Potential density

Potential density

The potential density of a fluid parcel at pressure P is the density that the parcel would acquire if adiabatically brought to a reference pressure P_{0}, often 1 bar (100 kPa). Whereas density changes with changing pressure, potential density of a fluid parcel is conserved as the pressure experienced by the parcel changes (provided no mixing with other parcels or net heat flux occurs). The concept is used in oceanography [http://oceanworld.tamu.edu/resources/ocng_textbook/contents.html] and (to a lesser extent) atmospheric science.

Potential density is a dynamically important property: for static stability potential density must decrease upward. If it doesn't, a fluid parcel displaced upward finds itself lighter than its neighbors, and continues to move upward; similarly, a fluid parcel displaced downward would be heavier than its neighbors. This is true even if the density of the fluid decreases upward. In stable conditions (potential density decreasing upward) motion along surfaces of constant potential density (isopycnals) is energetically favored over flow across these surfaces (diapycnal flow), so most of the motion within a 3-D geophysical fluid takes place along these 2-D surfaces.

In oceanography, the symbol ho_ heta is used to denote "potential density", with the reference pressure P_0 taken to be the pressure at the ocean surface. The corresponding "potential density anomaly" is denoted by sigma_ heta = ho_ heta - 1000 kg/m^3. Because the compressibility of seawater varies with salinity and temperature, the reference pressure must chosen to be near the actual pressure to keep the definition of potential density dynamically meaningful. Reference pressures are often chosen as a whole multiple of 100 bar; for water near a pressure of 400 bar (40 MPa), say, the reference pressure 400 bar would be used, and the potential density anomaly symbol would be written sigma_4.

Potential density adjusts for the effect of compression in two ways:
*The effect of a parcel's change in volume due to a change in pressure (as pressure increases, volume decreases).
*The effect of the parcels change in temperature due to adiabatic change in pressure (as pressure increases, temperature increases).

A parcel's density may be calculated from an equation of state:: ho = ho(P,T,S_1,S_2,...) where T is temperature, P is pressure and S_n are other tracers that affect density (e.g. salinity of seawater). The potential density would then be calculated as:: ho_ heta = ho(P_0, heta,S_1,S_2,...) where heta is the potential temperature of the fluid parcel for the same reference pressure P_0.

References

*cite book | author=John M. Wallace and Peter V. Hobbs | title=Atmospheric Science, An Introductory Survey, Second Edition | publisher=Academic Press | year=2006 | id=ISBN 0-12-732950-1
*cite book | author=Robert H. Stewart | title=Introduction to Physical Oceanography | year=2002 | http://oceanworld.tamu.edu/resources/ocng_textbook/chapter06/chapter06_05.htm


Wikimedia Foundation. 2010.

Игры ⚽ Поможем сделать НИР

Look at other dictionaries:

  • potential density —    1. The density of a unit of water after it is raised by an adiabatic process to the surface, i.e., determined from in situ salinity and potential temperature [22].    2. Density that would be reached by a compressible fluid if it were… …   Lexicon of Cave and Karst Terminology

  • Potential flow — streamlines around a NACA 0012 airfoil at 11° angle of attack, with upper and lower streamtubes identified. In fluid dynamics, potential flow describes the velocity field as the gradient of a scalar function: the velocity potential. As a result,… …   Wikipedia

  • Density (disambiguation) — Density and dense usually refer to a measure of how much of some entity is within a fixed amount of space. Types of density include: In physics, density of mass: Density, mass per volume Area density or surface density, mass over a (two… …   Wikipedia

  • Potential vorticity — (PV) is a quantity which is proportional to the dot product of vorticity and stratification that, following a parcel of air or water, can only be changed by diabatic or frictional processes. It is a useful concept for understanding the generation …   Wikipedia

  • Density functional theory — Electronic structure methods Tight binding Nearly free electron model Hartree–Fock method Modern valence bond Generalized valence bond Møller–Plesset perturbation theory …   Wikipedia

  • Density of states — Condensed matter physics Phases · Phase tr …   Wikipedia

  • Potential well — A potential well is the region surrounding a local minimum of potential energy. Energy captured in a potential well is unable to convert to another type of energy (kinetic energy in the case of a gravitational potential well) because it is… …   Wikipedia

  • Density logging — Well logging Gamma ray logging Spontaneous potential logging Resistivity logging Density logging Sonic logging Caliper logging Mud logging LWD/MWD v · …   Wikipedia

  • Potential der einfachen Schicht — Als Potential der einfachen Schicht wird eine von Karl Rudolf Koch in den 1970er Jahren entwickelte Methode bezeichnet, mit der durch Einführung von Flächenbelegungen die Berechnung des Erdschwerefeldes vereinfacht oder beliebig verfeinert werden …   Deutsch Wikipedia

  • potential-current density curve — poliarizacijos kreivė statusas T sritis Standartizacija ir metrologija apibrėžtis Elektros srovės tankio priklausomybės nuo elektrodo potencialo grafinis vaizdas. atitikmenys: angl. polarization curve; potential current density curve vok.… …   Penkiakalbis aiškinamasis metrologijos terminų žodynas

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”