From AMS Glossary
This mechanical production term S of the turbulence kinetic energy budget equation is (written in Einstein's summation notation)
and is almost always positive, that is, a source. In the fair-weather, turbulent atmospheric boundary layer, the terms associated with horizontal wind usually dominate, allowing the shear production term to be approximated as
) are the Reynolds stresses (turbulent kinematic momentum fluxes) in the (x, y) directions, (U, V) are the respective wind components, the prime denotes deviation from the mean, and the overbar denotes an average. The ratio of the buoyant to the negative of the shear production terms is also known as the flux Richardson number and is a measure of dynamic stability.