= Reference-buoyancy convention in quasi-geostrophic potential vorticity
The hydrostatic convention $b=f_0\psi_z$ may refer to full <buoyancy> or to its anomaly relative to $N^2z$. Subtracting the uniform reference profile changes $\psi$ by $-N^2z^2/(2f_0)$ and changes $q=f_0+\nabla_h^2\psi+(f_0^2/N^2)\psi_{zz}$ by the uniform constant $-f_0$. Horizontal velocity and <potential-vorticity gradients> are unchanged, so the advection and stability dynamics are identical. Stating the convention avoids mistaking this reference constant for a dynamical gradient.
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