Brunt-Väisälä frequency

In atmospheric dynamics, oceanography, and geophysics, the Brunt-Väisälä frequency, or buoyancy frequency, is the frequency at which a vertically displaced parcel will oscillate within a statically stable environment. In the atmosphere,

: $N\; equiv\; sqrt\{frac\{g\}\{\; heta\}frac\{d\; heta\}\{dz$, where $heta$ is potential temperature, $g$ is the local acceleration of gravity, and $z$ is geometric height.

In the ocean where salinity is important, or in fresh water lakes near freezing, where density is not a linear function of temperature,

: $N\; equiv\; sqrt\{-frac\{g\}\{\; ho\}frac\{d\; ho\}\{dz$, where $ho$, the potential density, depends on both temperature and salinity.

Context

The concept derives from Newton's Second Law when applied to a fluid parcel in the presence of a backgroundstratification (in which the density changes in the vertical). The parcel, perturbed vertically from itsstarting position, experiences a vertical acceleration. If the acceleration is back towards the initialposition, the stratification is said to be stable and the parcel oscillates vertically. In this case, N²>0 and the frequency of oscillation is given by N. If the acceleration is away from the initial position (N²<0), the stratification is unstable. In this case, overturning or convection generally ensues.

The Brunt-Väisälä frequency relates to internal gravity waves and provides a useful description of atmospheric and oceanic stability.