This sets up four separate grid for advection of u in x-direction (uu), advection of u in y-direction (uv), advection of v in x-direction (vu), advection of v in y-direction (vv). The previous approach with 2 grids was insufficient because, although uu and vv are both T grid based, they are one offset in different directions. The uv and vu grids are identical (X grid without i=0 and j=0) and could potentially be merged in the future. The current code passes the test_2d_transport test with 0 spread in U,V,z.
This sets up four separate grid for advection of u in x-direction (uu), advection of u in y-direction (uv), advection of v in x-direction (vu), advection of v in y-direction (vv). The previous approach with 2 grids was insufficient because, although uu and vv are both T grid based, they are one offset in different directions. The uv and vu grids are identical (X grid without i=0 and j=0) and could potentially be merged in the future. The current code passes the test_2d_transport test with 0 spread in U,V,z.