From AMS Glossary
The vertical kinematic flux of virtual potential temperature
, which when multiplied by the buoyancy parameter (g/Tv) yields a flux that is proportional to buoyancy, that is,
This is also a term in the turbulence kinetic energy budget, describing buoyant production or consumption of turbulence. It also appears in the numerator of the flux Richardson number.
Stull, R. B. 1988. An Introduction to Boundary Layer Meteorology. 666 pp.