From Glossary of Meteorology
A velocity scale w* for the convective mixed layer:
where g is gravitational acceleration, Tv is absolute temperature, zi is average depth of the mixed layer,
is the kinematic vertical turbulent flux of virtual potential temperature near the surface.
The velocity scale is typically on the order of 1 m s-1, which is roughly the updraft speed in convective thermals. This scale is often used in similarity theories for the convective mixed layer and was previously known as the convective velocity scale.
Stull, R. B. 1988. An Introduction to Boundary Layer Meteorology. 666 pp.