Potential density
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The Potential Density of a fluid parcel at pressure P is the density that the parcel would acquire if adiabatically brought to a standard reference pressure P0, usually 1 bar. The concept is used in Oceanography[1] and (to a lesser extent) Atmospheric Science.
In oceanography, the symbol ρθ (or more frequently the potential density anomaly σθ = ρθ − 1000) is often used to denote potential density. Furthermore, in Oceanography, the reference pressure P0 is often taken to be the pressure at the ocean surface (≈1 bar) which (for oceanography) is often approximated to be 0.
Potential Density adjusts for the effect of compression in two ways:
- The direct effect of a parcels change in volume due to a change in pressure.
- The indirect effect of the parcels change in temperature due to adiabatic change in pressure.
The second point demands a bit of clarification. Generally, a parcels density is calculated from an equation of state:
- ρ = ρ(P,T,S1,S2,...)
where T is temperature, P is pressure and Sn are other tracers that affect density (e.g. in water, Salinity). The potential density would then be calculated as:
- ρΘ = ρ(P0,Θ,S1,S2,...)
where Θ is the Potential Temperature of the fluid parcel for the same reference pressure P0.
[edit] References
- John M. Wallace and Peter V. Hobbs (1977). Atmospheric Science, An Introductory Survey. Academic Press. ISBN 0-12-732951-X.
- John M. Wallace and Peter V. Hobbs (2006). Atmospheric Science, An Introductory Survey, Second Edition. Academic Press. ISBN 0-12-732950-1.
- Robert H. Stewart (2002). Introduction to Physical Oceanography.
