u/bar boundary condition for nested grid in 3d setup

[stef]

Another question about the boundary condition for barotropic flow for nested grids. This applies to 3d setups.

In 'u/v2dbc_im.F', there are lines like:

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                cff=0.5_r8*GRID(ng)%on_u(Istr,j)*                       &
     &              (GRID(ng)%h(Istr-1,j)+zeta(Istr-1,j,kout)+          &
     &               GRID(ng)%h(Istr  ,j)+zeta(Istr  ,j,kout))
                cff1=GRID(ng)%on_u(Istr,j)/REFINED(cr)%on_u(m)
                bry_val=cff1*REFINED(cr)%DU_avg2(1,m,tnew)/cff

If I understand correctly, u/vbar is computed for all fine points separately, even those (3 or 5 or 7) filling the same coarse cell. This can yield strongly varying u/vbar if the topography varies strongly, i.e.

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cff

varies strongly and causes horizontal vorticity at the fine-grid-scale, even though the large-scale field is constant within the coarse cell.

May it be better to compute the integral cross-section 'A' of the fine grid columns filling the coarse cell, and then prescribe a constant u/vbar = DU_avg2 / A for all fine points within?