From AMS Glossary
The hypothesis that vorticity, and not momentum, is conservative in turbulent eddy flux due to the existence of pressure fluctuations.
This would apply especially if the turbulence were strictly two-dimensional. This hypothesis, together with that of the mixing length, leads to an expression for the variation of the shearing stress τ with height,
where K is the coefficient of eddy viscosity, and
the mean horizontal wind.