Monin–Obukhov length
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 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. A simple definition of the Monin-Obukhov length is that height at which turbulence is generated more by buoyancy than by wind shear.
The Obukhov length is defined by
where is the frictional velocity, is the mean virtual potential temperature, is the surface virtual potential temperature flux, k is the von Kármán constant. The virtual potential temperature flux is given by
where 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 is the height at which the buoyant production of turbulence kinetic energy is equal to that produced by the shearing action of the wind.
