= Topographic buoyancy boundary condition for quasi-geostrophic flow
{title2=$D_g[(f_0/N^2)\psi_z+b]=0$}
For a small steady lower-boundary height $b(x,y)$, impermeability gives $w=\mathbf u_g\cdot\nabla b$ at the reference plane. Substitution in the <buoyancy> equation $D_g(f_0\psi_z)+N^2w=0$ yields the displayed <boundary condition>. For nonzero Fourier modes in a uniform crossing current, this fixes $\psi_z(0)=-N^2b/f_0$. An upper <radiation condition> or decay condition is still needed to select the physical response.
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