# Equivalent potential vorticity

From AMS Glossary

## equivalent potential vorticity (EPV)

A form of potential vorticity typically used in diagnosing an atmospheric property known as conditional symmetric instability, which is related to slantwise convection. Diagnosing this property is particularly useful when forecasting the potential for narrow bands of heavier precipitation, especially for winter storms.
EPV is a function of the three-dimensional vorticity and the gradient of equivalent potential temperature, given by the following equation:

EPV = −(∇ ×u)∙∇θ_{e},

where ∇ is the three-dimensional gradient operator, **u** is the three-dimensional velocity vector relative to the rotating earth, and θ_{e} is equivalent potential temperature. When used as a diagnostic tool for symmetric instability, **u** is set to the geostrophic wind.

References:

McCann, D. W., 1995: Three-dimensional computations of equivalent potential vorticity. *Wea. Forecasting*, **10**, 798–802, doi:10.1175/1520-0434(1995)010<0798:TDCOEP>2.0.CO;2.

Moore, J. T., and T. E. Lambert, 1993: The use of equivalent potential vorticity to diagnose regions of conditional symmetric instability. *Wea. Forecasting*, **8**, 301–308, doi:10.1175/1520-0434(1993)008<0301:TUOEPV>2.0.CO;2.

*(Term added 5/23/2014)*