Turbulent Prandtl number (${\mathrm{Pr}}_t=K_m/K_b$)

The ratio of turbulent momentum diffusivity to buoyancy or heat diffusivity. Their ratio determines how the gradient Richardson number is related to the energy-budget Flux Richardson number. It is generally a closure-dependent function of the flow, not necessarily one.

An ideal correspondence between turbulent momentum and scalar transfer. In the local gradient-diffusion form used for a surface layer, the ideal analogy sets ${\mathrm{Pr}}_t=1$. It is an additional modeling approximation; measured buoyancy and momentum similarity functions need not be identical.