= Interfacial Richardson number
{title2=$\mathrm{Ri}_i=g'\delta/u^2$}
A local ratio of stabilizing buoyancy to shear across an interface of thickness $\delta$, using <reduced gravity> jump $g'$ and characteristic velocity $u$. The dimensionless value is $\mathrm{Ri}_i=g'\delta/u^2$. It is distinct from a whole-layer bulk <Richardson number> based on the total mixed depth. Local <entrainment> laws use this ratio when the transition thickness and velocity characterize the turbulence-producing region.
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