The Obukhov length
is used to describe the effects of buoyancy on turbulent flows, particularly in the lower tenth of the atmospheric boundary layer. It was first defined by Alexander Obukhov
Alexander Mikhailovich Obukhov was a Russian physicist and applied mathematician known for his contribution to statistical theory of turbulence and atmospheric physics. He was one of the founders of modern boundary layer meteorology...
in 1946, . It is also known as the Monin-Obukhov length because of its important role in the similarity theory developed by Monin and Obukhov .
The Obukhov Length
is defined by
is the frictional velocity,
is the mean virtual potential temperature
is the surface virtual potential temperature flux, k is the von Kármán constant
In fluid dynamics, the Von Kármán constant , named for Theodore von Kármán, is a dimensionless constant describing the logarithmic velocity profile of a turbulent fluid flow near a boundary with a no-slip condition...
. The virtual potential temperature flux is given by
is potential temperature,
is absolute temperature and
is specific humidity.
By this definition,
is usually negative in the daytime since
is typically positive during the daytime over land, positive at night when
is typically negative, and becomes infinite at dawn and dusk when
passes through zero.
A physical interpretation of
is given by the Monin-Obukhov similarity theory. During the day
it is the height at which the buoyant production of turbulence kinetic energy (TKE) is equal to that produced by the shearing action of the wind (shear production of TKE).