A generalized form of Lait's modified potential vorticity

Abstract

Ertel's potential vorticity P is in widespread use as a diagnostic of dynamical processes in the stratosphere. For a variety of applications, however, the exponential increase of P with altitude is problematic. For this reason, Lait proposed a modified potential vorticity ΠL, where a physically meaningful scaling is introduced that removes much of the altitude dependence of P. Here a generalized form of ΠL is proposed by introducing an additional degree of freedom in the scaling. This generalized modified potential vorticity Πg possesses the same conservation properties as ΠL itself and as the classic potential vorticity P but can be adjusted more closely to the specific situation under investigation. Comparison, over a large altitude range in the stratosphere, of fields of Πg with dynamical measures of the polar vortex edge and with observations of the long-lived trace gas N2O shows that Πg constitutes a more intuitively interpretable quantity than ΠL.