The grid-scale variables in the QTCM1 are located on a staggered C-grid  of dimensions , where the x (zonal, longitudinal) coordinate indexes from 1 to NX, and the y (meridional, latitudinal) coordinate indexes from 1 to NY. (there are certain exceptions to this description of the dimensioning, which are noted in Section 2.3.1). A schematic of the grid is given in Figure 2.1.
Figure: QTCM1 staggered C-grid. Note that j=0 only exists for v and variables dependent on v. See text in Section 2.3.1 for details.
The pair (i,j) denotes a grid point. For the standard version, and . Figure 2.1 abbreviates variables as follows:
These variables are all prognostic.
Thus, temperature is located at grid point (i,j), while zonal velocity is located at ( ), meridional velocity is located at ( ), and is located at ( ). Recall that the subscript ``0'' refers to the barotropic mode, while the subscript ``1'' refers to the baroclinic mode.
Other variables are also defined at the above locations. These include:
T. Variable is the surface temperature, and is diagnostic, except over land where it is prognostic.
T. Variable is the humidity, and is prognostic.
vort0. Variable is the barotropic streamfunction, and is diagnostic.
Although the C-grid has half-step variable indices, array indices in FORTRAN do not. Thus, we have adopted the convention that all half-indices are truncated when referred to in the QTCM1 code. For example:
In all other sections of this document, array indice naming follows the FORTRAN model code convention.
Below is a portion of the file qtcm.h which contains the array declarations for the main prognostic grid variables:
c baroclinic mode real*8 u1(NX,NY),v1(NX,0:NY),T1(NX,NY),q1(NX,NY) c barotropic mode V0; note only vort0 and u0bar are prognostic real*8 u0bar,u0(NX,NY),v0(NX,0:NY),vort0(NX,NY),psi0(NX,0:NY) c- Boundary inputs and derived; exception: Ts overland is prognostic real*8 Ts(NX,NY),qs(NX,NY),Ts0(NX,NY)