Whichever is easier for you.

[ekluzek]

From talking with Ufuk one resolution for this might be to remove the setting of corner coordinates. There was a problem in NCL in the past where they had to be set by hand. NCL probably does it correctly now so we could remove the manual setting of corners.

[ekluzek]

The 5x5_amazon grid is in CCW order, so the original version must have been this way. The current version goes back to 2013 though.

Applying the following difference I can reproduce the 5x5_amazon grid (with only a roundoff difference in center lat and lon).

--- a/tools/mkmapgrids/mkscripgrid.ncl
+++ b/tools/mkmapgrids/mkscripgrid.ncl
@@ -124,16 +124,16 @@ end
   do i = 0, nx-1
     lat(:,i)          =  latCenters;
     latCorners(:,i,0) =  latCenters - delY/2.d0;
-    latCorners(:,i,1) =  latCenters + delY/2.d0;
+    latCorners(:,i,1) =  latCenters - delY/2.d0;
     latCorners(:,i,2) =  latCenters + delY/2.d0;
-    latCorners(:,i,3) =  latCenters - delY/2.d0;
+    latCorners(:,i,3) =  latCenters + delY/2.d0;
   end do
   do j = 0, ny-1
     lon(j,:)          =  lonCenters;
     lonCorners(j,:,0) =  lonCenters - delX/2.d0;
-    lonCorners(j,:,1) =  lonCenters - delX/2.d0;
+    lonCorners(j,:,1) =  lonCenters + delX/2.d0;
     lonCorners(j,:,2) =  lonCenters + delX/2.d0;
-    lonCorners(j,:,3) =  lonCenters + delX/2.d0;
+    lonCorners(j,:,3) =  lonCenters - delX/2.d0;
   end do
 
   ; for some reason, "No_FillValue" isn't working in the case where imask=1

Here's the command to use to reconstruct it...

./mknoocnmap.pl -p -9.5,295. -name 5x5_amazon -nx 5 -ny 5 -dx 17.5 -dy 11 -verbose

[ekluzek]

Note, that if I let NCL set the corners, I also reproduce the 5x5_amazon grid, except only the corner latitudes are off by roundoff. So we might as well take out the calculation of the corners.

[ekluzek]

The one thing about removing the calculation of the corners, is that it then doesn't work for a single point grid (or a grid with a single latitude or longitude).