[MITgcm-support] SDIAGx and UDIAGx

[John Pender]

Hi All-
I have managed to confuse myself over the grid used by the diagnostic output variables discussed here:

	http://mitgcm.org/public/r2_manual/latest/online_documents/node267.html

I was under the impression that these outputs were on the C-grid but I find this is not necessarily the case.  I set up a small run with modifications to 
	code/diagnostics_fill_state.F
that wrote the exact same numbers (single layer) to each of

	SDIAG1, SDIAG2, SDIAG3, SDIAG4, SDIAG5, SDIAG6, SDIAG7, SDIAG8, SDIAG9,  and SDIAG10

in a series of 10 CALL DIAGNOSTICS_FILL( … ) commands, and another set of numbers (Nr layers) to each of

	UDIAG1, UDIAG2, UDIAG3, UDIAG4, UDIAG5, UDIAG6, UDIAG7, UDIAG8, UDIAG9,  and UDIAG10

in another series of 10 CALL DIAGNOSTICS_FILL( … ) commands.  If you look at, for instance, the UDIAG1 output you see (using ncdump for netdcf output) that the data is written to the center grid

	...
	double UDIAG1(T, Zmd000100, Y, X) ;
	…

but if you look at the UDIAG5 output you see that the x component is written to the face grid.

	...
	double UDIAG5(T, Zmd000100, Y, Xp1) ;
	…

The actual numbers written to file are the same for either case, so what is different is the presumed grid the data refer to.

Thanks very much,
John