Using potential vorticity tendency equations for diagnosing atmospheric dynamics in numerical models

The pros and cons of a number of PV tendency equations are assessed. Large cancellation between dominant terms in a commonly used fomulation can obscure results. A PV substance tendency formulation is free from the cancellation problem, but inaccuracies may appear due to the assumption of hydrostatic balance, and the fact that PV substance differs from true PV. An alternative class of PV tendency equations for use in NWP applications is proposed, free from the problems of the existing formualtions.