In atmospheric dynamics
, 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,
- , where is potential temperature, is the local acceleration of gravity, and 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,
- , where , the potential density, depends on both temperature and salinity.
The concept derives from Newton's Second Law when applied to a fluid parcel in the presence of a background
stratification (in which the density changes in the vertical). The parcel, perturbed vertically from its
starting position, experiences a vertical acceleration. If the acceleration is back towards the initial
position, 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.